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Anatomy of Fin Fish and Shellfish - COFS Jabalpur

Anatomy of fin fish and shellfish - Complete practical manual of COFS Jabalpur (BFSc 2nd year). Detailed diagrams, morphology, external & internal anatomy of important fishes, prawns, crabs, lobsters & molluscs. Best for fishery science students preparing for practical exams.

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SEED COURSE 1 UNIT 1 - Anatomy of Fin Fish 1.1 External Anatomy of Finfish Anatomically (Biologically) speaking a fish is composed of ten systems of bodily organs that work together to make up the whole individual. These ten systems cover the fish, handle its food, carry away wastes, they integrate the life processes of the fish and relate it to conditions in the environment. They also provide for breathing and for protections against injury. They support the body and enable movement and finally they work to perpetuate fish as species and through evaluation, as a major group of animals. The living species of fish are usually divided into three classes: the Agnatha, the jawless fishes, comprising the hagfishes and lampreys; the Chondrichthyes, the cartilaginous-skeleton fishes, such as sharks and rays; and the Osteichthyes, the bony-skeleton fishes, comprising all other living fishes. The skeletons of these three groups vary in fundamental ways. In the hagfishes and lampreys the backbone is basically a notochord, a rod like structure composed of unique notochordal tissue. In sharks and rays the notochord is surrounded and constructed by spaced rings of cartilage, the vertebrae, to form a backbone. 1.2. External Anatomy of Fishes 1.2.1. Body forms Commonly the fish body is Torpedo – shaped (fuci form) and often slightly to strongly avoid in cross section. In a perfectly “stream lined” body form (if head pointed trunk broadened and gradually tapering towards the tail) the greatest cross section is about 36% of the length back
SEED COURSE 2 from the anterior tip and gently sweep back to the tailed. However, all fishes do not have this typical body form. The shape of the body is variously modified depending on their habitat and made of living. There changes in shape are Globe shapes (Globiform – pufters of family Tetraodontidae) Serpentine (Snake like Angulli form – eels of family Anguillidae) Thread like in form (filiform – snipe eel Nemichthyidae). Some are strongly flattend from side to life (compressed butterfly fishes, chactodentidae and flounders – pleuronectidae) others flattened but greatly elongated trachipteriform – ribbon fishes, Trachipteridae) and till others flattoned form top to bottom/ depressed the skates and rays (Rajidae). In spite of the many variations the ground plan of body organization in fishes is bilaterally symmetry. The left and right halves of the body are basically mirror images of one another. The tail is an integral part of the body rather than an appendage only the fins are distinctly peripheral anatomical parts in most fishes. 1.2.2. Body Covering Fish is generally covered by a tough skin. It is continuous with the lining of all the body of the entire body opening and is transparent as it runs over the surface of the eye. Much of the diverse coloration of fishes is due to its colour cells and the slimy coating is due to its mucus cells. The skin in many fishes is devoid of scales, but in them it is armored by scales that develop in it. Scales range in size from microscopic to large. In thickness from tissue thin to very thick (in ornamentation from simple to complex in extent of body coverage from portal to complete). In structure from non-bony to bony loosely attached to very firmly attached. The types of scales may characterize the major fish groups. Scale morphology and the number of scales rows along or around the body frequently serve as specific and genetic characters. Living Agnathaus are scale less, shark and their relatives have dentinal placoid scales, Bony fishes have various types of bony scales. 1.3. Internal Anatomy of finfish
SEED COURSE 3 Internal anatomy of a bony fish: finned aquatic vertebrates animal with skin covered with scales. It lives in water and is usually oviparous. Brain: seat of the mental faculties of a fish. Esophagus: part of the digestive tract connecting the mouth to the stomach. Dorsal aorta: vessel in the back that carries blood from the heart to the organs. Stomach: part of the digestive tract between the esophagus and the intestine. Air bladder: pocket in which urine collects. Spinal cord: part of the nervous system that connects the brain to all other parts of a fish. Kidney: blood-purifying organ. Urinary orifice: opening for eliminating urine. Genital Orifice: opening related to the genital organs. Anus: end of the digestive tract. Gonad: hormone-secreting sexual gland of a fish. Intestine: last part of the digestive tract. Pyloric cecum: cul-de-sac related to the intestine. Gall bladder: small sac containing the bile. Liver: bile-producing digestive gland. Heart: blood-pumping organ. Gills: respiratory organ of a fish. Tooth: hard organ of a fish used to shred food. Eye: sight organ of a fish. Olfactory bulb: bulging part of the smell organ of smell of a fish 1.4. Structure of fins Appendages of fishes comprise of fins and cirrti (flaps of flesh) which attain extreme development in the sargassum fish (pterophryne) and the leafy sea dragan (phyllopteryx). The
SEED COURSE 4 fins are classified as median or paired. Median fins (Unpaired fins) Rayed fins in line with the median axis of a typical fish are these of the back (dorsal fin or fins) the tail (caudal fin) and the lower edge of the body just behind the vent (Anal fin). Although most commonly all of the median fins may be present, but some may be modified or absent in same fishes. Also developed in the median axis may be a rayless fatty adipose fin (as in the trouts – Salmonidae) off fins reduced to a few disconnected spines as in the stickle backs – Gasterosteidae) The anal fin may be modified into an intermittent organ, the gonopodium for use in copulation (as in the live bearers – Poecilidae). Paired fins Paired fins are the Pectorals and Pelvics (ventral) the pectorals are supported by the pectoral girdle that joins the skull. Although ordinarily present in fishes, the pectorals are wanting in such kinds as the lamprey (Petromyzonidae) and hag fishes (Myxinidae). They are greatly enlarged in the (Soaring) flying fishes (Exocoetidae) and flying characins (Gastropelecinae). In some cling fishes (Gobiesocidae) and Asiatic suckerbelly loaches (Gastromyzan) the pectorals serve as a part of the ventral hold fast organ.
SEED COURSE 5 The pelvic fins vary substantially in position and in adoptive modification typically their support is by a pelvic girdle anchored in the bully musculature. In soft rayed fishes (Malacopterygians) the peluics are abdominal in position, Eg. Clupeoids, Salmonids, cypriroids, but they also be situated arterially, just below the pectorals in a thoracic position (as in many spiny rayed species, the Acanthopterygians) or even under the throat in a jugular position (as in blennies). The pelvic are lacking in the lampreys and hag fishes and is lost in various other fishes,notably the eels (Angullidae etc). In the sharks and some relatives they are modified into claspers. In cling fishes (Gobiorocidae) sucker belly loaches (Gastromyzon) and certain other fishes, the pelvic from part of a hold fast organ that resembles a ruction cup on the belly. 1.5. Skin and Scales The skin of a fish consists of two layers. The outer layer is epidermis and the inner, the dermis or corium. The epidermis is composed superficially of several layers of flattened, moist epithelial cells. The deepest layers are a zone of active cell growth and multiplication. Here cell multiplication goes on all the time to replace from with in the outermost layers of cells as it is worm off and provide for growth. These epithelial cells from the epidermis are the first to close a surface wound. The dermal layer of the skin contains blood vessels nerves and cutaneous sense organs and connective tissue. The dermis plays the main role in the formation of sales and related integumentary structure. Scattered among the flattened cells of the epidermis are numerous openings of the tubular and flask shaped mucous gland cells that extent into the dermis. These cells secrete the slippery mucus that cover most fishes. The mucus lessons the drag an a fish when it swims through water.
SEED COURSE 6 As in other vertebrates, the skin of a fish is the envelope for the body and is the first lines of defense against disease. It also affords protection from, and adjustment to environmental factors that influence life, for it contains sensory receptors tunes to the surroundings of a fish. Further more, the skin has respiratory, excretory and osmoregulatory functions. 1.6. Scalation Outstanding among the special feature of the skin are the very prominent scales that most fishes have. However some do not have scales. Examples include the lampreys (petromyzonidae) and a number of cat fishes North America (F.W) (Ietaluridae). An intermediate structural category have seals only on a few places of the body for example: Paddle fish (polydon) three spines stickle bask Gasterosteus aculeatus and mirror carp. Then there are a few fishes that have very small deeply imbedded scales that they look scale less for example Fresh Water Eels (Anguilla) Brook trauts (Salvelius fontinalis) and the barbot (lota) of Cod family (F.W) (Gadidae) In arrangements scales are most often “imbricated” and thus overlap like roof tiles with free margin directed towards the tail in a manner that minimize friction with water. In some fishes such as barbot (lota) and F.W eels (Anguilla) the pattern is mosaic rather than overlapping one another, the scales are minutely separated or meet their neighbors only at their margins Scales shapes Although they comprise only a few basic structural types, Scales exhibit many modifications that are often characteristic of groups or species on the basis of shape one type is plate like (Placoid), with each plate carrying a small cusp, as common among the sharks (Elasmobranchil) a Second type is diamond shaped (rhombic) and found in gars (Lepirosteidae) and the bichirs (pdypterus). A third type of scale is called Cycloid because it is typically smooth disc like and more or less circular mostly found in soft rayed fishes. (Malacopterygi). In fourth type is ctenoid the posterior surface or margin is toothed or comb like and a characteristic of spiny rayed bony fishes (Acunthopterygil) However, some of the soft rayed fishes have ctenoid like contact organs on their scales, examples characins (characid) Killi fishes (cyprinodontidac). Some spiny rayed fishes have cepeloid scales exclusively. For example the brooke silverside labidestus. Many spiny rayed species exhibit both cycloid and ctenoid scales. Example: Common basses (Micropterus). Structural types
SEED COURSE 7 Structurally, there are two types of fish scales, placoid and Non-placoid. Non-placoid scales are basically of the three kinds – cosmoid, Ganoid and Bony ridged scales. I) Placoid scale Placoid scales also called dermal denticles have an ectodermal cap or covering of enamel like substances (as an human teeth) termed “Vitrodentine” beneath this is a thicker layer of entine with a pulp cavity and dentinal tubules emanating form it. Each scale has a disc like basal plate in the dermis with a cusp projecting outward from it though the epidermis. Placoid scales occur among the sharks and their relatives (chandrichthyes). II) Non – Placoid scales 1) Cosmoid scales: The cosmoid scales have a thinner, harder outer layer than the placoid ones. It is also termed as vitrodentine. The layer below this hard and non cellular and is called cosmine. The layer beneath is vascularised mid layer of perforated bony substances called “Iropedine” The growth of the scale is at the edge and beneath no ligivng cell layer covers the scale. The cosmoid scales found in (latimeria). 2) Ganoid Scale: In ganoid scale the outer layer is a hard inorganic substances called ganoine which is different from Vitrodentine. The layer below the ganoine is a cosmine like layer. The innermost layer is a bony layer called isopedine. The growth takes place at the edge, beneath and on the surface. Three types of scales are found in bichirs (pdypterus) and gars (Lepisosteidae). 3) Bony Ridges Scales: Bony –ridged scales are typically thin and transclucent lacking both dense enameled and dentinal layers of the three other kinds. Bony ridged scales characterizes the many living species of bony fishes (Osteichthyes) that have either cycloid or ctenoid scales. The outer layer of there scales is marked with bony ridges alternate with depression. The inner part or plate of the scale is made up of layers of criss – crossing fibrous connective tissues. The growth of the scales is both on the outer surface and from beneath. For ridges on the scale two terms are used ridges and circuli changes in the growth pattern of the individuals fish is reflected in the character and distribution of ridges. “Breeding” and “Yearmarks” can be identified on this basis in many species. In both cycloid and ctenoid scales,
SEED COURSE 8 a nuclear central zone can be recognized. This zone may be called as “focus” the scale. However this central position shifts due to differential growth of fore or after part of the scale, shifting the focus to more posterior and anterior margin. In many species grooves or radii radiate from or near focus towards one or more margins of the scale. The ctenoid scales have teeth on its posterior margins. Unit 2 - Cell structure, tissue and body organization 2.1. Organization: kind of construction of the body A cellular or protoplasmic level of organization: Body is not differentiated into cells. Example: Protozoa (subkingdom: Protozoa); All other animals except Protozoa come under the subkingdom: Metazoa (Body is composed of numerous cells) Cellular level of organization: The cells of the body are more or less loose and Independent. Example: Amoeba Tissue level of organization: In some animals, body cells form tissues. Example: Coelenterata and Ctenophora Organ system level of organization: In most of the animals, tissues combine into organs, which, in turn, combine to form organ systems. Example: Animals like Human being, Fish, Reptiles etc., Germ Layers: The fundamental cell layers laid down in an early embryo of the Metazoa are called germ layers. The Coelenterata have only two layers: Ectoderm and Endoderm. They are termed the diploblastic animals. Most other Metazoa possess three germ layers: Ectoderm, Mesoderm and Endoderm. They are known as the triploblastic animals. Example; Animals except Coelenterata 2.2. Symmetry It refers to the similarity in size, shape and number of parts on the opposite sides of a median line. Parts of an animal body are often so arranged that it is possible to cut it into two similar halves by one or more planes. Such animals are said to be symmetrical. Some animals, like Amoeba and Snail, cannot be divided into equal parts by any plane. They are said to be asymmetrical. The symmetrical animals show one of the three types of symmetry: spherical, radial and bilateral. Section – 1 Spherical (universal) symmetry: Body is divisible into similar halves by any plane passing through the center. Example: Seaurchin Section – 2 Radial symmetry: Body is with a number of similar parts radiating out from a central axis. The boy can be split up into equal halves by any plane passing through the middle from top to bottom. The animals with radial symmetry are called Radiata. Examples: Sponges and most coelenterates. In some animals, some parts of the body are single or paired rather than radial, so that only one or two planes through the longitudinal axis divide the animal into similar halves. Such a symmetry is called biradial. Examples: Comb-jellies, seanemones and seastars.
SEED COURSE 9 Section –3 Bilateral symmetry: In rest of the Metazoa, chief organs of the body are paired and are arranged on the sides of a central axis connecting the head with the tail. With the result, the body can be divided into two similar halves only by one plane. Such are termed as bilateral. The bilaterally symmetrical animals are termed the bilateria. A plane or section passing dorso-ventrally (vertically) through the middle of the anterior- posterior (longitudinal) axis is known as the Sagittal plane or section. This is the plane that divides the bilateral animal into similar and right and left halves. A plane or section running antero-posteriorly at right angles to the sagittal plane is called the frontal plane or section. It divides the body into dorsal and ventral halves. A plane or section passing dorso-ventrally at right angles to the antero-posterior axis is known as transverse plane or section. It divides the body into anterior and posterior halves. Section – 4 Digestive tract: The Mesozoa and parazoa lack a digestive tract. The Eumetazoa possess a digestive tract, and are termed the Enterozoa. Certain enterozoa, viz., coelenterata,Ctenophora and Platyhelminthes, have incomplete digestive tract, which has a single external opening, the mouth, that serves for intake of food as well as elimination of faeces. All other enterozoa have complete digestive tract, which has two external openings, the mouth for intake of food and the anus for elimination of faeces. Section – 5 Coelom: Some bilateria have no cavity in their body, except that in the digestive tract. Such forms are known as the acoelomates. All others possess a cavity in addition to the one in the digestive tract. The space between the two cavities is called the body cavity the nature and origin of this cavity varies in different groups. When lined by a mesodermal epithelium, the peritoneum, it is known as the true coelom, and when unlined by peritoneum, it is termed false coelom or pseudocoel. The animals with false and true coelom are respectively called the pseudocoelomates end eucoelomates. In Mollusca and Arthropoda the coelom is greatly reduced, and the blood fills the spaces between the internal organs. Such a body-cavity is called the haemocoel. The coelom is of great significance in animal evolution. It provides space for the development of various organ systems. Section – 6 Metamerism or Segmentation: Segments occur in three phyla: Annelida, Arthropoda and Chordata. Each segment is called a metamere or somite. Metamerism affects external and internal structures. It is said to be homonomous, if the somites are similar, as in earthworm (annelida); and heteronomous, is the segments are dissimilar as in prawn (Arthropoda).The segmentation in Chordates shows up clearly in the embryonic stage in te segmentally arranged mesodermal somites. In adult chordates segmentation is largely internal and is seen in their vertebrae, ribs, nerves and blood vessels. Unit 3 - Oral region and Associated structures 3.1. Adaptations for feeding (The feeding habits or feeding behavior of fishes is the search for and ingesting of food) The
SEED COURSE 10 diversity in feeding habits that fishes exhibit is the result of evaluation leading to structural adaptation for getting food from the environments. The structural adaptation of feeding organs such as 1) position and shape of the mouth 2) Presence or absence of “Teeth”. 3) Gap of the mouth etc., greatly help in predicting the nature of food and mode of feeding of the fish (in question). Section 1: Major feeding types : On the basis, fishes can be classified, according to their feeding habits as 1) Predators 2) Grazers 3) Food strainers 4) Food suckers and 5) Parasites. 1) Predators Fishes that feed on macroscopic animals, they usually have well developed grasping and holding teeth as in many sharks (Elasmobrabch) the Barracuds (Sphyraena), the pikes (Esox) gars (Lepisosteus). 2) Grazers In Grazing, the food is taken by bites or continual browsing (Grazing characterize many fishes that feed on planktons or on bottom organisms) (sometimes organisms are taken single or at other times in small groups) may feed on bottom or column – a bluegill (Lepomis macrochirus), Parrot fishes (scaridae) Butterfly fishes (chaetodontidae) browsing on coral reef. 3) Strainers (Filter feeders) Filter the water for plankton, here foods are selected by size and not by kind. Herring clupidae, Gizard, shads (Dorosoma), Paddle fish (polyodon) whale shark (Rhincodon) also have efficient food straining or filtering adaptation. The principal adaptation for filter feeding or strainers is the developemt of numerous, closely – set and elongated gill rackers. 4) Suckers The sucking into the mouth of food or food containing material is taken practiced by bottom feeding fishes such as the sturgeons (Acipenseridae) and suckers (catostomidae) 5) Parasites Parasitisum is perhaps the most unusual and highly evolved feeding habits among animals. They suck body fluid after rasping a hole in the sides of the body. Parasitic lampreys (Petromyzonidae), Sea lamprays (Petromyzon marinus) Pacific lampreys (Lamptra tridentata) 3.2. Major feeding types On the basis, fishes can be classified, according to their feeding habits as 1) Predators 2) Grazers 3) Food strainers 4) Food suckers and 5) Parasites. Predators Fishes that feed on macroscopic animals, they usually have well developed grasping and holding teeth as in many sharks (Elasmobrabch) the Barracuds (Sphyraena), the pikes (Esox) gars (Lepisosteus). Grazers In Grazing, the food is taken by bites or continual browsing (Grazing characterize many fishes that feed on planktons or on bottom organisms) (sometimes organisms are taken single or at
SEED COURSE 11 other times in small groups) may feed on bottom or column – a bluegill (Lepomis macrochirus), Parrot fishes (scaridae) Butterfly fishes (chaetodontidae) browsing on coral reef. Strainers (Filter feeders) Filter the water for plankton, here foods are selected by size and not by kind. Herring clupidae, Gizard, shads (Dorosoma), Paddle fish (polyodon) whale shark (Rhincodon) also have efficient food straining or filtering adaptation. The principal adaptation for filter feeding or strainers is the developemt of numerous, closely – set and elongated gill rackers. Suckers The sucking into the mouth of food or food containing material is taken practiced by bottom feeding fishes such as the sturgeons (Acipenseridae) and suckers (catostomidae) Parasites Parasitisum is perhaps the most unusual and highly evolved feeding habits among animals. They suck body fluid after rasping a hole in the sides of the body. Parasitic lampreys (Petromyzonidae), Sea lamprays (Petromyzon marinus) Pacific lampreys (Lamptra tridentata) 3.3. Oral adaptation for feeding Lips Suctorial feeders have an “inferior mouth” and fleshy modification of lips. Notable among these are the sturgeons (Acipenseridac) and suckeres (catastomidae). The lips of strurgeons and suckers are mobile and described as “Plicate” (having folds) or papillose (having small tufts of skin or papillose). Many sectorial feeders also have well developed barbells bordering the mouth.
SEED COURSE 12 Sucker mouth : Moxostoma (Redhorse) suctorial lips of free – living fishes may also serve as holdfast organs in fast flowing mountain streams. For example (Glyptorternum). In loach – like Gyriocheilid (Gyrinocheilus) has an extreamly specialized suctorial adaptation, in addition to suctorial lips the opercular structure has separate inhalent and exhalent device for respiration. 3.3.1. Modifications in the shape of mouth Among the grazers and suctorial feeders, there exist not only specially developed lips but also adaptations of other mouth parts. The Trumpet fishes (Aulostomidae) the cornet fishes (Fistularidae) and the pipe fishes (syngnanthidae) as well as many butterfly fishes (chaetodontidae) of coral reefs, have mouths that resemble elongated beaks. This adaptation is achieved by a protraction of the hyomandibular bone rather than by a lengthening of the lower jaw bones (denteries) themselves. Among these fishes, the method of feeding may be by suction as in the case of trumpet fishes, cornet fishes and pipe fishes or it may be a selective grazing action with sharp teeth when the long snout enables the butterfly fish to reach into small crevices of the coral. A peculiar structure among mouth modification has arisen in the half beaks (Hemiramphidae) where the lower jaw projects into a beak often a third of the length of the fish itself, with the mouth opening above it. Half beaks are usually surface feeding fishes. 3.3.2. Teeth Outstanding among the obvious oral adaptation for feeding in fishes are the teeth. They are
SEED COURSE 13 thought to have arisen from scales covering the lips, as represented in living sharks (squliformies) where the placoid scales of the skin visibly grade into teeth on the jaws. In bony fishes (Osteichthyes) Teeth are of three kinds, based on where they are found Jaw, Mouth and Pharyngeal. Jaw Teeth Jaw teeth are variously those on the maxillary and premaxillary bones above and on the dentaries below. In the roof of the oral cavity teeth are variously borne by the median vomer and by the palatine and ectopterygoid bones on each side. In the floor of the mouth the tongue often has teeth on it. Pharyngeal teeth occur as pads on various gill arch eolements in many species. In the carps (cyprinedae) and suckers (cjcatastromidae) the only teeth are those in deep in the pharynx (gut, mouth and eoesophagous) that develoed modification of lower elements after last gill arch, in clarius and labeo the teeth are modified for grasping, tearing, grinding and razor like cutting teeth have developed in predacious fishes. Kinds of jaw teeth Based on their form major kinds of Jaw teeth are 1) Cardiform 2) Villiform 3) Canine 4) Incisor and 5) Molariform. Cardiform Teeth Cardiform teeth are numerous, shrotfine and pointed such dentition with variations is found in many fishes that have multiple rowed teeth. For example American catfish (Ictaluridae) perches (percidae) and many sea bases (serranidae) Villiform teeth Villiform teeth are more or less elongated cardiform teeth. For example : Needlefishes (Belonidae) and Lion fishes as (Pterois). Canine teeth Canines are dog tooth like or fange like (long pointed tooth) they are elongated and subconical, straight or curved and are adapted for piercing and hodling for example walleyes (Alska pollock) (Stizostediosn). In certain fishes such as moreys (muraenidae) the canines are hinged (the hook) yield ot backward pressure but lock when moved forward and adaptation to retain living moving prey inside the mouth. Incisors teeth Incisors are sharp edged cutting teeth. In some fishes incisors fuse together in cutting beak as in parrot fishes (scaridae). Molariform teeth Molariform teeth are for crushing and grinding thus flat with prodruding denticles on the surface. These teeth are found in bottom dwelling fishes like skates and rays and some sciaenidae (drums). With in a single group the diversity of dentition on identical bones may vary largely. For example In carps and minnows (cyprinidae) the pharyngeal teeth range from sharp in carnivores such as in semotilus to molariform in the common carp (Cyprinus carpio). In general teeth are absent in plankton feeders and in some of the more generalized omnivore. They are present in increasing numbers of bones in more and more relative to its predatory behaviors. The premaxillary bones are toothed when jaw bones have little or no teeth. This is true for many soft rayed species such as bowfin (Amia) the gars (lepisosteus) the salmons and
SEED COURSE 14 trouts (Salmonidae). The maxillae are typically toothed in those soft rayed fishes as that carry premaxillary teeth. However tooth less in otherwise tooth – bearing spiny rayed fishes. 3.4. Gill Rakers Besides protecting the tender gill filaments form abrasion (a rubbing off or scrap) by ingested materially that are coarse in texture, gill rakers are specialization in relation to food and feeding habits. They are very stubby (short and thick) and unornamented in most omnivores for example sunfish (Lepomis cynellus). In many plankton feeders, the gill rakers are elongated, numerous are variously lamellate or ornamented to increase efficiency of filtering. Simple but very numerous rakers are possessed by gizzard shads (Dorosoma) and paddlefish (polyodon) Ornamentation of gill rackers are of taxonomic value. In pleuronectdae the gill rakers resembles feather having a main axis with lateral processes these lateral processes are branched. The adjacent gill rakers overlap to form a very fine sieve. Unit 4 - Gastro Intestinal Tract OR Digestive system and associated digestive glands 4.1. Gastro Intestinal Tract Esophagus Esophagus forms the beginning of the gastro intestinal tract/ In general the oesophagus is no distensible that it can accommodate anything the fish can get into the mouth and sometimes even accommodate the item if it happens to double on itself two or three times, on its way to the stomesh. The ocsgtagus is a short and narrow to be in a no. of Herbivorus and omnivorus fishes. (Cyprinus corpio, L.rohito,Tor tor etc. A large no. of mucus – secreting cells are scattered in the mucosa at taste buds are also present in some or species. 4.1.2. Stomach Frequently in literature the fishes are classified as stomach or stomach less fishes. 85% telecast have stomach 15% had no stomach. The stomach too shows various adaptations one of which is shape. In carnivore the stomach
SEED COURSE 15 typically is straight, elongate for example in gars (lepisosteus), bowfins (Amia), Pikes (Esox) and barracudas (sphyraena). In omnivorous species, the stomach is most often sac-shaped. A very special adaptation is the modification of the stomach into a grinding organ as in the sturgeons (Acipenser) gizzard shads (Dorosoma) and mullets (Mugil). Here the stomach is reduced in overall size but its wall greatly thickened and muscularized. The lining too is heavily strengthened with connective tissue and the lumen is very small. The organ is not for storage, mixing and primary digestion but rather a food grinder. Great dispensability is the adaptation of the stomach in the predatory deep sea swallowers (saccopharyngidae) and gulpers (Eurypharyngidae) enabling these fishes to take relatively huge prey. A remarkable modification of the stomach exists in the puffers (Tetradontidae) which can inflate themselves with water or air to assume often on almost globular shape. The adaptive value of this modification of the digestive tract is probably mainly one of defense, for many puffers and porcupine fishes have spines all over the body which can thus be created. Not all fishes have a stomach that is a portion of digestive tube with a typically acid secretion and a distinctive epithelial lining different from that of the intestine. In most of the herbivores and phytoplankton feeders the epithelial tissue of the esophagus grades directly into that of the intestine, thus termed stomach less. Though the primary criterian for being without stomach does not seem to be whether a fish is an herbivore or a carnivore but whether accessory adaptations for trituration and fine grinding of food exist either in the form of teeth or a grinding apparatus such as a gizzard. Where stomach exist most pronouncedly (strongly) in carnivores. They are characterized by a low pH and the prominent presence of pepsin (enzyme) among other digestive juices. 4.1.3. Intestine The intestine too has many variations. It is shortened in carnivores such as in the pike (Erox), perhaps because meaty foods can be digested easily. Where as in herbivores the intestine is highly elongated and several times the body length of the fish and in carps and certain catfishes. In sharks and other elasmobranchs the intestine has a coiled layer of absorptive tissue called spiral valve which increase absorptive surfaces for the relatively in short intestine.
SEED COURSE 16 4.1.4. Pyloric caeca (Fishes are only vertebrates that have appendages (Caeca) at the gastro-intestinal junctions). On the intestine of most bony fishes at the pyloric end of the stomach. There may be from one to many blind sacs or pylonic caeca or intestinal caeca. A few groups such as cat fishes (Ictaluridae) topminnows (Cyprinidae) and pikes (Esocidae) lack these structures. In such groups as flat fishes (Pleuronectiformies) the pyloric caeca are few usually not more than five (5). In others such as Mackerels (Scombridae), Salmons (Salmonidae) the number of these caeca may range to 200 or more. Generally caeca of different species vary considerably in size, state of branching and the connection with the gut. In sturgeons (Acipenseridae) the many caeca forma large mass, but only a single duct leads to the intestine. In salmon, each caecum communicates directly with the gut, the functions of pyloric caeca probably involve both digestion and absorption. 4.1.5. Rectum Morphologically there is no distinction between the rectumad intestine, the ilio-rectal valve is present in sciana, tetradon and muracnosox. 4.1.6. Anus It is the posterior opening of the alimentary canal or digestive system. The internal surface of the region near the rectum is covered with an epithelium rich mucous cells. The anus is made up of an inner circular and outer longitudinal muscle layer. The circular muscle is thick developed forming sphincter. The region of anus facing the anterior has the epithelium continues with the skin.
SEED COURSE 17 4.2.1. Associated digestive glands Liver Like other vertebrates, liver is an important organ in fishes which has both “secretary” and “storage” function. This is a large gland in all fishes, but sharks and rays may have extremely large livers comprising about 20% of the body weight especially in some pelagic sharks. The liver usually lies over or partially surrounds the stomach. It is typically bilobed, but may have only one lobe as in salmon or three as in mackerel. In Hag fishes the liver is in two distinct parts, with separate ducts leading to the gallbladder. Adult lamprey have no bile ducts or gall bladder, gall bladder is also absent in burbot (lota) but in most other fishes the gall bladder is present and function to store liver secretions. Ordinarily one hepatic ducts originates from each lobe of the liver and joins the cystic ducts from the gall bladder to form the “bile ducts” Liver function includes bile secretion and glycogen storage in addition to several other biochemical processes. Apart from their function fish liver also stores fats, vitamin “A” and “D” and the weary RBCs releasing hemoglobin for recycling into the body liver also helps production of urea and other nitrogenous compounds. 4.2.2. Pancreas Hag fishes have a small – pancreas with several ducts that empty into the bile duct. Lampreys have pancreatic tissue located through out the liver and intertinal wall. Among bony fishes the pancreatic tissue is usually diffused in or around the liver. This is especially true of spiny rayed fishes, in which the pancrease and liver incorporate in a “hapatapancrease”, in many of soft rayed bony fishes Pancreas is a distinct organ, in sharks and rays also the pancrease is a compact organ with two lobes. The pancreatic duct may reach the small intestine separately from the bile duct as in the sharks or may discharge into the bile duct as in the gar (lepisosteus). The pancreas secrets several enzymes, that are active in digestion. In addition the pancreatic islets have the endocrine function of producing insulin. Section 3: Spleen The spleen is usually recognized as a dark red structure lying on or behind the stomach to which it attaches by a band like ligament. Although it is associated with the digestive organs it has no digestive function, but rather is instrumental in blood cell formation (The function of red blood cell distraction is ascribed to spleen of higher bony fishes). In lampreys and Hag fishes, which do not have a compact spleen, spleen like tissue is diffused along the intestine. 4.2.4. Gas bladder The Gas bladder is a thin walled sac typically found in the upper part of the body cavity immediately below the kidney. In many fishes the shape is simple, usually some what torpedo shaped, but there are many variations, minnows and carps (cyprinidae) have anterior and posterior chambers, connected by an opening controlled by a splinter. Feather backs (Notopteridae) have air bladder divided laterally, but two chambers communicate anteriority most croackers (scieanidae) have unusual air bladders in that variously shaped sacs or branching caeca may be arranged along each side of the organ. In Herrings (chupeidae) the gas bladder has a posterior opening to the exterior near the anus. In the embryology of bony fishes the gas bladder originates as an outgrowth of the alimentary canal and remains attached to the esophagus or stomach via the pneumatic ducts such fishes
SEED COURSE 18 are known as physostomes. Some groups of fishes lose the connection to the alimentary tract. These fishes are known as physoclistous. Some of such fishes may retain the connection till larval or juvenile stages. Some bottom-dwelling stream fishes such as darter (Etheostoma) and Sculpin (cottus) lack the gas bladder. Various other bathy pelagic fishes have also lost gas bladder. Agnathus and cartilaginous fishes also lack gas bladder. Unit 5 - Circulatory System 5.1. Introduction Fishes are cold-blooded aquatic vertebrates and can be found in both saline and fresh water. The circulatory system of fishes is responsible for transporting blood and nutrients throughout the body. It has a closed circulatory system, i.e. blood travels across the body through the network of blood vessels. Unlike humans, fishes exhibit single cycle circulation, where the oxygen deprived blood comes to the heart, from where it is pumped to the gills and then circulated to the entire body. On the other hand, in mammals, the deoxygenated blood enters the heart, from where it is pumped into the lungs for oxygenation. Then, the oxygenated blood returns to the heart from the lungs, to be transported throughout the body. 5.1.1. Circulatory System of a Fish The circulatory system of fish is quite simple. Like mammals, the circulatory system of fish consists of a heart, blood and blood vessels. The heart of a fish is a simple muscular structure that is located between the posterior gill arches. It is enclosed by the pericardial membrane or pericardium. In most of the fishes, the heart consists of an atrium, a ventricle, a sac-like thin walled structure known as sinus venosus and a tube, known as bulbus arteriosus. Inspite of containing four parts, the heart of a fish is considered two-chambered. The blood contains plasma (the fluid portion of blood) and the blood cells. The red blood cells or the erythrocytes contain hemoglobin, a protein that facilitates the transport of oxygen to the entire body, while the white blood cells are an indispensable part of the immune system. The thrombocytes perform the functions that is equivalent to the role executed by the platelets in the human body, i.e. they help in blood clotting. Blood is circulated throughout the body with the help of blood vessels. The blood vessels are of two types, arteries and veins. The arteries are responsible for carrying oxygenated blood from the heart to the rest of the body, while the veins return deoxygenated blood from the different parts of the body to the heart.
SEED COURSE 19 5.1.2. Heart The heart is modified blood vessel exhibiting the three layers characteristic of arteries an inner lining (endocardium) of endothelium and elastic tissue on muscular layer (myocardium) that is very thick, especially in the ventricular region and an outer fibrous tunic (epicardium) on the surface of which is the visceral pericardium. The heart pulsates as a result of the response of the muscle cells to the electrolytes (Blood electrolytes) that infuse it. The rhythmicity of the pulsations is regulated by flexly by the autonomic nervous system except in hag fishes, in which no nerve fibers supply the muscle. The heart occupies the pericardial cavity (a sub division of the coelom anterior to the septum transversem). The heart of all vertebrates is built in accordance with a basic architectural pattern. It is demonstrated in its simplest form in hag fishes where its exhibits a series of four chambers sinus venous, atrium, ventricle and conus arterious through which flood flow in that sequences. Chondrychthyes has contractile, muscular and valved base the conus arteriosus to the ventral aorta where it leaves the ventricle. In higher bony fishes the plan is like that of the shark and its relatives, but the first section of the ventral aorta is typically thin-walled and valved bulbus
SEED COURSE 20 arteriosus and not contractile but elastic, alternately enlarging and shrinking in response to changes in blood pressure from alternate ventricular contraction, systole (contraction) and dia stole (relaxation) The ventral aorta in a fish is median in position, beneath the gills from its branch the afferent bronchial arteries to each gill pouch or arch. With in the gills, afferent bronchial break down into capillaries and collect again in to efferent vessel that form the dorsal aorta main vessel for distribution of blood to the body. 5.2.1.Blood Circulatory System Cyclostomata The largest number of seven to fourteen branchial arteries occurs in the lamprey – hag fishe group (cyclostomata) and depend on the number of gill pouches to be supplied. The branchial artery leads into gill filaments where it breaks up into capillaries in the lamellae, the capillaries reform into efferent vessels. In lamprey and hag fish the efferent branchial vessels rise dorsally one by one to join the mediam aorta. The first efferent branchial artery largely provides the blood supply for brain and head regions, although some arterial vessels principally the hyoid artery arise from the first afferent gill vessels. The Dorsal aorta is the main route of blood transport from the gills to the body it is unpaired behind the gills to the trunk region, it runs behind the notochord. The numerous branchial vessels that arise from he aorta and the corresponding veins are arranged/more or less to match the body segments. Lamprey and hag fish uniquely have valves to prevent back flow of blood to dorsal aorta. The venous systems in cyclostomes have elaborate sinuses the systems in the hag fishes also have secondary heart in the form of contractile bulbs along the way as in the veins of the tail. In lamprey posterior part of the digestive tract is supplied by vessel which have an artery placed in the lumen of a vein the blood from the intestine returns to the heart via the cardinal veins through suprearenal venous sinus. 5.2.2. Arterial System in Elasmobranchs Afferent branchial arteries The ventral aorta arises form the conus arteriosus and veins forwards along the ventral surface of the pharynx right upto the posterior border of the hyoid arch where it bifurcates into two branches each branch divide into first and second afferent branchial arteries. The first afferent branchial artery runs along the posterior border of the first branchial arch and supplied arterial branches to anterior and posterior gill lamellac of the first branchial arch of the third, fourth and fifth afferent branchial arteries arise form the ventral aorta almost equidistant from one another and runs along the outer border of the second third and fourth branchial arches. This supply the blood to the 3rd 4th and 5th gill arches respectively. Efferent branchial arteries The blood from the capillaries of gill lamellac is collected by a serious of blood vessel called the efferent branchial arteries. There are nine efferent branchial vessels on each side (may be less depending upon species) of these the first eight join in pairs to form four complete toops around the 1st four gill clefts. The 9th runs along the anterior border of the fifth gill cleft. The four loop and 5th efferent branchial arteries are joined by short longitudinal connectives running across the inter branchial septa. The efferent branchial subsequently join to form the
SEED COURSE 21 medium dorsal aorta. Arteries of the head The first efferent branchial and a small part from the dorsal aorta supply blood to the head. The first efferent branchial vessel gives rise to three branches the external carotid afferent spiracular and hyoidean. The afferent spiracular runs forward and gives rise to ophthalmic artery. This artery moves forward joins with external carotid to form cerebral artery. 5.2.3. Dorsal aorta and its branches The dorsal aorta runs backwards along the whole length of the body lying beneath the vertebral column. In the tail region it continues with in the haemal canal as caudal artery. Anteriorly it gives off several small branches to the roof of buccal cavity. The following arteries that arise from dorsal aorta, the celiac supplies blood to stomach and liver, the anterior mesenteric supplies the pancreas the intestine and the rectum the posterior mesenteres to the gonad. The parietal to the body wall the renel to the kidney (and the femoral to the griddle region). 5.2.4. Arterial system in Bony fishes Heart is similar to that of shark. But there is no “conus arteriosis” . However, the base of the ventral aorta is enlarged to form a non-contractile “bulbus arteriosis”. Arterial system The ventral aorta is comparatively shorter. It gives off 4 pairs of afferent brachial arteries through which venous blood is supplied to the gills ( 4 pairs) for purification. The blood from the gills is collected by 4 pairs of “efferent branchial arteries” In the bony fishes each gill arch has a single efferent artery unlike the condition in shark. The four efferent branchial arteries unite to form the lateral aorta on each side. The two lateral aorta are joined together both anteriorly and posteriorly forming a ring shaped vessel known as the “circular cephalicus”. In the anterior end, the external and internal carotids arise on each side form the circular cephalicus posteriorly the
SEED COURSE 22 two laterial aorta fuse together to produce the dorsal aorta which supplies all the organs of the abdomen and ends in the tail. 5.2.5. Venous system The blood distributed to the different parts of the body by the arteries and their braches through capillaries and returned to the heart by the veins. The veins differ in structure from the arteries in possessing thin walls and in forming wide irregular spaces called sinuses (during then curve) The Venus system can be devided into the following systems.  Anterior cardinal system.  Posterior cardinal system  Hepatic portal system  Ventral sinus  Cutaneous system. Anterior cardinal system: the anterior cardinal system consists of the following viens, the internal juglar veins it has the alfactory sinus, orbital sinus, post orbital sinus and anterior cardinal sinus. The orbital sinus finally opens into large anterior cardinal sinus. It opens into ductus cuaveri. The posterior cardinal system: This system consists of medium caudal vein two renal veins and two large posterior cardinal sinuses. The renal portal vein gives of branches to kidney recollect into renal veins to form the posterior cardinal sinuses. Hepatic portal system: This system has the hepatic portal vein anterior and posterior intestinal veins, gastric vein and the anterior and posterior gastric. These subsequently form hepatic sinuses which open into sinus venosus. Ventral veins: The ventral vein comprises two groups. 1) the anterior veins which discharges the blood into ductus curvier through the inferior jugular sinuses and the posterior veins which empty their blood through the subclavians. Cutaneous systems: The cutaneous system includes a dorsal, a ventral and two paired lateral veins. The ventral joins the abdominal veins. The laterals empties into the branchial vein.
SEED COURSE 23 Unit 6 - Respiratory system in fishes 6.1.1. Gills Gaseous Respiration in Fish Just like you and me fish need a constant supply oxygen in the form of O2 in order to run their metabolism. Without oxygen they can't turn their food into energy or make any new fish body. All the free oxygen on this planet on this planet, was, or is being, released into the air by plants, the atmosphere at the moment is about 21% O2. However oxygen will dissolve in water, in a similar sort of way that the bubbles in your Coca Cola are dissolved into the liquid, which is mostly water, that makes up the drink. Fish could of course breathe air like Seals and Whales, and some do, but if they wish to stay safe under the water for longer periods of time it would be much easier if they could get the oxygen they need from the water, and this is exactly what they do. In fact they were doing this long before any vertebrate animals learned how to breath the air. So fish live in the water and they breathe the water, to do this they have special organs called gills. Gills are wonderfully well designed, and they have to be because although the water does hold some oxygen it never holds any where near as much as the air, and to make things more difficult the amount of oxygen a body of water can hold decreases the warmer it becomes, and also, salt water holds less oxygen than fresh water. If the air above a body of water is 21% O2, this means that 210 parts per thousand are O2, but if we do the math for one of the figures below, such as cold salt water we see that it contains only 7.58 parts per thousand O2. What this means to the fish is that their gills need to be a lot more efficient at extracting O2 from the water than our lungs need to be at extracting it from the air. Fish solve the problems of extracting the O2 they need from the water they live in in a variety of ways. Firstly they have different life styles, obviously a fish that spends most of its life resting on the bottom of the ocean waiting for its dinner to swim by needs less O2 than a fish which actively chases smaller fish for its dinner. However most of the problem is solved in the design of the gills. A fish's gills are situated one set on either side of the body and near the back of the head They are open to the gullet at the front, and open to the external environment behind. They are designed so that water can flow continually passed them, coming in through the mouth, and/or the spiracle in sharks and their allies, and passing out through the single external gill opening in fish or through one of the 5 to 7 gill clefts in sharks and rays. In fish there is a bony plate protecting the gills, this is called the operculum, and it is hinged and has muscles attached to it so it can be regularly opened and closed.
SEED COURSE 24 This ability to have water continually passing over the gills is one of the major factors making gills more efficient than lungs. With lungs the air comes in, fills the space, and then has to be expelled before any more O2 rich air can be brought in. With gills there is no time wasted getting rid of the old air/water and no energy wasted reversing the direction of the flow. In sharks and rays the number of gills is usually 5, but there are some species with 6 or 7 sets, in fish the number of gills is 4 on either side of the body. Each gill is supported by a gill arch and protected by gill rakers. Each gill arch supports one set of paired gill filaments. The gill rakers help make sure that no extraneous material gets into the gill filaments to clog them up. Each paired gill filament in turn supports numerous lamellae (sing. lamella), extending out from both sides of the filament body. It's here in the lamellae that the uptake of O2 actually occurs. The lamellae are very fine structures, however there exact dimensions depend on the normal activity levels of the fish in question. The more active the fish the thinner they are and the less distance there is between them. Also the absolute thickness of the individual lamellae walls varies, this is important in considering to facility with which O2can diffuse from the water to the fish's blood, the thinner the membrane the more quickly. and easily the O2 can pass across it. Thus in sluggish fish like the American Brown Bullhead (Amerius nebulosus) the lamellae are 25 µ thick, 45 µ apart and the lamellae walls are 10 µ thick giving you 14 lamellae per mm, whereas in a highly active fish such as the Atlantic Herring (Clupea harengus) the lamellae are 7µ thick, 20 µ apart and the lamellae walls are >1µ in width, giving you 32 lamellae per mm. The presence of all these lamellae greatly increases the surface area of the gills, meaning that a large amount of water is available for gaseous exchange at any particular moment of time. In active fish, such as the Atlantic Mackerel (Scomber scombrus), which has nearly the same gill dimensions as the Atlantic Herring, there may be as much as 1,000 square mm of lamellae
SEED COURSE 25 surface for every gram of body weight. Such a fish weighing 1 kg will have approximately 1 square metre of gill surface area. Having such a large amount of gill area obviously helps the fish in its battle to extract enough O2 from the water it lives in. 6.2.1. Structure of gills 6.2.2. Gill slits There are six or seven pairs of gills in cartilaginous fishes while four pairs in bony fishes due to the loss of spiracle. Gill slits of bony fishes are covered by operculum while operculum is absent in cartilaginous fishes. In sharks gill slits are situated while in rays they are ventrally placed. A pair of spiracle is present in Elasmobranchii anterior to first gill which corresponds to a vestigial primitive first gill slit. Although spiracle is absent in bony fishes, in Actinopterygii it is replaced by a pseudobranch which is free in some fishes but skin covered in others. 6.2.3. Pseudobranch In carp and rainbow trout the pseudobranch is embedded in submucosal connective tissue of pharyngeal wall and shows a glandular appearance due to complete conglutination of branchial filaments. In some species, a pseudobranch with hemibranchs structure is located inside the operculum. However, in eel the pseudobranch is not present. it is also absent in cat fishes (Siluroidae) and featherback (Notopteridae). In glandular pseudobranch, abundant distribution of blood capillaries is found in the parenchyma enclosed by connective tissue. It contains acidophilic cells in mitochondria and endoplasmic reticulum and is rich in enzyme carbonic anhydrase. The pseudobranch regulates the flow of the arterial blood to the ophthalmic artery to increase the amount of blood carbon dioxide. In rainbow trout extirpation of pseudobranch indused melanophore expansion and body colour change, suggesting the secretion of a melanophore-aggregating hormone frome tissue. It also helps in metabolic gas exchange of retina and filling of gas bladder. Because of its direct vascular connection with choroid gland on the eyeball, the pseudobranch has been implicated in the regulation of intercellular pressure. The structure of gills has been studied extensively in Indian fishes by light, transmission and scanning electron microscopy. The gill comprises gill rakers, gill filaments (primary gill lamellae) and lamellae (Secondary gill lamellae).
SEED COURSE 26 A complete gill is known as holobranch. It consists of a bony or cartilaginous arch. The anterior and posterior part of each gill arch possesses plate like gill filaments. Each holobranch consists of an anterior (oral) and a posterior (aboral) hemibrnch. The architectural plan of teleostean gills shows hetergenity in their functional unit which is due to varied osmoregulatory, feeding and respiratory behaviour and to the physcochemical status of their environment. In teleost fishes, five pairs of branchial arches are present of which first four bear gill lamellae but the fifth is devoid of gill lamellae and transformed into the pharyngeal bone for mastication of food. It does not play any role in respiration. The gill arch is an important unit and bears primary (gill filament) and secondary lamellae. The branchial arch typically consists of paired pharyngobranchials, epibranchials, ceratobranchials, hypobranchials and a median unpaired basibranchial. The epibranchial and the ceratobranchial elements of each branchial arch bears two rows of gill filaments of the two hemibranchs of the holobranch., which are the seat gaseous exchange. It encloses afferent and efferent branchialarteries and veins. It is also provided by nerves. The branches of 9th (glossopharyngeal) cranial nerve innervate the first gill, while II, III and IV arches are supplied by the branches of vages (10th cranial nerve). It also contains abductor and abductor muscles. Inside it containes gill rakers, taste buds, mucous gland cells and sensory papillae. 6.2.4. Gill raker It occurs in two rows on the inner margin of each gill arch. Each gill arch is short stumpy structure supported by bony elements (fig3, a & b). The gill arch projects across the pharyngeal opening. They are modified in relation to food and feeding habits. The mucous cells of the epithelium help to remove sediments from the covering epithelium in order to enable the taste buds to function effectively and to sense the chemical nature of food passing through the gill sieve. 6.2.5. Gill Filaments (Primary gill lamellae) Each hemibranch consists of both primary and secondary lamellae. The primary gill filaments remain separated from the branchial septum at their distal end making two hemibranch in opposition which direct the water flow between the gill filaments. Amongst duel breathers the heterogeneity in the gill system is more pronounced particularly in the swamp eel, Monopterus, Amphipnous cuchia and climbing perch, Anabas testudineus. In Monopterus gill filaments are stumpy and are present only in second pair of gill and lack gill lamellae. According to Munshi
SEED COURSE 27 and Singh (1968,a) and Mnshi et al,(1990), the remaining three pairs are without functional lamellae. It is the modification for another way of exchange of gases. The gill filaments are blad-like structures supported by gill rays. The heads of the gill rays of both the hemibranchs are connected by ligaments (Yadav et al. 1993). They provided with two types of adductor muscle units in teleosts. The gill filaments are also lined by epithelium referred to as primary epithelium. The epithelium has glandular and non glandular part. 6.2.6. Lamellae (secondary lamellae) The each gill filaments is made up of secondary lamellae which are actual seat of exchange of gases. They are generally semicircular and lined up along both sides of the gill filaments. The lamellae frequency is directly proportional to the dimension and resistance of the gill sieve. The secondary lamellae are having two sheets of epithelium which are separated by space and through these spaces blood circulates. The epithelial sheets are separated by a series of pillar cells. Each cell consists of a central body and is provided with extension at each end. 6.3.1.Functions of Air bladder or Gas bladder As a Hydrostatic organ: It is important function of air bladder is hydrostatic. This is because of the fact that the bony fish can fill up or empties the bladder at will. This help them to maintain their body density closely to that of the surrounding environment thus the air bladder actually act as “float” in the body cavity. When the gas content in the bladder is increased, the fish becomes lighter and it rises to higher levels and conversely when diminished the fish sinks deeper in water. As a respiratory organ: fishes which have to live in draught condition or foul water, the bladder sometimes acts as a subsidiary or accessory respiratory organ. In some deep water marine fishes the air bladder acts as a store house of oxygen. In some gill breather also store or in their air bladder for emergency purpose. As a sound, producing organ: A large number of fish species uses the air bladder for sound production. By producing characteristics sound, the fish probably communicates with the apart sex. Sound is also produced when fish “apprehends any danger”. As a sensory organ: It is viewed that the wall of the air bladder is highly sensitive to water pressure as well as temperature fluctuations. Thus air bladder acts as a barometer or monometer or a Hydrophone. Unit 7 - Nervous system in fishes 7.1. Introduction As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, the brain, and spinal cord, are the primary integrating mechanisms. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for
SEED COURSE 28 appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain. The nervous system consists of Brain, Spinal Cord and the nervous to co-ordinate various activities (Peripheral nerves and Autonomic nervous) of the body. Fish brain is an enlarged anterior end of the spinal cord. It is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception such as sight, hearing, or smell (olfaction). Its parts progress linearly from a “forebrain” region (enlarged cerebral hemisphere and the connecting tween brain), through the “Midbrain” with its swellings (the optic lobes) to the “hind brain” (cerebellum and medulla) and continue toward the caudal fin with the spinal card (In embryonic) the forebrain is called “Prosencephalon” the mid brain “mesencephalon” and hind brain – Rhombencephalon, the brain and spinal cord are whitish and soft. (The brain is housed in cranium of the skull). The spinal cord runs length wise of the fish in neural canal of the vertebral column. The cerebral hemisphere (forebrain) and cerebellum are more prominent in sharks and relatives (Chondrichthyes) and bony fishes (Osteichthyes) than in the lampreys and hag fishes (Cyclostomes). The “mid brain” prominent in the cyclostomes is also prominent in the Chondrichthyens. However, that of the higher bony fishes (Actinopterygi) is often very large. The cavities of the brain are continuous with that of the spinal cord. Being highly complex life forms fish need a brain and a nervous system to control their body's actions. The nervous system of fish, much like ours, is composed of a central co-ordinating brain, a spinal cord and many, many nerves. 7.1.1. The Brain Generally speaking fish have small brains in relationship to their overall body weight. Elasmobranchs (Sharks and Rays) in general have a slightly larger brain for the same body mass as Teleosts (Bony Fish), however there is great variety within the teleosts scientists have learned something quite surprising about the Elephantnose Fish (Gnathonemus petersii). The brains of cyclostomes (Hagfish and Lampreys) are simple but specifically evolved to suit their lifestyles. For instance the optic lobe is well developed in the visually oriented Lampreys but indiscernible in the blind Hagfish. In both however the medulla is large and the cerebellum small. Together the cerebellum and the medulla make up the hind brain.
SEED COURSE 29 The medulla controls the operations of the inner organs such as heart rate, blood pressure, digestion and waste disposal. It is also a relay centre for many nerves sending messages to and from the mid and or forebrain. The cerebellum controls motor co-ordination (but it does not initiate motor activities). This means it controls the timing and interaction of muscles once a muscular action has been initiated. The cerebellum is also important in maintaining equilibrium. The mid-brain of a fish consists mostly of the optic lobes, which vary greatly in size between species in accordance with their dependance on sight, and in some species the optic lobes may be so large they completely cover the forebrain. In fish the mid-brain is important in sorting out incoming information and it is also the main centre of learning (whereas in mammals it is the forebrain that is the main centre of learning). The forebrain of fish is dominated by the olfactory lobes which extend forwards and may be placed at the end of stalks. These olfactory lobes are large in the cyclostomes and very large in the elasmobranchs reflecting the importance of smell to these to groups of fish. The teleosts, for whom sight is often the most important sense have smaller olfactory lobes. In many elasmobranchs and some teleosts there exists a cerebrum or pair of cerebral hemispheres These also seem to be predominantly involved with the sense of smell (in mammals the cerebrum is much larger and involved in planning and learning). The pituitary also arises out of the forebrain, it plays and important role in the regulation of metabolism. A fish's brain never completely fills the cranium, the cavity in the skull where it lies protected. The remaining space is filled up with a gelatinous material. Finally as in all vertebrates the brain, plus the gel, are surrounded by a membrane that helps keep foreign matter and micro- organisms from contacting this most important organ. The forebrain in fishes denoted to the reception elaboration and conduction of smell impulses. The mid brain consists of the optictectum and all vision senses are received here. This optic tedium is great functional importance in the central nervous system (CNS) and has several layers of nerve cells. Hind brain controls the swimming equilibrium, maintenance of muscular tomes and orientation in space in the anterior most regions. The brain divisions have developed as the sensory integrations. It is the centre to which lead the sensory nerves except those of smell (I) and sight (II) 7.2. Nerves Apart from the brain and the spinal cord the fish body is supplied with a vast network of nerves, the electric wires of the body along which messages travel. Nerves are built of of numerous neurons and neurons are a one-way system, messages either travel to or from the brain or the spinal cord along a particular neuronal path, but never both ways. Those nerves that arise from the spinal cord are called spinal nerves and those which arise from the brain are called cranial nerves. Normally there is one pair of spinal nerves (left and right) for each vertebrae, thus long thin fish with many vertebrae such as eels will have many more pairs of spinal nerves than a much shorter fish such as a gobi. In fish there are 10 pairs of cranial nerves all with well defined roles.
SEED COURSE 30 7.2.1. Peripheral nervous system There are two kind of nerves, spinal and cranial nerves. The former take their origin from the spinal cord and are metamerically arranged, that is to lay, their number is same as that of the vertebrae. Spinal Cord: The spinal cord, or nerve cord is similar in all fish. It is a thick sheath of nervous material that runs from the base of the brain back along the fish's body through, and protected by, the neural canal of the spinal column. Normally it extends the full length of the fish's body, but a notable exception to this is the giant Sunfish (Mola mola) wherein the spinal cord is actually shorter than the brain. It serves as the basis of many simple responses and as the major link to the brain for sensory input and brain-mediated responses. Cranial nerves The cranial nerves arise from the brain and Ten (10) pairs of them are typically present in a teleost (twelve in higher vertebrates). They are as follows: 1) Olfactory 2) Optic 3) Oculomotor 4) Trochlear 5) Trigeminal 6) Abducens 7) Facial 8) Acoustic or Auditory 9) Glorrophoryngeal 10) Vagus. Olfactory nerve: It is purely sensory one connecting nasal organ with olfactory lobe. It is a special sensory nerves conveying smell impulses to the brain. Optic nerve: Likewise sensory and supplies to the eyes. In bony fishes, the two crosses each other below the brain immediately after leaving the optic lobes. The nerve from the left lobe joins to the right eye and vice versa. It supplies the visual impulses. Oculomotor nerve: Arises from the lower surface of the brain and innervates four of the six striated muscles of the eye ball. Trochlear nerve: Arises form the dorsolateral side of the brain between the optic lobes and the cerebellum and supplies the superior oblique muscle of the eye ball. Trigeminal nerve: Arises from the lateral side of the medulla oblongata and supplies the snout and upper and lower jaws. It is a mixed nerve and is divided into three important branches – the ophthalmic, maxillary and mandibullar. Abducens nerve: Arise from the ventral side of the medulla oblongata, a little behind the trigeminal nerve. It also enters the orbit and supplies the posterior rectus muscle that moves the eye ball. The Facial nerve: has independent origin from the side of medulla oblongata behind the trigeminal, but soon joins the later to form the trigemino – facial complex which divides into three branches, the supra–orbital, infra orbital and hyomandibular. The Auditory nerve or acoustic nerve: Arises form the side of the medulla oblongata behind the facial and supplies nerves to the “inner ear”. The Glassopharyngeal nerve: Arises from the ventro – lateral aspect of the medulla oblongata behind the auditory and enters the first gill slit. It is a mixed nerve and supplies a part of the lateral line system, taste buds in the pharynx and the muscles of the first gill slit. The Vagus: The vagus nerve arises behind the glasso-pharyngeal and has an extensive distribution. It divides into branchia – visceral trunk and a lateralis branch. The branchio visceral trunk divides into three branchiolis nerves and a visceralis branch. Each branchial branch gives off a slender branch to supply the muscles of the gills. The visceralis branch supplies various
SEED COURSE 31 organs of the viscera. The lateralies is a stout nerve that runs up to the end of the tail along the lateral line canal and innervates it by a several branches. Unit 8 - Urino – Genital System 8.1. Introduction The fish, like most animals, begins life as an egg and as in other invertebrates, the single cell egg can not develop unless it is fertilized by a sperm produced by a male. Fish sperm is most commonly referred to as milt. Eggs may be fertilized either externally or internally. External fertilization takes place when the egg is penetrated by the sperm after the egg leaves the female’s body. Most fish are reproduced by this system. Internal fertilization occurs when the male introduces the sperm into the female’s body, where it makes contact with and fertilizes the egg. Some sharks are ovoviviparous; that is, the egg is fertilized internally and held within the female without attachment to her until it is ready to be extruded alive. In other species, such as some of the sharks and the sculpin, and the skate, the egg is penetrated by the sperm inside the female’s body, but it does not hatch until some time after being released from the female. Reproduction and associated activities in fish are generally referred to as spawning. The spawning season, or breeding period, is that time when the eggs of the female and the milt, or sperm, of the male are ripe. This period may last only a few days or it may extend into weeks and even months. Fish that live in tropical waters of fairly constant temperature may spawn year round. 8.2. Reproduction The reproduction in fishes is bisexual, hermaphrodite or parthenogenic the most predominate process is bisexual reproduction. In such fishes the sexes are separate. Eg :- The individuals are dioecious. In few fishes both sexes are present in same individuals i.e., such fishes are hermaphrodite. Eg. Perca, Stizortadian, Micropterus. In some fishes, juvenile hermaphroditism has been noticed. In Poecillia fermosa parthenogenesis occurs, actually the correct process is gynogenesis i.e., the development of young without fertilization. Members belonging to class Pisces show a variety of sexuality from synchronous hermaphroditism, protogynous and protandrous hermaphroditism (consecutive hermophroditism) to gonochorism (Dioecious). Types of Reproduction: Synchronus hermaphroditism : In synchronus hemophroditism, both testis and ovary mature at the same time with a possibility of self fertilization. Eg. Scrranus subligirus Family – Serranidae, Cyprinadontidae, Maenidae, Labridae, Ipoopidae etc. Consecutive hermaphroditism: In Consecutive hermaphroditism the fish may be first a female with functional ovaries and consequently a functional male, such hermaphroditism is called “Protogynous hermaphroditism” and when the fish first a functional male and then a functional female such hermaphroditism is term “Protandrous hermaphroditism”. Protandrous - Gonostamatidae, Sparidae, Labridae. Protogynous – Synbranchidae, Serranidae, Maenidae. Gonochorism (Dioecious):
SEED COURSE 32 Gonochorism means when both the sexes, male and female are two separate functional individuals (unisexual the male and female reproductive organs born on different individuals). 8.3.Sexual differences The characteristics of sexual differences or sexual dimorphism that unable identification of the sexes is classified as primary and secondary. Primary sexual characters are those that are concerned actually with the reproductive processes. Testis and their ducts in the male and ovaries and their duct in the female constitute primary sexual characters. The secondary sexual characters themselves are really of two kinds those which have no primary relationship with the reproductive act at all, and those which are definitely accessory to reproduction. A genital papilla is present in the male fishes of lampreys (Petromyzonidae) darters (Etheostoma nigrum) white bass (Morone chrysops). “Tubercles” appear on various areas of the body surface in many males during sexual maturity. For example, smelts (Ormerus), some minnows (Cyprinidae). The “fins often” provide characteristic distinctive of males: on an average they are larger than in female. In some fishes, the “caudal fin” may show sexual dimorphism, for example the lower lobe as greatly extended in the males of the sword tail (Xiphopherus helleri) and somewhat enlarged in white sucker (Catastomus commersoni). Obviously “colouration” in fishes often serves as a mark of sexual distinction and recognition; it is termed sexual dichromatism. In general, the males are brighter or more intense in colour than the females. For example, in orange spotted sunfish (Lepomis humilis) Bowfin (Amia calva), the male has a darker eye spot on the tail region. Other example are Wrasses (Labridae)and parrot fishes (Scaridae). Several different head characteristics also serve to distinguish the sexes among fishes for example, In Salmons and Trouts (Salmonidae) the breeding makes typically develop a knobby hook or kype near the tips of both the upper and lower jaw.
SEED COURSE 33 An accessory sexual characters marks the males of several species in which the anal fin becomes enlarged into an “intermittent copulatory organ”. This organ designated as the “gonopodium”, occurs in such fishes as the mosquito fish (Gambusia affinis) the guppy (Lebistes) and other males of live bearing top minnows (Poecilidae). Pelvis fins are variously modified in the sharks and their relatives (Elasmobranchi) as intermittent structures, the “myxopterygia” (Claspers) that help to ensure internal fertilization, which is widespread in this group of fishes. A few accessory reproductive structures among females serve as sexual characteristics. An outstanding example is the egg laying tube or ovipositor in the female of the European betterlings – Rhodeus amarus.
SEED COURSE 34 8.4. Reproductive system Gonads The gonads of fishes are usually elongated structures suspended by mesentries from the dorsal aspect of the abdominal cavity. Their relationship to the kidney and their ducts differs widely among groups. 8.4.1. Lampreys The gonads are single, suspended from the midline and reach most of the length of the body cavity. Though no sperm ducts or oviducts are present at spawning but both the eggs and sperm are shed into the body cavity from which they exit through paired abdominal pores to the urogenital sinus. A prominent urogenital papilla is developed in mature specimen. 8.4.2. Hag fishes As in lampreys, a single elongate gonad is present in both sexes; the testis is irregular and
SEED COURSE 35 lobate, without a sperm duct and sperm released into the body cavity. Unlike the lampreys, hag fishes produce large eggs with tough shells. There is no oviduct. Both eggs and sperm reach the exterior through an abdominal pore just behind the anus. In hag fishes the gonads are suspended by a mesentery from the gut. 8.4.3. Sharks The testes are paired and usually placed anteriorly in the body cavity suspended dorsally by means of a “mesorchium”. Often right testis is larger than the left, sperm discharges into a central canal network that communicates with the anterior part of the kidney through efferent ducts tranversing the mesorchium. The front part of the kidney is modified into a glandular “epididimis” where the archinephric ducts receives the efferent ductles and runs down posteriorly. Just behind the testis the kidney is modified into “Leydig gland” in which the tubules secrete a “seminal fluid” into the archinephric duct. As the archinephric ducts runs down along the kidney as the vas deferens, it enlarges into a seminal vesicle from which a sperm sac opens dorsally. The vesicles and sperm sacs open into the urinogenital sinus, which in turn empties into the cloaca. From the cloaca sperm enter the grooves of the claspers, through which they can be transferred to the female. Ovaries are paired, but the left one may be greatly reduced in size in some species. Like the testes, they are placed well anteriorly in the body cavity; each is suspended by a mesovarium. The oviducts open anteriorly to the ovaries and usually have a common mouth or funnel. Eggs are released into the coelom, proceed into this funnel and then traveled down the oviduct to the region of the shellgland (Nidamental gland) where fertilization occurs and a horney shell or membrane as secreted. In oviparous species, the shell is tough and protects the developing embryo. In viviparous species, the shell is slight or vestigial and the young develop in the posterior, uterine portion of the oviducts. 8.4.4. Bony Fishes In most of the bony fishes testis are whittish, lobulate organs, lying along the gas bladder, although in some groups such as Salmonids, the organs are smooth and entire without lobules. In most of the forms, there is no connection at all between the reproductive openings to the exterior for the two systems with the urinary pore posterior to the genital pore. In some the sperm ducts connects with the urinary system in a urogenital sinus located at the
SEED COURSE 36 posterior end of the body cavity. Ovaries of bony fishes are typically saccular and continue with the oviduct. Interesting exceptions to this system involves the more primitive forms viz., Acipenser, Polydon and Amia all shed eggs from the incompletely covered ovaries into the body cavity. Eggs are caught by a coelomic funnel opening partially from the back of the ovary and conveyed through the oviduct. Other exceptions are seen in the Smelts, Salmonids, Eels and few others, none of which have saccular (A hallow or bag or a pouch and a flexible structure in the body). Ovaries continue with oviducts. Smelts have coelomic oviduct with funnels opening behind the ovaries. Salmonids show incompletely enclosed ovaries and extrude eggs though a very short funnel leading to a pore just anterior to the urinary pore. Eels have no funnels; the eggs simply pass out through a pore. The ovaries of fishes are usually well separated, but fusion of the right and left organs can be seen in some Percoids (fishes of the family Parciformes). In large mouth bars, the ovaries join posteriorly to produce a “V” shaped structure. Ovaries of the yellow perch, Perca flavaseens are so completely fused so as to give the appearance of a single organ. This ovary is fused to the body wall just posterior to the anus and eggs are extruded when this area ruptures, so those oviducts are not functional. The rupture of the body wall heals soon after oviposition. 8.5. Excretory system The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney. The kidney, gills, and skin play an important role in maintaining a fish's internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion). The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but that of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable (like those of the kidney) of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water in their food. Marine fishes must conserve water, therefore their kidneys excrete little water. To maintain their
SEED COURSE 37 water balance marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. By reabsorption of needed water in the kidney tubules, they discharge a more concentrated urine than do freshwater fishes. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Some marine fishes, at least, can excrete salt by clusters of special cells in the gills and intestine. There are several teleosts-for example, the salmon-that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment. Marine lampreys, hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood. 8.5.1. Structure of Kidney “Kidney is the organ through which most of the metabolic wastes are excreted”. The kidneys of fishes are reddish brown, soft elongated paired structures lying ventrally to the vertebral column. Though paired, the two kidneys show multiple shapes in different fishes due to fusion at various positions. Ventrally the kidneys are covered by coelomic epithelium. The teleostean kidney may be divided into a “head kidney” and a “trunk kidney” but this differentiation sometimes is not possible by external observations. Anatomically fish kidney is of two types – Pronephric and Mesonephric. In fishes with kidney distinguishable to a head and trunk kidney show the pronephric part in the head region (generally non functional in terms of excretion and mesonephric part in the trunk region. Generally there are no conspicuous differences in shape between the two sexes. “Ogawa” (1961a) has classified the marine teleostean kidney into five (5) configurational classes.  Type I: The two sides of the kidney are completely fused throughout. No clear distinction between trunk and head kidney. Ex. Clupeidae (Herrings).  Type 2: The middle and posterior portions only are fused. Clear distinction between head and trunk kidney. Ex. Plotosidae (Marine catfishes), Angulidae (Eels).  Type 3: Posterior portion only is fused, anterior portion represented by two slender branches, clear distinction between head and trunk kidney. Most marine fishes have this type of kidney. Ex. Bellonidae, Mugilidae, Scombridae, Carringidae and Pleuronectidae.  Type 4: Extreme posterior portion only is fused and head kidney is not recognizable. Ex. Syngnathidae (Pike fishes).  Types 5 : The two kidneys are completely separate. Eg: Lophidae (Angler fishes) Ogawa has observed that all the freshwater teleost species that he examined can be grouped into the first 3 or the 5 groups described above. Examples are as follows. Type 1 – Salmonidae – Salmons and trouts. Type 2 – Cyprinidae – Carps and minnows Type 3 – Cyprinodontidae (Killi fishes), Gastrostidae and Cottidae. Generally the anterior part of the kidney (Head kidney) is pronephric and possesses lymphoid, haematopoitic internal and chromofin tissue. The nephron is generally absent in this region but when present it is devoid of renal corpusele and glomerulis. In most fishes the pronephrons is
SEED COURSE 38 traditional and it is taken over by the mesonephros in the later period of the life (Adult stage). In other words the head kidney in majority of fishes is not functional and in some other it is functional only in the early stage of the individual (life). Some teleosts, however, show the extension of posterior kidney (trunk kidney) towards the anterior kidney. In this part of the kidney typical nephrons are present with glomerulli, renal corpuscle, renal tubule and collecting ducts. The variable amount of haematopoitic and pigment cells are distributed among the tubules and vascular spaces in the trunkate region. The two archineptric ducts are always fused. This fusion may occur at the posterior end of the kidney or at some point between the kidney and the urinary papilla. Dilation of the orchinephric duct may found a bladder like enlargement (Urinary bladder) in many bony fishes. Unit 9 - Endocrine system 9.1. Introduction The vertebrate endocrine system is present in it essence already in Lampreys and Hag fishes,further similarities with higher vertebrates appear in both the shark and bony fishes. Fishes possess a well developed endocrine system comparable to that of other vertebrate .The recent demonstration of tissue with parathyroid like function (Rasquin and Rosenbloom 1954) has filled a gap formerly thought to exist in the list of fish endocrine organs. Major contributions in the fish endocrine have also been made in studies of physiology of migration (Fontain,1954,Oliverean 1954)and reproduction (Bretsch-neider,Wit 1947,Docid 1955,Hoar,1957). Important Endocrine Glands in Fishes. Following are important endocrine glands found in fishes  Pituitary gland.  Thyroid gland.  Chromaffin tissue (Supra renal body, medullary tissue represent the adrenal).  Interrenal tissue (Adrenocortical tissue).  Gonads.  Ultimobranchial tissue.  Intestinal mucosa. Following are main tissue with possible endocrine function:  Carpuscles of stanius.  Pineal organ.  Thymus  Urohypophysis  Pseudobranches. 9.2. Pituitary gland The arrangement and organizations of the component parts of pituitary gland varies greatly in different species.Olivereace,1954,has recently discussed the terminology and the homologus of the different area.the pituitary gland or the hypophysis develops from two parts as follows:
SEED COURSE 39  Neurohypophysis derived from the brain.  Adenohypophysis which develops as an ectodermal up-growth the roof of the buccal cavity (Rathke’s pouch). The Hypophysis of the ventral side of the brain just behind the optic chaisma and is attached to the infundibulam by means of stalk.In some fishes the stalk is absent and the gland is pressed closely against the floor of the brain. Adenohyphophysis The adenohypophysis is divided into pars distallis and pars intermedia. Of these the parsis further divided into rostral pars distalis (Pro-adenohypophysis)and posterior distalis (meso adenohyphophysis).the pars intermedia corresponds to the meta adenohypophysis of the older terminology. Neurohypophysis The region consists of a large number of nerve fibres that arises from cells bodies in the hypothalamus and enter the hyphophysis,where they branch repeatedly and ramify in the pairs intermedia. Pituitary in Elasmobranch The neurohypophysis of this group is diffused and intermingled with the pars intermedia.the two parts are often been collectively referred to as neurointermediate lobe.Neurosecretory axons are arose from the paraoptic nuclei and lateral hypothalamus terminate in the neurohypophysis but,these are absent from the anterior infaundibulam floor.a few such fibres termed the saccus vasculosus are folded and highly vascularised structure laying posterior to the neurohypophysis. The pars distalis lies below the infandibulam and is divisible in the procentral and rostral zone.the ventral lobe is a peculiar feature of the Elasmobranches adenohypophysis and varies greatly in size and shape among different species. 9.2.1. Pituitary in teleosts In teleosts the adenohypophysis consists of a pars intermedia and an intermediate lobe which is sometimes divisible topographically into a rostral and procentral region.This region of pars destalis appears to contain cell and the cell types characteristic of the mammalian anterior lobe .some workers have homologues the rostral pars distalis with the tetradopods pars tuberalis,but it is not certain that this is true. the neurohypophysis of teleost as in Elasmobranch is diffused and interdigites with the cells of pars intermedia and to a lesser extentwith the cells of pars distalis.Neurosecretory fibrous axons from the paraoptic nucleus and the lateral tubular nucleus terminate in all parts of neurohypophysis but their secretions have also been found in both regions of pars distalis. The Saccus vasculosus is well developed in many teleosts but it does not appear to be supplied by neurosecretory tract.
SEED COURSE 40 9.2.2. Thyroid gland In elasmobranchs,thyroid is an eucapsulated organ usually located near the point where the afferent branchial arteries leave the ventral aorta. the thyroid is typically an unpaired organ lying below the pharynx. In teleosts the thyroid follicles are scatterd along the ventral aorta.thyroid follicles are often found in the head kidneys of the platy fish (platypolcitus).tyhe number increasing with the age.in a few teleosts such as ‘Bermuda’ Parrot fish, the follicles are arranged to form a compact unpaired gland. 9.2.3. Chromaffin tissue Adrenal tissue are present in all vertebrates from cyclostomata to mammals but profound differences are encounted in the arrangement of the functional components i.e.,steroid producing cells and catecholamine producing cells.in Elasmobranches the stereo genetic tissue is composed into several well form bodies lying between the caudal ends are kidneys.these are interregnal glands.paired aggregation of chromaffin pairs are present between kidneys.the two components of the adrenal are typically separated through small cells of chromaffin cells and have been described in the interregnal gland of the ray.Great variation is found among actinopterygian fishes(teleost) with respect to the condensation and dispersion of adrenal tissue.these are generally located within or just anterior to the cephalic kidney and occur around the post cordinal veins and their branches. 9.2.4. Interrenal tissue ‘Baecker’-1928 and ‘Dettus’-1940 have described the interregnal tissue of many fishes.Abion-1940 has made an extensive review of the literature.In Elasmobranchs interregnal is located posteriorly in one or more compact masses between the opisthonephric and is
SEED COURSE 41 discreate enough to permit surgical removal.In teleosts the interregnal gland is located historically.here it is confined to the cranial regions of the head kidney. 9.2.5. Gonads In fishes, the maturation and possibly also the elaboration of gametes requires the presence of sex harmones in addition to secondary sex characteristics such as coloration,breeding tubercells and the maturation of gonopodium depends upon the pressure of endocrine substances from the sex glands. Testis Testis is made up of lobular units.in teleost lobular unit may be short and long tubules and internally divided space with their apex at the center of the organ and their broader ends directed towards the periphery.In most of fishes however the com-plex network of elongated and intergreatly divided lobules, and lobularspaces are bound together forming a compact organ. Septum and covering contain much elastic but no muscle.the lobules open into a spermatic duct which may be long and tortuous with a lining of secretory epitheliumor may be straight and simple. Interstitial tissue and serotoli cells.Seasonal variation and the amount of interstitial and connective tissue cells of gonads have been described in details.in this species the development of interstitial tissue probably the source of testicular androgen considers with the appearance of secondary sexual characters and complex breeding behavior. Sertoli cells have been described in both Elasmobranch and teleost and show cyclical changes in relation to spermatogenesis.this is noted sperms are often discharged at the stage.
SEED COURSE 42 Ovary The ovary is made up of numerous ovarian follicles embedded in connective tissue.the arrangement of ovarian follicles within the supporting tissue varies greatly in different groups.The ovary in most teleost fishes is a hollow sac-like organ into which extend numerous ovigerous folds lined by germinal epithelium. The germ cells, the endodermally derived oogonia, multiply mitotically and get transformed into nonyolky primary oocytes whose, nuclei are arrested at the prophase of the first meiotic division until maturation. These processes can take place even in the absence of the pituitary (Barr, 1968; Hoar, 1969). Primary oocytes, covered generally by two layers of follicle cells, an outer thecal layer and an inner granulosa layer, undergo vitellogenesis when yolk is deposited in the. ooplasm. During maturation, the first polar body is given cut and the second meiotic division is arrested at metaphase. The eggs are spawned by the fish at this stage and the second polar body is released only after fertilization. In some fishes, ovulation and- spawning occur almost at the same time, whereas in ethers (rainbow trout and milkfish) ovulated oocytes are retained in the ovarian or peritoneal cavity and spawning takes place much, later.
SEED COURSE 43 9.2.6. Islets of Langerhans Islets of Langerhans, also called islands of Langerhans, irregularly shaped patches of endocrine tissue located within the pancreas of most vertebrates. The islets consist of four distinct cell types, of which three (alpha, beta, and delta cells) produce important hormones; the fourth component (C cells) has no known function.Within the pancreas of higher forms of sharks,islets of Langerhans develop from the tubules of the otherwise called digestive gland.islets tissue in the actinopterygian fishes tend to be concentrated.There is usually one islet or sometimes two large islets in the region of the bile duct.in agnathan fishes pancreatic islets have been observed in the lamprey but not in the Hag fish.The endocrine pancreas is present in most fish as islet of Langerhans and is associated with the exocrine pancreas. In some species the islets are very large and may be grossly visible (Brockman bodies). During the spawning season the size and number of islet will increase in some fish. 9.2.7. Thymus In the actinopterygian fishes a rather massive thymus is observed in the medial wall of the branchial cavity.this thymns apparently arises from contributions of several of the branchial pouches.the thymus of Latimeria is a comparable to that of the actinopterygian but is lobular.in the cyclostomes all of the gill pouches give rise to thymus tissue dorsally.the anterior pouches also have ventral anlagen. In the adult only the dorsal parts remain. 9.2.8. Ultimobranchial Body An ultimobranchial body is said to be present in the sharks,holocephalans, and some ray-finned fishes.in some of the teleosts,there is flat discoidal mass lying in the connective tissue between the floor of the oesophagous and the sinus venosus. This organ secretes calcitonin (lowers serum calcium levels) that acts with hypocalcin (secreted by the corpuscles of Stannius) to regulate calcium metabolism.
SEED COURSE 44 9.2.9. Intestinal mucosa The intestinal mucosa being producing harmones which join with nervous controls to regulate the pancreatic secretions.the intestinal harmones are secretin and pancreozymin.secretin produces flow of the enzyme carrying liquids from the pancreas and pancreozymin enhances flow of the zymogens.both harmones are found in the anterior portion of the small intestine and other tissue with possible endocrine function. 9.2.10. Corpuscles of Stannius The Corpuscles of Stannius, which occur in the kidney of many actinopterygian fishes, have often been viewed as potentially endocrine. these arise as many (40 or) more)diverticulate from the nephric ducts in the anterior part of the opisthonephros (segments 9 to 12) of Amia and Lepisosteus or, in teleosts, as a few or even a single pair of diverticulae in the posterior region of the kidney.These are islands of eosinophilic granular cells located in paired organs on the ventral surface of the kidney. This organ secretes a protein called hypocalcin (teleocalcin) that acts with calcitonin to regulate calcium metabolism. The parathyroid glands are absent in the sharks and in the actinopterygian fishes; their function is taken over by other endocrine organs (Corpuscles of Stannius). 9.2.11. Pineal Gland The pineal organ,outgrowth from the roof of the brain,is recognized as a photoreceptor in fishes (it is vestigial in the hag fish), but its glandular structure suggests endocrine activity.removal of the pineal organ of the guppy (Lebistes) is followed by reduced growth rate,skeletal abnormalities,are marked stimulation of both pituitary and thyroid glands.The pineal gland is a light sensitive neuroendocrine structure that lies in the anterior brain and is a well-vascularized organ. This gland secretes melatonin that may play a role in controlling reproduction, growth, and migration. Unit 10 - Skeletal system 10.1. Introduction The skeletal system of vertebrates is composed of bone and/or cartilage. Bone tissue is found only in the Subphylum Vertebrata. Some of the lower vertebrates do not possess bone, but all the higher vertebrates do. As such, bone is often thought of as being typical of vertebrates. In vertebrates, bone functions as a supporting tissue, a calcium reserve and as a hemopoietic (blood forming) tissue. The skeleton is the basis of form and support of the vertebrate body. Muscles attach to the skeleton and vital organs are surrounded and protected by skeletal elements. As you examine the skeletons of the perch and the rat you should note a number of basic changes that have occurred in the evolution of the vertebrate skeleton. Some of the changes involve a reduction in the number of bones in the skull and a reduction in the number of ribs. Correlated with the move from an aquatic to a terrestrial environment are the increase in the complexity of the limbs and limb joints, the development of the pectoral and pelvic girdles and the strengthening of individual bones to support the weight of the organism on land. The central structure of support in the lower vertebrates, the notochord, is progressively replaced functionally by elements of the vertebrae. Although the notochord runs the length of the vertebral column in fish, in many it has been greatly restricted by the vertebrae. In adult tetrapods, the only remnant of the notochord is
SEED COURSE 45 the gelatinous material found in the intervertebral discs between successive vertebrae of the vertebral column. In the study of the skeletal system in the perch and a more advanced vertebrate, the rat, you should try to determine which skeletal features are signs of typical evolutionary advancement and which may be specializations due to the animal’s way of life. The skeleton of vertebrates is broadly divided into two parts: the axial skeleton consisting of the skull, vertebrae and ribs; and theappendicular skeleton consisting of the pectoral and pelvic girdles and the bones of the appendages.
SEED COURSE 46 10.2.1. The Axial Skeleton Skull The skull of the perch is actually a double structure consisting of two “boxes” of bone, one enclosed by the other. The outer skull is an armour of dermal bone. Primitive extinct bony fish had dermal bony armour covering most of their bodies. In the modern fish, the outer skull is virtually all that remains of this armour. Dermal bone forms, as its name implies, in the dermis of the skin and is not proceeded by a cartilage structure. The inner skull is composed of endochondral bone. Endochondral bone develops under the dermis and replaces existing cartilaginous structures. Hence the name “endochondral” denotes the bony tissue develops “within” existing cartilage structures. Elements of the inner skull form the cranium or brain case. The perch skull consists of many small bones. You are not responsible for knowing the identity of these bones, but look closely at the skull to see the inner endochondral skull encased by the outer dermal skull. 10.2.2. Vertebral Column A series of endochondral bones called vertebrae form the vertebral column. Vertebrae have several common features. The large spool-shaped central portion of each is the centrum. Extending through the middle of each centrum is a canal for the passage of the notochord. As mentioned previously, many fish retain a notochord throughout life. Above the centrum, an arch of bone surrounds and protects the spinal cord. A dorsal projection, the neural spine, extends outward from the vertebral column.The fish vertebral column is divided into two subdivisions: the trunk and the tail (caudal). Although fish do not have a neck, the first two trunk vertebrae are modified. These vertebrae lack ribs. The rest of the trunk vertebrae possess ribs. Caudal (tail) vertebrae possess a ventral portion which forms a hemal arch which surrounds blood vessels.
SEED COURSE 47 10.3. Appendicular Skeleton and Fins Using the preserved perch and the skeleton, locate the following structures: Median Fins The dorsal fins of the perch have fin rays for support. The anterior dorsal fin has ossified fin rays which provide stiff support, while the fin rays of the posterior dorsal fin are not ossified and are flexible. Only the first two fin rays of the anal fin are ossified. The caudal fin is composed entirely of soft, unossified fin rays. Pectoral Girdle and Fins The pectoral fins are attached to a bony girdle, the pectoral girdle. The pectoral girdle is composed of a number of fused elements. The girdle is also fused to the skull (the head and trunk of the perch move as a unit). The fins are supported by soft fin rays.
SEED COURSE 48 Pelvic Girdle and Fins The pelvic fins are attached to the pelvic girdle, which is composed of two bony pelvic plates. The plates may be fused along the midline. The pelvic girdle is not attached to the vertebral column nor to the pectoral girdle, but is free-floating (embedded in muscle only). Only the medial fin rays are bony, the rest are the typical soft unossified type. Unit 11 - Sensory System in fishes 11.1. Introduction A fish’s eyes are adapted or modified for underwater vision, but they are not very different from human eyes. Fish do not have true eyelids. Human eyelids prevent the eyes from becoming dry and also protect against dirt. A fish’s eyes are always covered by water; therefore, they require no lids. The metallic-looking ring, called the iris, encircling the dark center, or lens, of the fish’s eye cannot move as it does in the human eye. The human iris can expand or contract, depending upon light conditions. Because light never attains great intensity underwater, a fish needs no such adaptation. The big difference between a human eye and the eye of a fish occurs in the lens. In humans it is fairly flat or disc like; in fish, it is spherical or globular. Human eyes are capable of changing the curvature of the lens to focus at varying distances—flatter for long- range focusing and more curved for shorter range. Although the eye of a fish has a rigid lens and its curvature is incapable of change, it can be moved toward or away from the retina (like the focusing action of a camera). Fish can distinguish colors. There are indications that some kinds of fish prefer one color to another and also that water conditions may make one color more easily distinguished than others. Many kinds of fish have excellent vision at close range. Fish that live in the dusky or dimly lit regions of the sea commonly have eyes that are comparatively larger than the eyes of any other animal with backbones. Fish that live in the perpetual darkness of caves or other subterranean waters usually have no eyes, but those inhabiting the deep sea, far below the depth to which light rays can penetrate, may or may not have eyes. The reason that most deep-sea fish have well-developed eyes is the prevalence of bioluminescence. Deep-sea squid, shrimp, and other creatures, as well as fish, are equipped with light-producing organs. The light they produce is used to recognize enemies or to capture prey. Many fish with poor vision have well-developed senses of smell, taste, and touch. Improbable as it may seem, a fish does possess nostrils. Four nostrils are located close to the top of the snout, one pair on each side. Each pair opens into a small blind sac immediately below the skin. Water, carrying odors, passes through the sacs, which are lined with the receptors of smell.
SEED COURSE 49 Some fish, including sharks, possess an extremely acute sense of smell. Fish have taste organs located in the skin of their snouts, lips, mouths, and throats. A fish’s tongue, unlike the human tongue, is flat, rigid, and cartilaginous and moves only when the base below it moves; nevertheless, it does possess taste buds that indicate to the fish whether to accept or to reject anything taken into its mouth. There is a close relationship between the senses of smell and taste in fish, just as in humans. Many types of fish are first drawn to food by its odor. Although fish obviously do not possess outer ears as humans do, they are still capable of hearing. A fish possesses only an inner ear, found in the bones of the skull. In many fish, these ear bones are connected to their air bladders. Vibrations are transmitted to the ear from the air bladder, which acts as a sounding board. The lateral-line system, a series of sensory cells usually running the length of both sides of the fish’s body, performs an important function in receiving low-frequency vibrations. Actually, it resembles a “hearing organ” of greater sensitivity than human ears. The typical lateral line is a mucus-filled tube or canal under the skin; it has contact with the outside world through pores in the skin or through scales along the line or in-between them. A nerve situatedmat intervals alongside the canal sends out branches to it. In some cases, the lateral line extends over the fish’s tail, and in many fish it continues onto their heads and spreads into several branches along the outer bones of their skulls, where it is not outwardly visible. The fish utilizes its lateral line to determine the direction of currents of water and the presence of nearby objects, as well as to sense vibrations. The lateral line helps the fish to determine water temperature and to find its way when traveling at night or through murky waters. It also assists schooling fish in keeping together and may help a fish to escape enemies. Many fish are noisy creatures. They make rasping, squeaking, grunting, and squealing noises. Some fish produce sounds by rubbing together special extensions of the bones of their vertebrae. Others make noises by vibrating muscles that are connected to their air bladders, which amplify the sounds. Still other fish grind their teeth, their mouth cavities serving as sound boxes to amplify the noises. Many fish make sounds when they are caught. Grunts and croakers got their names from this habit. Since fish have a nervous system and sense organs, it would appear that Smell receptors are located in the nostrils, and water (carrying odors) is drawn into sacs that are lined with the organs of smell. Olfactory nerves connect the nostrils and brain. they could feel pain. The fish’s brain is not highly developed, however. There is no cerebral cortex (the part of the brain in higher animals that stores impressions), and so the fish has little or no memory. It is not uncommon, for example, for an angler to hook the same fish twice within a short time. Many fish are caught with lures or hooks already embedded in their jaws. Fish are essentially creatures of reflex, rather than of action produced or developed by using the brain. In all probability, physical pain in fish is not very acute, and if any impression of pain is made in the brain, it is quickly lost.
SEED COURSE 50 “A number of highly specialized organs, which receive physical and chemical stimuli from the environment, are called as sense or sensory organs”. They are associated with the nervous system. Most obvious sense organs are eyes, nose, ear and skin which receive stimuli for vision, olfaction, audition and touch respectively. Less obvious are Lateral line, sensory crypts and papillae, ampullae of Lorenzini. The visual stimuli involve changes in light intensity and quality and acoustical ones are received through the inner or lateral line. Chemical stimuli are those experienced through either small or taste organs. Pain is probably not experienced as a strong sensation by fishes, though forceful or noxious physical or chemical stimuli evoke violent reactions. 11.2. Nares One or two nares (nostrils) on each side of the snout leading to a build sac represent the organs of smell externally among fishes. However, in the lampreys and hag fishes the nostril is single and median. Most fishes have the narial openings at the top and sides of the snout. In other such as the sharks, rays and skates the nares are on ventral surface of the snout. In dog sharks, the narial apertures by a flap of tissue into incurrent and excurrent pores to complete separation of incoming and outgoing water through development of a separate pore for each current, in most teleost there is often value for classification in the fleshy valuular tissue of the teleosts. In living lungfishes (Dipnoi) the external incurrent nares communicate by passage with excurrent nares that have come to lie in the mouth cavity. 11.3. Eyes The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are
SEED COURSE 51 extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences. The essentially lidless eyes that cannot be closed are situated in orbits, one on each side of the midline of the fish head. Most often the eyes are lateral with partially independent fields of vision and movement. In many bottom dwellers, including the skates (Rajidae), most Sculpine (Cottidae) and Goosefishes (Lophidae), the eyes are dorsal in adults of flounders and their relatives both eyes on one side of the head, the eyes are variously reduced or absent in cave fishes. Visible through the transparent skin that covers the eye and through the transparent cornea of the eyeball are a. The opening of the pupil of the eye and through it, the spherical crystalline lens inside the eye ball b. The colored, washer – shaped iris surrounds the pupil.
SEED COURSE 52 11.4. Skin organs Numerous microscopically small opening of skin sensory organs are developed on the surface of the fish body. In most fishes a series of these pores, extending along each side in a single row from the head to the caudal fin comprises the lateral line. The lateral line system forms branches about the head, including one above and one below the orbit of each eye. In some fishes like Pikes, Esox lucius and Silver sides, the pores and their sensory organs are not linearly concentrated on the body but are rather widely scattered. In sharks and their relatives, there are extra and specialized parts of the lateral line system, especially in the snout region. In few fishes taste buds occur in the skin and intigumentary tactile sensory structures. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain. An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head and down the midside of the body where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure.
SEED COURSE 53 The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.
SEED COURSE 54 11.5. Inner ear The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see the fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. It is thought that many fishes communicate with each other in a crude way by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways. In fishes the middle ear apparatus is absent and only inner ear is present which is concerned with two senses. i.e., hearing and balancing. It is contained partly in the auditory capsule. In sharks the anterior vertical canal joins with horizontal canal to form a “crus”. The pars inferior which receive sound is composed of two vesicles. The anterior sacculus and posterior legena. In bony fishes, the utriculus, succulus and legena comprise lapilles sagitta and astericus respectively. These are calcified structure and are secreted by ectoderm. Besides the sound detection, the inner ear functions to “orient “or “balance” the animal providing it feeling of direction in which gravity is acting when suspended in lightless, pelagic habitats. Minnows, carp, catfishes and other teleosts connects the auditory system to the swim bladder with a chain of small bones called “Weberian ossicles”., The ossicles connect the pulsating swim bladder wall with Y – shaped lymph sinus. 11.6. Lateral line system The lateral line system, found in many fishes and in some aquatic amphibians, is sensitive to differences in water pressure. These differences may be due to changes in depth or to the currentlike waves caused by approaching objects. The basic sensory unit of the lateral line system is the neuromast, which is a bundle of sensory and supporting cells whose projecting hairs are encased in a gelatinous cap. The nueromasts continuously send out trains of nerve
SEED COURSE 55 impulses. When pressure waves cause the gelatinous caps of the neuromasts to move, bending the enclosed hairs, the frequency of the nerve impulses is either increased or decreased, depending on the direction of bending. Neuromasts may occur singly, in small groups called pit organs, or in rows within grooves or canals, when they are referred to as the lateral line system. The lateral line system runs along the sides of the body onto the head, where it divides into three branches, two to the snout and one to the lower jaw. A swimming fish sets up a pressure wave in the water that is detectable by the lateral line systems of other fishes. It also sets up a bow wave in front of itself, the pressure of which is higher than that of the wave flow along its sides. These near-field differences are registered by its own lateral line system. As the fish approaches an object, such as a rock or the glass wall of an aquarium, the pressure waves around its body are distorted, and these changes are quickly detected by the lateral line system, enabling the fish to swerve or to take other suitable action. Because sound waves are waves of pressure, the lateral line system is also able to detect very low-frequency sounds of 100 Hz or less. An interesting adaptation of the pressure-sensitive systems is seen in the modified groups of neuromasts called the ampullae of Lorenzini, which are found in sharks and certain bony fishes. The ampullae of Lorenzini act as electroreceptors and are able to detect electrical charges, or fields, in the water. Most animals, including humans, emit a DC field when in seawater. This is presumably caused by electrical potential differences between body fluids and seawater and between different parts of the body. An AC field is also set up by muscular activity (contractions). A wound, even a scratch, can markedly alter these electrical fields. The cat shark, Scyliorhinus, is known to catch prey by using its ampullae of Lorenzini to detect the electrical field generated by flatfish (plaice) buried beneath the sand. It provides “distinct touch sense” of objects. The lateral line system is an integral part of the acoustico laterials system which includes ear. It involves sensory lines distributed on the head and body, pit organs and Ampullae of Lorenzine.
SEED COURSE 56 Ampullae of Lorienzine are present in the head region of sharks and rays. They are small sac like structure, which open outside by small pores. The Ampullae of Lorienzine are sensitive to mechanical and weak electrical stimuli as well as change in salinity. The receptors of lateralline system are called “neuromast” Each consists of individual hair cells with an attached “cupula”. Fig: Diagrammatic comparison of various sensory epithelia of the acoustico - lateralis system Most of the lateral line organs of the head region are innervated by sensory fibres of the lateralis anterior root of cranial nerves VII (facialis). The rest of the organs of systems are innervated by the laterials posterior root of the vagus (X).
SEED COURSE 57 Unit 12 - Arthropoda 12.1. General characters of Arthropoda  Arthropods are triploblastic, bilaterally symmetrical, metabolically segmented animals.  Body is covered with a thick chitinous cuticle forming an exoskeleton.  Body segments usually bear paired lateral and jointed appendages.  Musculature is not continuous but comprises separate 'striped muscles.  Body cavity is haemocoel. The true coelom is reduced to the spaces of the genital and excretory organs.  Digestive tract is complete; mouth and anus lie at opposite ends of the body.  Circulatory system is open with dorsal heart and arteries but without capillaries.  Respiration through general body surface, by gills in aquatic forms, tracheae or book lungs in terrestrial forms.  True nephritic are absent. Excretion by coelomoducts or Malpighian tubules or green or coxal lands.  Cilia are entirely absent from all parts of the body.  Sexes are generally separate and sexual dimorphism is often exhibited by several forms.  Fertilization is internal. Development is usually indirect through larval stages.  Parental cave is also often well marked in many arthropods.
SEED COURSE 58 12.1.1.Shrimps - External Anatomy External Anatomy As a decapods crustacean, the white shrimp, Penaeus (Litopenacus) sctiferus, is rather primitive. It has been selected for use here because it illustrates not only the structure of a shrimp, but also the generalized body plan of a decapod crustacean. A large portion of the white shrimp, as of any other shrimp, consists of muscle and shell, or exoskeleton. In fact, the largest of the three natural divisions of the body; namely, the abdo¬men or "tail," consists of little other than mus¬cle and shell. Two main masses of muscle are the "meat" of the shrimp's tail: (1) the rela¬tively small dorsal abdominal muscles, which lie above the intestinal tract, or gut, and above the dorsal abdominal artery, both of which are removed in preparation for eating, and (2) the large ventral abdominal muscles, which extend from either side of the intestinal tract and dorsal abdominal artery ventrally to both sides of the ventral abdominal nerve cord.
SEED COURSE 59 For swimming quietly, the white shrimp uses its five pairs of abdominal appendages, known as pleopods. But when the shrimp moves rapidly, it does so by contracting its ventral abdominal muscles and curving forward its tail fan, which is composed of a centrally situated telson and a pair of lateral appendages known as uropods. The powerful thrust exerted by tail and tail fan upon the water propels the shrimp back¬ward with extraordinary speed. The tail of the shrimp returns to its normal, more or less elongated, position by the contrac¬tion of the dorsal abdominal muscles, which act as extensors. The tail's flexibility results from deep folds of thin, soft chitin that link the six segments of the tail to one another. Within the cephalothorax of the white shrimp are large portions of the digestive, circulatory, nervous, and reproduc¬tive systems. The long digestive tract, or gut, has three main subdivisions known, respectively, as foregut, midgut and hindgut. Food particles picked up by
SEED COURSE 60 the mouth parts are ground by the mandibles and swallowed, whereupon they enter the narrow, tubular, muscular esophagus, which is the initial portion of the foregut. Lined with chitin, the esophagus nonetheless can accommodate large amounts of food since it has one anterior and two lateral folds loosely filled with connective tissue. When these folds become unfolded, the esophagus can distend greatly. 12.1.2. Digestive system From the esophagus, food particles enter the anterior chamber, the second portion of the chitin- lined foregut. Many authors have called this the cardiac stomach. The an¬terior chamber has lateral longitudinal folds that permit it to expand when filling with food. The anterior chamber also has ventrally situated longitudinal ridges that lead back to the openings of the midgut glands in the caudad part of the posterior chamber, frequently called the pyloric stomach. Walls of the anterior chamber contain a triangular structure consisting of a median tooth and a row of tooth-like denticles along each side. When food enters the anterior chamber, the muscles that insert on the chamber alternately contract and relax, thereby causing the median tooth to move against the denticles and lateral ridges. In so doing, this grinding apparatus, termed the gastric mill, breaks down the food into very fine particles. While food is within the anterior chamber, it is mixed with digestive juices that flow forward ventrally from the posterior chamber. The juices enter the caudad part of the posterior chamber viaducts that originate in the lumen, or cavity, of many-branched tubules constituting the paired midgut glands. Thus, the lumen of the tubules is continuous with the lumen of the gut. Digestion of food takes place partly in the anterior chamber, partly in the posterior chamber, and partly in the tubules of the midgut glands. In the posterior chamber, there is a filter formed by two lateral ridges and one ventral median ridge densely covered with hair-like setae. Owing to this filter, only fluid and minutely divided food particles can pass from the posterior chamber into ducts leading to the midgut glands and thence into its branching tubules for further digestion. From the midgut glands, end products of digestion are readily absorbed into the hemolymph. Fine indigestible material within the midgut glands is forced back into the posterior chamber and then into the straight, unlined, tubular portion of the midgut. Here end- products of digestion enter the hemolymph via the many small blood vessels connecting the tubular portion of the midgut with the dorsal abdominal artery just above. Here also the fine indigestible material is mixed with larger indigestible particles that had been filtered away from the openings of the midgut glands and had passed directly from the posterior chamber into the tubular part of the midgut. Within the midgut, indigestible material is packaged into long fecal pellets and enclosed within a membrane, the pen¬trophic membrane (from the Greek, pen, around; troplio, feed), which is secreted by epithelial cells of the midgut and is mucoid in nature. Strong peristaltic contractions of the mid- gut push-the fecal pellets along to the chitin-lined hindgut, which is enlarged as a rectum. A series of rapid contractions by the rectum then forces the fecal pellets out of the body by way of the anus. At the junction of midgut and hindgut in the sixth abdomi¬nal segment, the midgut gives rise to a diverticulum, called the posterior midgut cecum by some authors and the hindgut or rectal gland by others. The function of this organ is not known, but its cells appear to be secretory. Presumably the midgut would be distinguishable from the foregut and hindgut by its lack of chitinous lining. Thus, the esophagus, anterior chamber, and cephalad portion of the posterior
SEED COURSE 61 chamber are lined with chitin and are clearly foregut. The caudad portion of the posterior chamber also is lined, although incompletely, with chitin; yet this is midgut. Dorsally and laterally, the chitinous lining of the foregut in the white shrimp (and in many other decapod crustaceans) extends into the midgut well past the openings Of the midgut glands; ventrally a caudad extension of the chitinous lining separates the openings of the midgut glands and also covers the epithelium of more posterior portions of the midgut for some distance. These caudad extensions of the chitinous lin¬ing probably direct sand and other indigestible particles to the peritrophic membrane for packaging without damage, en route, to the delicate area around the openings of the midgut glands. 12.1.3. Circulatory System The heart of the white shrimp has three pairs of small openings known as ostia. Through these ostia, the blood flows into the heart from the surrounding area, which is termed the pericardial sinus, or pericardium. Valves prevent the blood from leaking out through the ostia as the heart contracts. Instead, the blood is driven into major arteries, most of which run forward to supply blood to the sense. organs and to vital organs within the cephalothorax. How¬ever, the sternal artery runs to war ventral region of the shrimp, where—it gives rise to a ventral thoracic artery that supplies blood to the thoracic appendages and to the thoracic portion of the ventral nerve cord. The dorsal abdominal artery leaves the heart posteriorly and supplies blood to the gut, the abdominal muscles, and the abdominal portion of the ventral nerve cord. 12.1.4. Nervous System The nervous system of the white shrimp consists of a brain (supraesophageal ganglion), which is situated dorsally in the head, two circumesophageal connectives that pass on either side of the esophagus and are connected with each other by the tritocerebral commissure, and a ventral nerve cord, which runs posteriorly the entire length of the shrimp and at more or less regular intervals is swollen into bulbous ganglia. The entire central nervous system of the white shrimp, as of other decapod crustaceans, is fundamentally "ladder-type" in structure, but in most regions the two longitudinal halves of the "ladder" have fused. As a consequence, the word "gan¬glion" generally refers to a pair of laterally fused ganglia. The brain receives nerves from sense organs of the head, notably the eyes and antennae, and supplies nerves to the muscles that operate these sense organs. In the ventral nerve cord, the first ganglion (subesophageal ganglion) and the remaining ventral ganglia (five in the thorax and six in the abdomen) receive nerve fibers from sensory cells widely dis¬persed through the body of the shrimp and supply nerves to muscles that move the mouth parts, thoracic legs, pleopods, and tail. In addition, lying on the circurnesophageal connectives is a pair of connective ganglia, or stomatogastric ganglia (stö-m¬to-GAS-trik; from the Greek, stoma, mouth; gaster, stomach). The connective ganglia and the stomadeal ganglion on the anterior surface of the çsophagus combine to form the stomadeal system, which supplies nerves to the esophagus and the foregut. In the forward part of the cephalothorax of the white shrimp, situated on the second, or antennal, segment are the kidneys, which because of their location are often called an¬tennal glands. Each kidney is made up of a small dorsal portion that lies above the brain and a large ventral portion lying beneath the brain. The two portions of each kidney are connected with each
SEED COURSE 62 other by lateral arms. Part of the ventral portion extends into the antenna on the same side of the animal. A short duct from this portion of each kidney leads to the exterior through an excretory pore, which lies at the base of the antenna on its inner (medial) side. In higher shrimps, the Caridea, a bladder also is present. There are 19 pairs of gills in Penaeus setiferus. Three pairs occur in each thoracic segment, except the first and last, where there is one pair. In any given segment the gills may be attached to the base of the limb, to the flexible membrane between limb and body, or to the body wall. A gill of the white shrimp consists of a primary supporting axis known as a rachis (RAY-kis; from the Greek, rhakhis, spine, or backbone), from which secondary supporting structures emerge at right angles. On the secondary supporting structures are many gill filaments that in turn protrude at right angles. Each secondary supporting structure with its attached filaments nests against the preceding one. In caridean shrimps, the gill filaments are flattened and plate-like and protrude directly from the primary supporting structure; this type of gill also is found in crabs. 12.1.5. Respiratory System In shrimps, as in all other decapod crustaceans, the gills lie within two branchial chambers, each of which results from a deep lateral fold of the carapace. The beating of a leaf-like flap, the gill bailer, or scaphognathite, causes water to enter the branchial chamber from below and behind that is, through opening between the thoracic legs and in front of the abdomen. The water leaves the -branchial chamber through 'a channel, directed toward the head, in which lies the beating gill bailer. As the water circulates through the branchial chamber, an exchange of gases takes place between the water and the blood in the gill filaments. At the same time there is a discharge of excess salts from the blood into the water and an uptake of needed salts from the water into the blood. 12.1.6. Reproductive System In the white shrimp the most conspicuous components of the female reproductive system are two ovaries that extend, partially fused, from the anterior of the foregut pos¬teriorly to the tail fan. The portion of each ovary that is within the cephalothorax consists of a forward-projecting lobe, which lies close to the esophagus and chambers of the foregut, and seven finger-like lateral lobes, which are situated above the midgut gland and beneath the heart. This arrange- ment makes the heart resemble a saddle straddling the ovaries. The abdominal portion of the ovaries consists of two lobes, lying above and to the sides of the intestine and below and to the sides of the dorsal abdominal artery.
SEED COURSE 63 Emerging from each ovary at the sixth lateral lobe is an oviduct. Coursing ventrally, each oviduct opens to the ex¬terior a genital pore situated medially on the basal segment of the third thoracic leg. The opening is concealed within an ear-shaped protuberance covered with setae. Externally and posterior to the genital openings of the female lies a structure that is adapted for receiving a packet of sperms, or spermatophore (sper-MA-t5-phôre; from the Greek, sperma, seed; phoros, bearing) from the male during mating. Known as thelycum (THEL--cüm; from the Greek, thelys, female), this structure consists of several lobes and protuberances bearing stiff bristles. The male reproductive system of the white shrimp includes a pair of partially fused testes that lie in a position quite similar to that of the ovaries in the female. Each testis has an anterior lobe projecting forward over the chambers of the foregut and six lateral lobes that lie over the midgut gland and under the heart. In place of a long abdominal lobe as in an ovary, each testis has a short posterior lobe. A pair of ducts known as the vasa deferentia emerge from the main axis of the testes at their posterior margin, course ventrally, and open to the exterior at the genital pores situated medially on the basal segment of the fifth pair of thoracic legs. Each vas deferens has four distinct regions: a short, narrow proximal portion; a thickened, doubly flexed medial portion; a long, narrow tubular portion; and a much dilated, muscular terminal ampoule. Within the terminal ampoule the sper¬matophore is formed.
SEED COURSE 64 The spermatophore of the white shrimp roughly resembles a pod. It consists of two halves, each of which contains sperms enclosed within a sheath and surrounded by chitin. The thoracic legs of the male shrimp presumably assemble the spermatophore immediately after each half is expelled from the terminal ampoule of the corresponding vas deferens. The legs place the spermatophore within the trough of the petasma, a structure that results from modification of the first pair of pleopods. The petasma consists of stiffened longitudinal rods and folds of soft chitin that, when unfolded, result in a broadly inflated male copulatory organ.
SEED COURSE 65 During mating, the male shrimp uses the petasma to thrust the spermatophore against the thelycum of the female. Here bristles on protuberances of the thelycum overlap the spermatophore, thus helping to secure it. Two lobes known as "wings" on the spermatophore become anchored in a groove on the ventral surface of the female between her third and fourth thoracic legs. Despite these devices for securing the spermatophore, it is easily dislodged, and spermatophore ¬bearing females of white shrimp are not commonly caught in shrimp trawls. A pair of light-colored, pad-like structures situated just posterior to the thelycum are believed to play no role during impregnation. Lobsters 12.2.1.External Anatomy In general body plan a lobster does not differ greatly from a shrimp. A lobster has the same type of muscular abdomen, or tail, which undergoes sudden flexion by contraction of the large ventral abdominal muscles and more leisurely extension by contraction of the smaller dorsal abdominal muscles. As in shrimp, the tail of a lobster provides the animal with its surest means of escape- jetlike propulsion backward.
SEED COURSE 66
SEED COURSE 67 In lobster as in shrimp, a carapace covers the head and the thorax and, except for the presence of the cervical groove, obscures the boundary between these two regions. In lob¬sters, the cephalothorax is commonly called the "body," while in shrimp this same region is known as the "head." Despite similarities in body plan, it is quite easy to distin¬guish a lobster from a shrimp. Even as an adult, a shrimp is relatively small and its shell is somewhat fragile. An adult lobster, on the other hand, may reach very large size and acquire an extremely hard shell. Furthermore, a lobster is compressed dorsoventrally (from top to bottom), not laterally (from side to side), as is a shrimp. True lobsters have yet another distinguishing characteristic: their first pair, of thoracic legs is modified as large claws, or chelipeds. In some species, such as the American lobster, Homarus americanus, one large claw, the crusher, is much heavier than the other claw, known as the pincer, or the biting, cutter, or ripper claw. The crusher of the American lobster occurs about as frequently on the right side of the body as on the left. These are large & claws are lacking in the spiny or rock, lobsters. As in a shrimp, many vital organs of a lobster are situated under the carapace within the cephalothorax. Here are the chitin-lined foregut, at least a portion of the midgut, and the midgut glands. Here also lie the brain, heart, gills, excretory organs, and a large part of the male and female reproductive organs. 12.2.2. Digestive System In both true lobsters and spiny lobsters, the gastric mill is more highly developed than in the white shrimp. The gastric mill of the lobster is largely restricted to the region of the foregut in
SEED COURSE 68 which the large, thin-walled anterior chamber gives way to-.the much smaller, thick-walled posterior chamber. At the constriction between the two chambers, three movable teeth, one median and two lateral, are attached to small, hard skeletal plates known as ossicles. These teeth chew the food, which arrives in the anterior chamber as long, stretched, but unchewed pieces. A well-developed gastric mill is a useful device enabling a decapod crustacean, when safely hidden from its enemies, to chew s food at leisure, after having swallowed it in large pieces. The gastric mill is least developed in such decapod crustaceans as the shrimps, in which the mouthparts chew the food quite thoroughly before the food enters the esophagus. In the walls of the anterior chamber, a lobster has many ossicles in addition to those of the gastric mill. These addi¬tional ossicles serve as a place of attachment for muscles that move the foregut and thereby enable the ossicles of the gastric mill to grind the food. Once the food has been ground thoroughly, it passes through a setose filter that prevents all but the finest particles from entering the mid gut glands through ducts that open into the unlined caudad portion of the posterior chamber. The midgut of the American lobster is long, extending back to the last abdominal segment, where it connects with the chum-lined hindgut, which has become modified as an en¬larged rectum. A posterior midgut diverticulum, or cecum, arises just in front of the junction of midgut and rectum. Undigested wastes are egested from the rectum through the anus. In spiny lobsters, the midgut is very short, while the hindgut is long and contains many longitudinal folds. No enlarged rectum is present, the terminal portion of the hind- gut being narrow and very muscular. By their contraction the muscles of the hindgut force undigested (fecal) material out through the anus. In the foregut, midgut, and midgut glands of the American lobster, digestion of food takes place through the action of digestive enzymes that are secreted by the midgut glands. These glands are also the principal site for absorption of digested food and for storage of reserve food materials. Chefs call the midgut glands of the lobster the tomally; accumulated food reserves make the tomally rich and flavorful when cooked. The tomally can easily be recognized, for it is
SEED COURSE 69 soft, large, and many-lobed, and, in color, i-1 green, bright yellow, yellow-green, or yellow- brown. 12.2.3.Circulatory System The circulatory system of a lobster is not very different from that of the white shrimp. In lobsters as in shrimps, the heart lies under the middorsal surface of the cephalothorax just in front of its junction with the abdomen. Three pairs of ostia allow blood that has collected within the pericardium to flow into the heart when the organ relaxes. During contraction of the heart, the ostia close and prevent the blood from flowing back into the pericardium. From the heart;-the blood flows forward through several arteries to vital organs within the cephalothorax. The blood also flows into the dorsal abdominal artery and its paired branches in each segment. These supply blood to the ven¬trally situated flexor muscles and the dosal1y situated exten¬sor muscles of the abdomen. A sternal artery carries blood to the gonads, then courses ventrally to give rise to the ventral thoracic artery and the ventral abdominal artery. In lobsters, as in shrimps, the ventral thoracic artery carries blood to most thoracic appendages and to the thoracic portion of the ventral nerve cord. In lobsters, but not in shrimps, the ventral abdom¬inal artery extends through the abdomen, supplying blood to the last two pairs of thoracic legs, the ventral nerve cord, the posterior part of the hind gut, and the tail fan. 12.2.4. Nervous System The central nervous system of the American lobster differs little from that of the white shrimp. Lobsters, like shrimps, have a brain, or supraesophageal ganglion, composed of several fused paired ganglia. Running ventrally and posteriorly from the brain are two circumesophageal connectives, a slight swelling on each connective as it passes the esophagus marking the position of the stomatogastric, or connective, ganglia. Behind the esophagus, the connectives are joined by the small tritocerebral commissure. Due to fusion of the first two thoracic ganglia with three cephalic ganglia to form the subesophageal ganglion, the thoracic portion of the ventral nerve cord in the American lobster contains only five additional ganglia (ganglia of the last two thoracic segments have fused with
SEED COURSE 70 each other). The abdominal portion of the Ventral nerve cord contains six gan¬glia, one in each segment. This arrangement of thoracic and abdominal ganglia is similar to that in the white shrimp. In spiny lobsters, the thoracic ganglia have undergone greater fusion. Dr. C. J. George and his co-workers at Wilson College, Bombay, India, reported that in the thorax of Panulirus polypizagus there are only two ganglionic masses. The larger, anterior ganglionic mass has resulted apparently from fusion of nine pairs of ganglia (three cephalic, six thoracic), while the smaller, posterior ganglionic mass has come from fusion of two thoracic pairs. Yet in its abdomen, Panulirus polyphagus retains the original number of six ganglia. As in shrimps, the brain of lobsters receives nerves from sense organs of the head, notably the eyes and antennae. Ganglia of the thorax supply nerves to the mouth parts and thoracic legs. Abdominal ganglia furnish the nerve supply to flexor and extensor muscles of the abdo¬men, to the intestine, and to the abdominal appendages. 12.2.5. Excretory System The kidneys of lobsters, like those of shrimps, are known as antennal glands. More compact than in shrimps, the kidneys of lobsters have a pale olive-green hue and thus are often called green glands. They lie on each side of the body, below and in front of the foregut. Urine that is formed in a glandular portion passes into tubes that enter a duct leading from a dorsally situated bladder. There is no direct connection be¬tween bladder and glandular portion, so the bladder
SEED COURSE 71 can be filled only when urine backs up through this duct. Subse¬quently, the urine is released to the exterior via the same duct, which opens on the basal segment of the antenna. 12.2.6. Respiratory System Gills of lobsters are of a type known as trichobranch (TRIK-o-brank; from the Greek, thrix, hair; branchia, gills), for they are composed of numerous filaments arranged, plume- like, around a central axis. As in shrimps, on any given thoracic segment there may he as many as four pairs of gills, one pair on the basal segment of the limbs, two pairs arising from the soft membrane linking the limbs to the body, and one pair on the side of the body just above the limbs. In the American lobster the full complement of gills occurs at the base of the second, third, and fourth thoracic legs, with fewer pairs on the remaining thoracic segments except the first, which lacks gills. In all, there are 20 pairs of gills in the American lobster. On each side the gills lie within the branchial chamber, which is formed, as in shrimps, by a deep lateral fold of the carapace. Access to the brachial chamber is through very small openings between the appendages and two larger Openings, both ventral, one at the posterior end of the branchial chamber and the other at its anterior end. In a channel at the anterior
SEED COURSE 72 opening is the leaf-like flap known as the gill bailer, or scaphognathite, which by its rapid beating drives water forward in the channel and out of the branchial chamber. At the same time the current thus established within the bran¬chial chamber causes water to enter the ventral and posterior openings, principally the latter. Every few minutes, the gill bailer reverses is beat for a few strokes, thereby causing the current of water to flow in the opposite direction. By this reversal of current, silt and other debris that may have settled on the gills are loosened and can be flushed from the chamber.
SEED COURSE 73 12.2.7. Reproductive System In the American lobster the ovaries of the female appear in the form of a letter H, with the cross bar at the forward margin of the heart and with longitudinal lobes extending forward and backward through much of the animal. The stage of ovarian development is apparent from the color, bright yellow or flesh-colored early in development, then salmon, light green, and finally a rich dark green by maturity. After cooking, the mature, egg-filled ovaries are bright red and are known as the coral. From the ovaries, paired ribbon-like oviducts emerge at a level just below the heart, then quickly narrow as they run outward to the body wall and downward to the base of the third pair of thoracic legs, where they terminate on the inner surface of the basal segment. Externally and medial these openings is a triangular, bluish structure extending from the base of the third to just beyond the base of the fourth pair of thoracic legs. This is the seminal receptacle, a small pocket in the exoskeleton that receives sperms from the male during mating.
SEED COURSE 74 In males of the American lobster the testes, which are pale tan-grey in color, may be H-shaped, like the ovaries of the female, or longitudinally paired, without a cross bar. From the testes, paired ducts, the vasa deferentia, emerge beneath the heart, at approximately the same place that the oviducts emerge from the ovaries. Like the oviducts, the vasa deferen¬tia run outward to the body wall before turning downward. At this point they become S-shaped, with their posterior margin thickened and glandular, capable of secreting a gelatinous material that coats
SEED COURSE 75 the sperms as they pass through the duct. The vasa deferentia then become briefly bulbous and muscular and, following this, narrow and thin-walled, forming an ejaculatory duct that opens at a papilla on the inner surface of the base of the fifth (and last) pair of thoracic legs.
SEED COURSE 76
SEED COURSE 77 The most obvious external difference between male and female American lobsters lies in the shape of the first pair of abdominal appendages. In the male these are the copulatory pleopods, relatively long, hard, grooved, and tapering. In the female these pleopods are small and soft. Yet there are other sexual differences, for mature males are heavier and have lager claws and a longer, more swollen carapace than have mature females. Also in spiny lobsters, the sexes can he separated by differ¬ences in the abdominal appendages. In males the pleopods have one leaf-like terminal segment. In females the pleopods have two terminal branches, those of the first pleopods being leaf-like, while those of more posterior pleopods have one ref1ike branch and one rod-like branch used for attachment of eggs. In addition, the fifth pair of thoracic legs in male spiny lobsters terminates in a single, simple segment like that of more anterior legs, whereas in females the fifth pair of thoracic legs terminates in a small claw used in cleaning the attached eggs. Mature males tend to be larger than mature females. Before leaving the subject of structure in American lobster, we may give some thought to coloration, for it can be surpris¬ingly variable and frequently serves a protective function, enabling a lobster to blend with its background. Normal, or "wild-type," coloration of American lobsters is mottled olive-green or dark blue-green above, with small black or-green-black spots and often red tubercles and spines. On some lobsters the sides of the body and tail, as well as large portions of the claws, may be dusky orange, often dot¬ted with green-black. Other lobsters are almost entirely dusky orange, with green-black spots. Such variations in color exist among lobsters of widely differing sizes, from the one-pound individuals commonly purchased in fish markets to the lobsters of 10 to 15 pounds or more that are caught on the southeastern part of George's Bank and in areas to the south. American lobsters may be of other colors as well. Some, known as calico, or leopard, lobsters, are light yellow with purple-blue marbling or spots. Other lobsters are rich indigo blue, with bright, clear blue on the sides of the body and on the extremities. Sometimes lobsters are pale red, hardly dis¬tinguishable from the cooked animal when seen from above. Yet, whatever their color topside, live American lobsters tend to be very lightly pigmented, or even cream-colored, underneath. Occasionally, fishermen catch American lobsters that are cream-colored above as well as below, but with dark eyes and often with red pigment on the underside of the claws. Or such a cream-colored lobster may have faint traces of blue in its shell, as did one that was exhibited in Boston at the New England Aquarium. Such lobsters are frequently called al¬binos, although true albinos lack all pigment in eyes and shell. True albino American lobsters apparently have never been taken. No single factor is responsible for the differences in color of American lobsters. The basic color pattern is inherited, just as are color and texture of hair in man and other mammals. But in an American lobster the actual color that develops may depend partly upon the type and strength of illumination to which the animal is exposed and even more upon its diet. Thus, Professor F. H. Herrick, who in 1895 published .a classic monograph on the American lobster, observed that bluish coloration in this animal can result from prolonged exposure to sunlight. Recently, John T. Hughes and George C. Matthiessen of the Massachusetts Division of Marine Fisheries reported that lobsters held for a period of years at the Division's lobster
SEED COURSE 78 hatchery and rearing facility in Oak Bluffs, Martha's Vineyard, and fed primarily quahaugs, clams, scallop viscera, and alewives, turned a deep sky-blue color, which eventually faded into a pale blue—grey. When, however, these bluish lobsters were then fed exclusively on green crabs, they reverted somewhat to the wild-type colora¬tion after the next molt and became identical in coloration with the wild-type following the second molt. Color in all decapod crustaceans results primarily from the presence of pigments known as carotenoids (after carrots, from which they were first isolated) in the tissues and shell. The major carotenoid of decapod crustaceans is astaxanthin, which is bright red in color. When combined, or conjugated, with protein, the red color of free astaxanthin is replaced by a color characteristic of the particular conjugated protein that is present. For example, in the shell of American lobsters, the most abundant pigment usually is a conjugated protein of astaxanthin that is blue. Eggs of American lobsters contain a green conjugated protein. Green crabs about to molt have a green conjugated protein in the old shell and a brown one in the epidermis and pigmented layers of the new shell. The reason that diet plays such an important role in de¬velopment of color in American lobsters and other decapod crustaceans is that carotenoids present in the conjugated pro¬teins of these animals have to be either ingested or produced in the animal's body from ingested carotenoids. These pig-ments cannot be synthesized from noncarotenoid material, except by plants. Shrimps, lobsters and crabs turn red when they are cooked because heat breaks down the linkage between astaxanthin and protein, and the astaxanthin is freed. Shrimps, lobsters and crabs that are red before being cooked do not have free astaxanthin, but rather an astaxanthin —protein complex that is red in color. With regard to coloration, there is an important difference between shrimps, on the one hand, and lobsters, crayfishes and crabs, on the other. This concerns the way in which the colors are
SEED COURSE 79 manifest. Shrimps have a light, fragile, quite trans¬parent shell, through which the underlying integument is visible. In the integument are numerous pigment-containing cells known as chromatophores. Under the influence of cer¬tain hormones that originate within the central nervous sys¬tem and are released into the hemolymph, the pigments within the chromatophores either concentrate in the center of the cell or migrate to the periphery, as the case may be. Chromatophores have many branches, and thus a cell in which the pigments are dispersed looks very different from one in which the pigments are assembled into a tiny mass at the center. Furthermore, the area covered by chromatophoral pigments when they are dispersed is much greater than when they are concentrated, so the degree of pigment dispersion largely determines the overall coloration of a shrimp. This may change, rapidly and frequently, in response to changes in illumination and color of background, a fact that explains why common names for shrimps often include some that are descriptive of very different colors. In lobsters, crayfishes and most crabs, the shell is thick, strong, and largely opaque, due to pigments that are deposited within the shell. Hence, in these decapod crustaceans, the color of the animal is fairly constant, depending primarily upon the color of pigments within the shell rather than upon the degree of dispersion of pigments within the chromatophores. Only in. certain restricted area is the shell of a lobster, crayfish, or crab more or less transparent, and here the color of the underlying pigments can be seen. In a few crabs, notably the fiddler crab Uca pugilator and the ghost, or sand, crab Ocypode, the shell is fairly light and semi¬transparent, and overall coloration results largely from pig¬ments within the chromatophores. Sometimes the left half of an American lobster (or of its close relative, the European lobster, Homnrus gammarus) may be of one color and the right half quite a different color. Professor Herrick and several later investigators described a number of such particolored lobsters: light yellow/bright red; dark green/pale red; blue/white; green-black/light orange; dark green/sky blue; dark blue/light red; dark green/red; white-red/purple.-blue. In one case a bilateral difference in color of American lobster was correlated with a bilateral difference in sex. In 1959, Dr. Fenner A. Chace, Jr., and Dr. George M. Moore described an American lobster that on its left side was orange, with mot¬tling and spots of dark green-brown and on its right side was similarly mottled and spotted but mostly in shades of blue over a light, blue ground color. Externally, the lobster ap¬peared female on the right side and male on the left side. When the lobster was dissected, it was found to have well¬ developed female reproductive organs on the right side and male reproductive organs on the left. In three earlier reports by other scientists, American lobsters having both male and female reproductive organs were described, but in no case was the bilateral difference in sex associated with a bilateral difference in coloration. Crab 12.3.1.External Anatomy We have seen that although crabs appear to be tailless, they have a very small tail, which they keep tucked underneath their body. Due to its small size, this tail and its appendages cannot be used for locomotion. The thoracic legs of a crab are used for walking. In certain crabs, including
SEED COURSE 80 the blue crab, the last pair of thoracic legs is flattened and paddle-shaped and is used for swimming. While the tail of shrimp or lobster is among the meatiest and most succulent portions of the animal, the tail of a crab contains little meat. The dorsal abdominal muscles are small and very weak, being used solely to extend the tail backward. Virtually the only time at which these muscles are used is during mating, when the abdomen of both male and female must be drawn backward to permit the transfer of sperms.
SEED COURSE 81 The ventral abdominal muscles of crabs are somewhat heavier and stronger, particularly in the mature female. While carrying eggs, she used these muscles to curl her broad, rounded abdomen over the mass of eggs. When not carrying eggs, she uses these same muscles to hold her abdomen tightly in a depression on the ventral surface of her body. Male crabs have a “locking device” consisting of small tubercles on the fifth thoracic segment that secure the triangular or T- Shaped abdomen in a depression on the ventral side of the thorax Covering both head and thorax of a crab dorsally is a hard carapace. Thus, the boundary between the two body regions is obscure and, as in shrimps and lobsters, one generally speaks of a cephalothorax rather than of the two separate regions. The cervical groove, indicating the
SEED COURSE 82 boundary be¬tween head and thorax, lies just behind the center of the carapace, where it runs generally forward and to each side. Ventrally, the boundary between head and thorax is well marked, as is the division of the thorax in to segments, al¬though only the last five may readily be visible. Also the attachment of the thoracic legs to the exoskeleton is clearly apparent, one pair on each of these last five thoracic seg¬ments. The first pair is modified as chelipeds, or claws, while the remaining four pairs are adapted for 'balking or, in some cases, for walking and the last pair for swimming. In two families of primitive crabs (Dromiidae, Dorippidae), the last pair or last two pairs of thoracic legs ar held dorsally, often supporting a piece of sponge or bivalve shell or some other type of sheltering material. The cephalothorax of a crab is characteristically short and broad and, in some species, greatly extended to the sides. In the blue crab, Callinectes sapidus, the paired, widely expanded
SEED COURSE 83 branchial regions of the carapace terminate in a long, sharp lateral spine. Here the exoskeleton turns sharply inward and downward, to end just above Ube legs. As a result of anterior— posterior compression and lateral expansion, the branchial chambers are short and wide. Within these cham¬bers the gills are, of necessity, arranged in a broad oval, rather than linearly as in shrimps and lobsters. Indeed, at their base the most anterior pairs of gills in a blue crab "face" forward. Contrary to widespread popular belief, crabs can walk for¬ward or diagonally, and some species do so quite often. But usually crabs move sideways, particularly when hurrying. The attachment of one pair of chelipeds and four additional pairs of thoracic legs within the short space available at the side of a crab favors sidewise movement over forward move¬ment. When a crab runs sidewise, the legs on the leading side pull the body by flexing, while the legs on the trailing side push the body by extending. One genus, the semiterrestrial ghost crab, Ocypode, can run at great speed. In tests by Dr. Dennis R. Hafeman and Dr. J. I. Hubbard, the species Ocypodc ceratoplithalrna ran at an average speed of 1.825 meters per second, or over four miles per hour, on the firm sand of a tidal beach. When on the hard deck of a ship, the crabs ran even faster, the average speed being 2.33 meters per second, or 5.2 miles per hour. These are the highest recorded speeds for any crustacean. During the tests the crabs did not use their last (fifth) pair of thoracic legs or their chelipeds, except for balancing. The second, third, and fourth pairs of thoracic legs did the moving, with the second and fourth legs on the leading side usually being extended first, to be followed by the third leg. On the trailing side, the same sequences occurred, but with a phase lag of about a third of a cycle. Some observers have reported that when Ocypode is run¬ning, it does so with one side leading for a while. The crab stops abruptly, rotates its body, and then runs with its other side leading. The process of rotation is repeated. In this way, the flexor and extensor muscles of the legs on each side are alternately used and rested. In the anterior portion of the cephalothorax of a crab are the mouth parts, grouped around the opening to the esophagus. These mouth parts are generally similar to those of shrimps and lobsters. The outermost pair is the third maxillipeds, used for holding food. Under and in front of these are two more pairs of maxillipeds and two pairs of maxillae, also used for holding food, and a pair of mandibles, or jaws, which push the food into the esophagus. 12.3.2. Digestive System The foregut of crabs, like that of lobsters, has in its walls many ossicles, or small hard plates and projections, that ar¬ticulate with one another in a complicated way and serve as a place of attachment for muscles that move the foregut. Ac¬cording to American biologists Robert Pyle and Eugene Cro¬nin, the blue crab has in or associated with its foregut at least 50 ossicles and over 80 muscles. These effect a churning action of the foregut and a grinding of the gastric mill that break down particles of food that have been swallowed. The gastric mill of crabs resembles that of lobsters in consisting of one dorsal and two lateral teeth situated at the constriction that separates the large anterior chamber of the foregut from the smaller posterior chamber. The midgut originates approximately where ducts from the midgut glands enter the posterior chamber. Behind this chamber, the midgut appears as a small tube, scarcely three- eighths of an inch in length in a full-grown blue crab. The midgut glands consist of three pairs of lobes, one pair extending forward and to the sides, a
SEED COURSE 84 second pair extending laterally toward or over the gills, and a third pair leading back toward and, in some species, into the abdomen. The midgut glands may fill much of the body cavity, although their extent at any one time depends largely upon their content of food reserves and water. As in shrimps and lobsters, digestion of food in crabs takes place partly in the anterior chamber of the foregut, partly in the posterior chamber, and partly within tubules of the mid- gut glands. A bristly filter in the ventral wall of the posterior chamber prevents all but the most finely divided material' from passing up the ducts into tubules of the midgut glands. Since the lumen of the midgut glands is continuous with that of the midgut, these glands are diverticula of the midgut. In the blue crab, another diverticulum arises from the mid- gut just behind the posterior chamber. A pair of tubes, known as midgut ceca, runs from the dorsolateral surface of the midgut forward and laterally, ending in coils that lie just above the first large lobe of the midgut glands. These ceca are translucent and difficult to see in dissection. Lining the lumen of the midgut ceca are cells like those lining the midgut. Both groups of cells probably function in the absorption of food. The hindgut makes up the remainder of the digestive tract. It runs between the lobes of the mid- gut glands, under the heart, and into the abdomen, where it follows a straight course to its posterior opening, the anus. Only a slight swel¬ling is present in the most posterior portion of the hindgut, hardly enough to-justify calling, this region a rectum. The entire hindgut is lined with chitin. In the second or third abdominal segment, the hindgut of the blue crab gives rise to a cecum. From its origin on the left side, the cecum runs forward and over the hindgut, terminat¬ing in closely packed coils on the right side. The function of this cecum is not clear, but this organ may be involved in the regulation of salts in the hemolymph when a crab is exposed to dilute media.
SEED COURSE 85 12.3.3. Circulatory System In a crab, circulation of blood takes place much as in a shrimp or lobster. Arteries carry blood dorsally from the heart forward into the head and viscera and backward into the abdomen. A sternal artery carries blood ventrally, where main branches direct it both forward and back. Further branching of main arterial vessels leads the blood into thin- walled capillaries, where exchange of gases and foodstuffs between blood and tissues can occur. The blood collects in venous sinuses, goes to the gills, and then enters the pericar¬dial sinus surrounding the heart. Here, when the heart re¬laxes and its three pairs of ostia open, the blood enters the heart. In keeping with the breadth of a crab's body, the heart is broad, filling much of the pericardial sinus, or pericardium. In the small tail of a crab, the ventral abdominal artery is of relatively small size compared with the same artery in the muscular tail of a lobster. 12.3.4. Nervous System The nervous system of all crabs, except: the most primitive, has undergone a high degree of fusion. All ventral ganglia are fused into a single thoracic ganglionic mass, which lies near the floor of the cephalothorax and through which the sternal artery descends. From the periphery of the thoracic gan¬glionic mass, nerves radiate out to the appendages all the way from the mandibles to the last thoracic legs. An abdominal nerve emerges posteriorly at the midline and supplies the muscles and appendages of the tail. Connecting the thoracic ganglionic mass with the brain, or supraesophageal ganglion, are the two long, large nerves that pass on either side of the esophagus and are known as the circumesophageal connectives. Slight swellings on the con¬nectives mark the position of the stomatogastric, or connec-tive, ganglia that supply nerves to the foregut. The trito¬cerebral commissure links the two connectives in crabs, as in shrimps and lobsters. The kidneys of crabs lie on the interior ventral surface of the body, just posterior to a position between each antenna and the corresponding eyestalk on the same side. Due to their color, which is pale green, yellow, or green-brown, the kid¬neys are also called green glands; due to their position on the second, or antennal, segment, they are also often called antennal glands.
SEED COURSE 86 In structure, the antennal glands of crabs are similar to those of lobsters. There is a glandular portion, which secretes urine and regulates salts, and a large, many-lobed, thin- walled bladder, in which urine is temporarily stored. The main lobe of the bladder lies above the glandular portion of each kidney, but the remaining lobes extend out in several directions. Because of the delicacy of the lobes, it is almost impossible to see them unless they are fixed in alcohol or injected with India ink or a powdered dye, such as carmine. Urine passes from the glandular portion of each antennal gland upward into the bladder and then to the exterior via a duct. The opening of the duct, which lies at the base of the antenna, is covered by a calcified, movable cover, called an operculum. 12.3.5. Respiratory System The gills of crabs differ from those of lobsters, where each gill consists of many filaments arranged, plume-like, around a central axis. In crabs two rows of closely set, leaf-like plates or lamella are attached to the central axis of all or, in same species such as the blue crab, all but one pair of gills. In the blue-crab one anterior pair of the gills has only one row of lamella. Gill of crabs known as phyllobranchs, after the Greek words phylion, meaning leaf, and branch, meaning gill. There are eight gills on each side of a blue crab's body.
SEED COURSE 87 Water enters the branchial chambers of crabs primarily through an anterior opening above the base of each claw, or cheliped, and to a much less extent through openings at the base of the other thoracic legs. In the blue crab, when the chelipeds are raised and held forward, the opening at the base of the chelipeds is very large and nearly circular. When the chelipeds are folded against the body, the opening is a wide slit, which becomes narrower when the third maxillipeds are brought close to the midline, for a flange at the base of each third maxillipeds reduces the width of the slit. Bristle-like setae arising from the basal portion of the cheliped filter some of the water entering the slit. In crabs, the opening at the base of the last pair of thoracic legs may or may not be important in the entry of water. According to Dr. Arudpragasam and Naylor, who studied pathways of gill ventilation in several species of crabs, the more flattened and shortened the body of a crab and therefore the more it diverges from the elongated body and laterally facing gills of a lobster or shrimp, the more important in the entry of water are the anterior openings at the base of the chelipeds and the less important are posterior openings. As in shrimps and lobsters, the current of water through the branchial chambers of crabs is maintained by the beating of the gill bailer, which lies in the channel at the anterior exhalant opening of each branchial chamber. After entering through the openings at the base of the chelipeds and, to a less extent at the base of the thoracic legs, the water passes under the gills, up between the gills, over the gills, and out through the exhalant aperture. Periodically, as a result of reversal in the action of the gill bailer, the direction of the respiratory current is reversed. This aids in cleaning the gills of debris and tends to divert water back over the gills that lie in the posterior part of the branchial chambers. 12.3.6. Reproductive System The ovaries of a female blue crab are connected to each other just behind the foregut and extend forward and back¬ward through the body. Thus, in blue crabs, as in American lobsters, the ovaries appear roughly H-shaped. In early stages of its development, each ovary of a blue crab is thin and white, with a short lateral arm. It still appears this way im¬mediately after the female has shed her shell and, as a soft crab, has mated. But ovarian growth starts soon thereafter and, several months later, results in a very large ovary, which is orange because the eggs are full of orange yolk. Each ovary now may extend far out to the side of the body and into the first abdominal segment. From the ovaries, paired oviducts run forward and down¬ward for a short distance, then widen to form an oval-shaped structure known as a seminal receptacle.. Here are stored the sperms that the female blue crab receives from the male blue crab during mating. Each seminal receptacle slants backward and downward and then narrows into a short tubular vagina, which runs ventrally to an opening on the sixth thoracic segment. Although the oviducts and the dorsal portion of the seminal receptacles are soft and unlined, the ventral portion of the seminal receptacles and the vagina are hard, being lined with chitin. At the time of ecdysis, this lining is shed, along with other portions of the exoskeleton.
SEED COURSE 88 In an immature blue crab the seminal receptacles are small and white. Yet, in a mature crab immediately after copulation, the seminal receptacles are enormously distended, at times equal in size to the heart; and they are pink in color, due to the presence of a gelatinous "sperm plug" that keeps the sperms secured within the receptacles. Later, after the sperm plug has been absorbed, the receptacles are again white. At the time of ovulation, when ripe eggs are released from the ovaries and move down the oviducts, sperms fertilize the eggs either within the oviducts or within the seminal recepta¬cles. When fertilized eggs emerge from the vagina, they be come attached to the pleopods of the female and remain there until ready to hatch into the first larval stage. Yet many sperms remain within the seminal receptacles and many eggs within the ovaries, so usually a second "laying" occurs, after which the ovaries appear collapsed and grey or brown in color as they begin to degenerate. Yet even now, enough sperms remain within the seminal receptacles to fertilize several more batches of eggs, were the eggs able to ripen. In an immature blue crab the seminal receptacles are small and white. Yet, in a mature crab immediately after copulation, the seminal receptacles are enormously distended, at times equal in size to the heart; and they are pink in color, due to the presence of a gelatinous "sperm plug" that keeps the sperms secured within the receptacles. Later, after the sperm plug has been absorbed, the receptacles are again white. At the time of ovulation, when ripe eggs are released from the ovaries and move down the oviducts, sperms fertilize the eggs either within the oviducts or within the seminal recepta¬cles. When fertilized eggs emerge from the vagina, they be come attached to the pleopods of the female and remain there until ready to hatch into the first larval stage. Yet many sperms remain within the seminal receptacles and many eggs within the ovaries, so usually a second "laying"
SEED COURSE 89 occurs, after which the ovaries appear collapsed and grey or brown in color as they begin to degenerate. Yet even now, enough sperms remain within the seminal receptacles to fertilize several more batches of eggs, were the eggs able to ripen. In the male blue crab the testes consist of a pair of slender, convoluted, opaquely white arms lying on the dorsal surface of the midgut glands. Medially, the terminal portion of each arm passes around the posterior end of the foregut and joins with the other arm to form a short cross-bar. Just anterior to the cross-bar a tiny tube, the vas efferens, connects each arm of the testes with a much-coiled vas deferens. The vas efferens is difficult to find since it is concealed within the testis and the coils of the vas deferens. The vas deferens consists of several portions. The first known as the anterior vas deferens, is white and tightly coiled and lies close to the middorsal line between the foregut and the heart. Here the sperms are gathered in egg-shaped bun¬dles, called spermatophores, and stored. In the second .portion, the median vas deferens, the coils form a large mass and appear pebbled pink, due to their content of material that subsequently is deposited in the seminal receptacles of the female during copulation and forms a sperm plug.
SEED COURSE 90 The third portion, the posterior vas deferens, is long, con¬voluted, greenishly translucent, and almost empty except during passage of the spermatophores. The final portion, the penis, is a short, translucent tube at the base of the last pair of thoracic legs. The penis lies permanently within a groove in the first pair of abdominal appendages, the copulatory pleopods. These, in turn, are inserted into the seminal recep¬tacles of the female during copulation. Also fitted into the groove on the copulatory appendages of the male is his sec¬ond pair of pleopods, which during copulation act as pistons to push the spermatophres along the groove, where they break up and release the sperms.
SEED COURSE 91 The T-shaped abdomen and elongated, grooved copulatory pleopods of the male blue crab are his most distinguish¬ing external sexual characteristics. In contrast, the abdomen Of the mature female blue crab is broad and rounded, and her pleopods are relatively short and fringed with hair-like setae, to which eggs are attached during development. Coloration also serves to separate the sexes. Normal coloration of both male and female blue crabs consists of a dark blue-green or gray-green carapace and bright blue and blue-green legs, with scarlet markings. Except for the appendages and the female abdomen, the underparts are white. In the male, the greater portion of the chelipeds, or claws, is blue-or gray-green, with dull purple "fingers," whereas in the female there is more blue on the chelipeds and the "fingers" are bright red. It may interest the reader to learn that just as there are blue American lobsters, so also there are blue blue crabs. Some years ago a specimen of blue dab' was described as having a carapace of robin's egg blue and appendages of pale blue with traces of pale red. The under surface of the body was white. Also, just as parti-colored American lobsters exist, so do parti- colored blue crabs. One such specimen was described as being gray on the left side and brownish on the right. A tendency toward albinism occurs in blue crabs, as it does in American lobsters. Unit 13 - Mollusca 13.1. General characters  Molluscs are essentially aquatic, mostly marine, few freshwater and some terrestrial forms.  The body is soft, unsegmented, bilaterally symmetrical and consists of head, foot, mantle and visceral mass.  The body is clothed with a one layered often ciliated epidermis.
SEED COURSE 92  Body is commonly protected by an exoskeleton calcareous shell of one or more pieces, secreted by the mantle.  Head is distinct, bearing the mouth and provided with eyes, tentacles and other sense- organs excel: the Pelecypoda and Scaphopoda.  Ventral body wall is modified into a muscular flat or plough-like surface, the foot which is variously modified for creeping, burrowing and swimming.  Mantle or pallium is a fold of body wall that leaves between itself and the main body mass, the mantle cavity.  Visceral mass contains the vital organs of the body in a compact form taking the form of a dorsal hump or dome.  Body cavity is haemocoel. The true coelom is generally limited to the pericardial cavity and the lumen of the gonads and nephritic.  Digestive tract is simple with an anterior mouth and posterior anus but in gastropods, scaphopods , and cephalopods the intestine becomes U-shaped brin  ging the anus to an anterior position.  Pharynx contains a rasping organ the radula except in Pelecypoda.  Circulatory system is open except in cephalopods which show some tendency towards a closed system.  Respiratory system consists of numerous gills or ctenidia usually provided with osphradium at the base. Lung is developed in terrestrial forms.  Excretory system consists of a pair of metanephridia which are true coelomoducts and communicate from pericardial cavity to the exterior by nephridiopore.  Nervous system consists of paired cerebral, pleural, pedal and visceral ganglia joined by longitudinal and transverse connectives and nerves.  Sexes usually separate (dioecious) but some are hermaphroditic. Fertilisation is external or internal.  Development is either direct or with metamorphosis through the trochophore stage called veliger larva. 13.2.Unio or Lamellidens or Anodonta Class - Pelecypoda Distribution The Freshwater Mussel is a familiar representative of the phylum Mollusca. The family Unionidae is widely distributed all over the world and includes nearly all the large freshwater mussels or clams. The family consists of several genera and nearly 1,000 species of which a good number are represented in India. The commonest species in England is the Swan Mussel (Anodonta cygnea). The types commonly dissected in India are Unio and Lamellidens marginalis. The description that follows will apply in general to almost any freshwater mussel. 13.2.1. Habits and Habitats The freshwater mussels are found in ponds, lakes, rivers and streams, some in quite and others in flowing waters. They occur more abundantly in waters containing lime as this material is necessary for the production of their shell. They live nearly buried in the mud or sand at the bottom, with only the posterior tips of their shell valves exposed. They may also occur wedged in between the rocks and stones, with the valves slightly spread, and the two siphons exposed. They burrow and crawl slowly by extending their large muscular and ploughshare like foot between the two valves, or through the gape of the shell, and larvae a furrow to mark the path
SEED COURSE 93 they have followed. They may migrate to shallow places by night and retire to deeper places by day, and may change the habitats with the seasons. 13.2.2. External Features Shape and size The size varies from 5 to 10 cm. in length. The body is soft, unsegmented, bilaterally symmetrical and battened from side to side. It is sandwiched between the two valves of tie shell. When viewed from the side to: likely looks oval, with a blunt anterior end, the slackest part being in the middle of the body, near the dorsal side becoming gradually thinner toward the ventral edge. Shell The animal IS completely surrounded by a hard calcareous shut. It is brownish in colour. Valves: The shell consists of two separate, equal and lateral pieces called values, covering the right and left sides of the body, respectively. The shill of a mussels made of two vales, is called bivalve as distinguished from the univalve shell of a snail, made of a single pied. In a smile flew, the shell is opal, the anterior and being rounded and the posterior mere pointed. Hinge-ligament: Write two valves of tie shill are united tomato along the aerial id: in a straight bilge line by an external, browns tough, illicit - and note' calcareous or horny hinge-ligament made of conohlolin. The gape of the shell is ventral. The Elastic ligament draws the valve: together dorsally and causes them to gape ventrally. Umbo: Dorsally, in front hinge ligament of the dingo and nearer the anterior dorsal pillar end, there is a whitish knob-like pert re swelling in each valve, called the umbo, which are the thickest and the oldest portion of the shell. It is the first part or the shell the develop in the late veliger larval stage. It is usually corroded by the action of carbonic and hurdle acids in water. Since entitle edge tie umbo is directs d anteriorly, it is possible to determine the right and left posterior Lines of Growth: The outer surface of each shell valve presents a number of concentric lines around the umbo as centre and running parallel to the free large of the shell. These are the lanes or benign of growth, representing intervals between successive growth stag's.
SEED COURSE 94 The shell increase in size by the deposition of new rings around the outer rim of the shell each year. Rings that are far apart indicate periods of rapid growth, owing to a plentiful food supply; periods of restricted growth are represented by close-fitting rings. The rings representing the dormant periods of the winter months are more conspicuous so that it is possible to estimate the age of a clam by these rings. Hinge teeth In Unio and Lamellidens, the inner surface of each valve possesses dorsally along the hinge line, small sharp ridges and teeth-like projections separated by grooves or sockets. These are known as hinge teeth. The teeth of one valve fit into the corresponding sockets of the other, thus holding the valves from slipping out of the position. They differ in number, shape and degree of development in different types of mussels and are entirely absent in Anodonta. The hinge-teeth close the shell with amazing precision and the margins of the shell articulate perfectly like the edges of a well-made locket. This precision is necessary because, if the shell valves did not join tightly, the clam would be much more vulnerable to its many enemies. Muscle scars The inner surface of each valve also bears characteristic markings or impressions, indicating the former attachment of muscles. Near each end, anteriorly and posteriorly, is a large and oval scar of the adductor muscle, that of the posterior muscle being larger than that of the anterior. Near the impression of the anterior adductor muscle are two small impressions, the dorsal and posterior one left by anterior retractor muscle, and the ventral and posterior by protractor muscle. A small impression of a posterior retractor muscle also lies dorsal to the impression of the posterior adductor muscle. Running inside and parallel to the free ventral margin of the valve, from one adductor impression to the other, is a fine groove or line, the pallial line, which marks the attachment of the muscle fibres from the edge of the mantle (retractor pallial muscles). If a clam is molested, the foot at once withdraws in- side the shell, the two valves of which are slowly and lightly shut by the powerful anterior and posterior adductors, thus barring the entry of in-truders. The attempt to pull apart the two valves of the shell may not succeed unless a thin- bladed knife is first inserted through the gape of the shell to severe the large adductor muscles. The star¬fish, however, has a novel way of opening the shell valves. It circumvents the clam,
SEED COURSE 95 attaches its tube feet to the shell valves and exerts a steady pull. Sooner or later, the adductor muscles of the clam become exhausted and relaxed so that the shell opens. 13.2.3. Microscopic structure of shell When viewed in a cross section, the shell presents three distinct layers: periostracum, prismatic layer and nacreous or pearly layer. Periostracun It is the outermost, greenish-brown, thin, translucent, horny layer, formed by a chitin-like horny organic substance, the conchiolin. It is secreted first by the edge of the mantle. It is rough and serves to protect the underlying calcareous layer, from being dissolved by the corroding action of weak carbonic acid in water. It gives the exterior of the shell most of its colour. It is often eroded or worn away from older parts of the shell, like umbo, where the median prismatic layer becomes exposed. Prismatic layer. The middle or prismatic layer is also secreted by the mantle-edge. It consists of minute prisms or crystals of calcium carbonate, prismatic separated by thin layers of chiolin, arranged perpendicularly nacre to the surface of the shell It gives strength to the shell. Nacreous or pearly layer The innermost layer called nacre or "mother-of-pearl', is secreted by the whole outer surface of the mantle and present a smooth, iridescent or lustrous surface. It consists of alternate of the shell and the mantle, layers of calcium carbonate and conchiolin, laid down parallel to the surface of the shell. The mantle deposits nacreous layers over any irregularities that occur either in the shell or over loose particles that may lodge in the mantle itself. The result is the formation of a pearl. The proportion of the CaCO3 in the entire shell varies from 89-99, whereas that of phosphate of lime from 1-2%. The chitin like-horny organic base, called conchiolin provides a sort of membranous framework.
SEED COURSE 96 13.2.4. Pearl formation A pearl is the result of any injury to the mollusks. It is secreted by the mantle as a means of protection against some foreign body. Whenever a foreign body, such as a grain of sand or a small parasite, such as a larval stage of a fluke, gets between the mantle and the shell it becomes enclosed in a sac of mantle epithelium which is thus irritated. The irritation stimulates the mantle epithelium to secrete thin concentric layers of mother of pearl around the foreign body. The amount of deposition is in direct proportion to the degree of irritation, At the end of several years, a pearl will be formed. Pearls are often found in clams and edible oysters but these are not nacreous and therefore or little value. The most precious pearls are found in the pearl oyster (Pinctacla vulgaris), which is closely allied to the freshwater mussel. The Japanese have developed a technique of producing pearls artificially by inserting foreign bodies such as glass beads, into the mantle of oysters which are retained in wire cages or crates until pearls are produced which can be later removed and sold in the market. It requires 3 to 4 years to produce a pearl of considerable size but a large one requires 7 years. Cultured pearls are genuine pearls but less valuable than uncultured pearls which can be identified by experts. Imitation pearls are beads coated with an iridescent substance called pearl essence that is obtained from the scals of fish. 13.2.5. Mantle or Palliun Beneath the shell, the soft body of the mussel is enveloped in a thin, semi-transparent and soft covering of skin, called the mantle or pallium, which secretes the shell. It also consists to lateral halves, the mantle lobes or folds, which are continuous dorsally. Each mantle lobe is a thin sheet of tissue, closely applied to the inner surface of the valve. The ventral free border of each mantle lobe is thickened and contains muscles which insert upon the pallial line.
SEED COURSE 97 At the posterior end, the mantle lobes are thick¬ened, muscular and form two short tubes or siphons. The current of water enters through the ventral incurrent or inhalent siphon and leaves through the dorsal excurrent or exhalent siphon. The inhalent siphon is wider, with a fimbriated or papillated margin and formed simply by coming together of the two mantle lobes. The exhalent siphon is narrower, with smooth margin, and formed by the fusion of the two mantle lobes. On the postero-dorsal side, the two mantle lobes also form a dorsal mantle pore. Histologically, the mantle consists of—  an outer columnar epithelium beset with numerous unicellular glands secreting nacre,  a middle fibrous connective tissue, and  an inner ciliated epithelium containing mucus-secreting cells. 13.2.6.Mantle cavity The space enclosed between the two lobes of the mantle is the mantle or pallial cavity. On removing one mantle lobe, the mantle cavity and its organs (visceral mass, gills, foot, etc) are exposed. Visceral mass The soft body or visceral mass occupies the dorsal parts of the mantle cavity. It is dark in colour and contains various organs including the digestive, circulatory, excretory, and reproductive systems. In a freshly-killed animal, a greenish-brown digestive gland is visible in the antero- dorsal region, a pericardial cavity containing the heart in the mid-dorsal region and dark- coloured paired kidneys below the pericardium. Head The freshwater mussel lacks a distinct head as it would not be of much use to an animal that lives with its anterior end buried in mud. The eyes and tentacles are absent. The large mouth opens beneath the anterior adductor muscle, bordered by a pair of broad, lamellar labial palps
SEED COURSE 98 on each side, while the anus' lies above the posterior adductor muscle. Gills A pair of long, double plate-like gills hangs freely in the mantle cavity, one on each side from the visceral mass. The gills have a sieve-like structure, perforated by minute pores and covered by cilia. Their line of attachment to the visceral mass forms a continuous horizontal partition, dividing the mantle cavity into a large ventral infra-branchial chamber, and a small dorsal supra- branchial chamber. Foot or podium The antero-ventral hatchet-like prolon¬gation of the visceral mass, hanging down in the mantle cavity forms a large, muscular, extensile foot or podium which is adapted for burrowing. It is laterally compressed and terminates below into an elongated keel. The thick basal part of the foot contains a portion of alimenty canal, the digestive gland and the gonad. The foot can be extended by blood pressure and by the muscular action of a pair of pedal pro¬tractor muscles. It can be withdrawn into the shell by the action of anterior and posterior retractor muscles. 13.2.7. Locomotion The foot is the chief locomotory organ and its size and shape are always changing in the living animal. The wedge-shape foot is adapted for progression in the mud or sand at the bottom of the river of lake where it lives. In a buried clam, the shell valves slighty agape ventrally and through this opening the fleshy foot protrudes and burrows through the mud like ploughshare. As the mussel wants to move, the pointed foot is extended forward, as far as possible into the mud by the contraction of the protractor muscles. An influx of blood now takes place into the cavity of the foot so that its tip swells up, becomes td and acts as anchor. Sphincter muscles round veins prevent the return of blood from foot. Next, contraction of the retractor muscles pulls the body of the mussel forward through or deeper into mud. The blood is forced out of the foot which 1'ns down again and can be extended forward in the mud. The repetition of these movements of the foot results in a slow progression of the animal, while a narrow wedge- shaped path is left behind. 13.2.8. Body Wall The whole external surface of the body is covered by a single layer of epithelial cells, which is mostly ciliated, especially on the gills, the labial palps and the internal surface of the mantle. The skin of the foot contains glandular cells, while glandular and sensitive cells are abundant on the mantle edge. Beneath the epidermis, internal spaces of the visceral mass are occupied by the connective and muscular tissues.
SEED COURSE 99 13.2.9. Body Cavity The general body cavity is a haemocoel filled with blood. The true coelom is schizocoelic and greatly obliterated by the connective tissue, unstriped muscle fibres and blood sinuses. It is represented only by three small cavities—(l) a single ovoidal chamber, the verlcardium, which lies dorsally, containing the heart and a part of the intestine, and lined by the coetomic epithelium; (2) the gonocoels or the cavities of the gonads; and (3) the urocoels or the cavities of the excretory organs. Musculature The muscles are mainly of the slow contracting, unstriped type and arranged in distinct bands or sheets. The two shell valves are closed by the contraction of two large, strong, cylindrical transverse muscles, situated one close to either end dorsally and passing across the body from one valve to another. They are called anterior and posterior adductor muscles. When these muscles relax, the elastics hinge ligament opens the valves. Near these muscles, are two smaller muscles, the anterior and posterior retractor muscles, which run from the foot to the shell and serve to withdraw the foot, during locomotion. A small protractor muscle, close behind the anterior adductor, serves to compress the visceral mass, thus causing the protru¬sion of the foot. The complex intrinsic muscle of the foot also serves as a protractor of that organ. The delicate palliàl muscles, inserted upon the shell all along the pallial line, serve to retract the edge of the mantle. 13.2.10. Respiratory System Respiration is aquatic and carried on simultaneously w the feeding process. The respiratory organs are the gills the mantle. Gills or ctenidia The freshwater muscle respires the oxygen dissolved in water by a pair of gills or cienidia or branchiae. On each side of the foot is a single gill, hanging the mantle cavity between the mantle and the visceral ma like a flattened, plate-like structure. The great length of i gills has become possible due to the large siz of the man cavity into which they extend antero- posteriorly. Structure of a ctenidium Each ctenidium is compos of two more or less rectangular plates or laminae, one innerand other outer. Each gill lamina is a hollow double-fold, formed of two thin parallel plates or lamellae, an inner and an outer one, united together at their anterior, ventral and posterior edges, but free dorsally. Each gill lamina thus forms an elongated narrow bag, opening dorsally into a supre branchial chamber. The lamellae are joined together by vertical cross partitions or inter-lamellar junctions, so that the thin space between the two lamellae is divided, at regular intervals, into a series of vertical narrow compartments or water tubes. The water tubes or each gill lamina are closed ventrally, but join a common supra-branchial chamber, dorsally. Each gill lamella consists of a large number of close-set, thin, vertical gill bars or gill filaments, which impart vertical striations to the outer surface of the lamella. The adjacent gill filaments are connected by small bridge-like, horizontal bars, the inter- filamentar functions, which impart horizontal striations to the laminae. The filaments of both the lamellae are continuous at the free ventral edge so that each lamina appears V-shaped in a transverse section, and the ctenidiurn of each side resembles a W. The gill, lamellae have a porous or sieve-like structure, being perforated by minute but frequent openings, the inhalent ostia, bounded by filaments and their junctions and
SEED COURSE 100 leading into the water tubes. Thus, the structure, of each gill-plate is very complex like a piece of basket-work. The ctenidia of the mussel are of the eulamellibranch type. Each ctenidium is bipectinate, with a central ctenidial axis, from which filaments arise in two rows, one on either side. The filaments of each row are folded in the middle to appear V-shaped in section. Of the two arms of V, one is descending, the other ascending. At the angle of the fold each filament is notched. The notches of all filaments form a continuous food groove, that extends the whole length of the underside of each lamina of the ctenidium. The gill filaments are composed of connective tissue. They are strengthend by chitinous rods and covered by a ciliated epithelium. The cilia are of three types, those present on the outer ridge-like faces of the filaments are called frontal cilia, those on lateral parts are lateral cilia while those lying in between are the latero-frontal cilia. Attachment of ctenidia The mode of attachment of Sills to the body determines the course of water current in the body. The gill axis or ctenidial axis remains fused to the dorsal wall of the mantle cavity throughout, but becomes free near the posterior end of the body. The outer lamella of the outer lamina is