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Scoliodon laticaudus: Spadenose shark -Notes for T.Y.B.Sc. Sem VI

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Scoliodon laticaudus a spadenose shark is for the TYBSc course Semester-VI – USZ0601of University of Mumbai

Scoliodon laticaudus a spadenose shark is for the TYBSc course Semester-VI – USZ0601of University of Mumbai

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  • 1. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod SCOLIODON LATICAUDUS Spadenose shark Systematic position Kingdom: Animalia Phylum: Chordata Class: Subclass: Order: Family: Genus: Chondrichthyes Elasmobranchii Carcharhiniformes Carcharhinidae Scoliodon Species: Synonyms Carcharias muelleri J. P. Müller & Henle, 1839 Carcharias palasoora Bleeker, 1853 Carcharias sorrahkowah Bleeker, 1853 Carcharias sorrakowah Cuvier, 1817 S. laticaudus Reference Book: ‘On the spadenose shark, ‘SCOLODON LATICAUDUS’ -by R. V. Ranade Habit & habitat Scoliodon laticaudus is marine; brackish; demersal; amphidromous shark. It is typically found in coastal waters at 10–13 m (33–43 ft) deep, often close to rocky bottoms and in lower reaches of tropical rivers. It is uncertain, however, if this species can live in perfectly fresh water for extended periods. They form large unisexual shoals for feeding purpose. It feeds on shrimps, small benthic fish, cephalopods and crabs. Viviparous, with an unusual columnar placenta; litter size varies from 3 to 14. Distribution Indo-West Pacific, Scoliodon laticaudus is found in Eastern Africa near Somalia, Tanzania, Mozambique. Scoliodon laticaudus is native of Bangladesh; Borneo; Cambodia; China; India; Indonesia; Japan; Macao; Malaysia; Myanmar; Oman; Pakistan; Thailand; Philippines; Singapore; Bangladesh; Sri Lanka; Taiwan, Province of China; Thailand; Viet Nam. Apparently absent from Australasia and Oceania. Kasim (1991) reported that the annual recorded catch of Spadenose Shark in the Veraval coast, India from 1979-1981 averaged 823 tons. These were taken mostly by trawl and gillnet fishing as by-catch. Along Mumbai coast spadenose are found to about five kilometers area. External characters Body moderately stout, fusiform in shape and can be distinguished into head, trunk and tail; color bronze grey above, white below, fins sometimes darker than body; no conspicuous markings. Head broad, greatly depressed, eyes and nostrils small. First dorsal fin extends up to or beyond middle of pelvic fin. Pupil vertical and nictitating eyelid is in anterior and lower corner of eye. Nostrils are closer to mouth than the snout. Mouth is ventral, crescentic, lined by indistinct upper jaw but a well-defined lower jaw. Gills are five pairs and without operculum. Two dorsal, a pair of pectoral, a pair of pelvic, one ventral and one caudal fin are present. Caudal is heterocercal and bears a crescent shaped depression called as precaudal pit. In males the inner margin of each triangular pelvic fin is rolled forming a finger-like tapering appendage, the clasper 1
  • 2. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod or myxopterygium. Shark grows maximum about 74 cm, but mostly found smaller in size. Body size at birth is 12 to 15 cm. Skin The integument provides a characteristic form to the body. It also protects the enclosed organs. Skin is very tough, slimy and firmly attached to underlying muscle coat. Skin is comprised of outer epidermis and inner dermis. Epidermis is derived from ectoderm and dermis from mesodermal layer of the embryo. The epidermis has stratified epithelium and consists of basal germinativum layer of cuboidal cells followed by 45 layers of progressively flattened cells and cornification is not complete. The outermost layer forms a squamous epithelium. In the epidermis may have mucus secreting glands. Unicellular mucous glands are present in the dermis. Dermis has no layers but is made of scattered cells embedded in connetive tissue, and contains chromatophores. They are of two types 1) black, stellate and highly branched melanophores 2) rounded or oval yellowish xanthopohres. Exoskeleton Exoskeleton is comprised of tiny, microscopic placoid scales also known as dermal denticles because their structural organization is similar to that of the vertebrate teeth. Placoid scales cover entire body of shark except inner margin of the claspers. It is suggested that close placement of placoid scales channel water resulting in a laminar flow that acts to reduce friction and also make shark hydro-dynamically quiet while stalking prey. Each placoid scale is made up of a basal plate and a crown. Basal plate is embedded in dermis and has an opening of pulp cavity on its ventral side which leads to the spines of crown and supply nutrition from dermis for growth. The tridentate crown projects outside the skin and is protective to skin. The outermost layer of crown is made of enameloid or vitrodentine material. The inner layer is called as dentine. 2
  • 3. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod Arrangement of placoid scales in shark Structure of placoid scales side view Endoskeleton Endoskeleton of Scoliodon laticaudus is formed of axial skeleton and appendicular skeleton. Axial skeleton includes vertebral column and skull. Vertebral column of Scoliodon laticaudus is made of amphicoelous type of vertebrae i.e. they have concavity on either side of the centrum. Vertebrae are divided into two groups, anterior group of trunk vertebrae and posterior group of caudal vertebrae. 1. Trunk vertebra It comprises a neural arch around spinal cord, a centrum and a pair of ventro-laterally placed transverse processes. Transverse processes in trunk vertebrae articulate with cartilaginous ribs about half an inch length. The top of neural arch is known as neural spine. The vertebral neural plates are in the adult fused with their respective centra, and are notched behind for the exit of the ventral (motor) roots of the spinal nerves. The intervertebral neural plates are 1. Neural spine, 2. Neural arch, 3&12. Neural canal, 4 &13. Concave centrum, 5. Focus, 6. Haemal arch, 7. Haemal canal, 8. Haemal spine, 9. vertebral Neural plate, 10&11. Inter-vertebral neural plate, 15&16. Transverse processes 3
  • 4. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod polygonal pieces alternating with the vertebral neural plates; they are notched behind, but at a more dorsal level than are the vertebral neural plates, for the exit of the dorsal or sensory roots of the spinal nerves. 2. Caudal vertebra In caudal vertebra the transverse processes instead of projecting laterally, are bent inwards beneath the centrum and meet and fuse in the mid-ventral line to form the haemal arch. Haemal arch encloses a haemal canal and is projected ventrally as haemal spine, supporting the caudal fin. The posterior part of vertebral column is bent upward to support asymmetrical tail, known as heterocercal tail. 3. Skull Skull consists of a cranium with a flattened floor and a more irregular and incomplete roof. Its sides are expanded in front owing to the olfactory capsules, and behind owing to the auditory capsules, while in the middle they are deeply hollowed to form the orbits or eye-cavity. Dorsal view Ventral view The dorsal and ventral boundaries of the orbits are respectively formed by the prominent supra-orbital and suborbital ridges. Posteriorly it has a wide opening, the occipital region. Anteriorly it gives off three long processes which fuse together terminally to form the rostrum (two dorso-lateral and one ventro-median cartilages). Behind the olfactory capsules comes a large, nearly circular, hole, the anterior frontanelle above cerebrum on cranium. Behind above cerebellum chanber is a small dorsal opening called parietal fossa. Olfactory capsule lies behind rostrum. The olfactory capsules have no ventral walls, and are separated from one another by the internasal septum. It encloses the olfactory chambers and is open ventrally. Behind the olfactory capsules comes a large, nearly circular, hole, the anterior frontanelle, slightly behind which are the two ophthalmic foramina. The dorsal and ventral boundaries of the orbits are respectively formed by the prominent supra-orbital and suborbital ridges. Behind are the auditory capsules, each of which is marked by a pair of prominent ridges, which lodge respectively the anterior and posterior vertical semicircular canals of the ear. A number of ventral structures disconnected or only loosely connected with the cranium. These together constitute the visceral skeleton forming the jaws and supporting the gills. 4
  • 5. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod Appendicular Skelton of shark includes pectoral and pelvic girdles 1. Pectoral Girdle Two pectoral girdles are attached to the coracoid bar on either side on the ventral side of the shark just behind the gills. The paired scapular cartilages, and their extensions, the suprascapular cartilages, extend dorsally from their base at the lateral ends of the coracoid bar, near the fin, on either side of the body in the muscular wall. The glenoid surface is the surface of articulation between the fin base and the coracoid bar. Proximally fin is comprised of propterygium, mesopterygium and metapterygium from outer to inner side which articulates distally to their corresponding radials. outline of the distal part of is supported by horny fin-rays called as ceratotrichia. 2. Pelvic Girdle Pelvic girdle is present in the posterior side of the body to which the pelvic fins are attached. The pelvic girdle is connected directly to the vertebral column in the sacral region. The pelvic girdle has two equal halves which are known as 'ossa innominata'. Each as innominatum is formed by three bones. They are the dorsal bone ilium, the ventral bone ischium and the antero-ventral bone pubis. The pelvic girdle has a depression at the junction of the three bones. It is termed as acetabulum into which the head femur of the hind limb articulates and forms a ball and socket joint. Form acetabulum it projects a long curved shaft, basipterygium. Basipterygium gives out a series of radiales. Posteriorly basipterygium is attached with claspers in male shark. 1. Ischio-pubic bar of pelvic girdle 2. Basipterygium 3. Clasper 4. Radiale Fig. 8. Dorsal view of the pelvic girdle and fins of a male Scoliodon laticaudus 5
  • 6. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod Digestive system Buccal cavity is the area enclosed by the jaws (mandibular arch) and the cartilage of the throat (hyoid arch). The triangular sharp teeth are arranged in several rows beginning at the outer edges of the upper and lower jaws. Behind the functional teeth are additional rows folded downward ready to replace any that are lost. The pharynx is the portion of the alimentary canal posterior to the hyoid arch and has five pairs of gill and also contains the tongue in dogfish. The shark can bring water into its pharynx to the gills by way of the spiracle and mouth. The raised floor of the oral cavity is referred to as the tongue. The tongue of the shark is practically immovable and without muscles. It is supported anteriorly and posteriorly by cartilage and hence referred as false tongue. Posteriorly it narrows to form the esophagus. The esophagus is the short hollow tube connecting the pharynx to the stomach. The esophagus is the thick muscular tube extending from the top of the cavity connecting the oral cavity and pharynx with the stomach. The esophagus is lined with papillae, which form a tight seal to keep water out. The esophagus leads into the "J"-shaped stomach. The upper portion, the cardiac region, continues as the main body, and ends at the duodenal end. The mucosa is the inner lining of the stomach. The rugae are longitudinal folds that help in the churning and mixing the food with digestive juices. A circular muscular valve, the pyloric sphincter, is located at the far end or pyloric end of the stomach. It regulates the passage of partially digested food into the intestines. The pancreas is located on the duodenum and the lower stomach. The secretions of the pancreas enter the duodenum by way of the pancreatic duct. The liver is the largest organ lying within the body cavity. Its two main lobes, the right and left lobes extend from the pectoral girdle posteriorly most of the length of the cavity. A third median lobe much shorter lobe and contains the green gall bladder along its right edge. The dark, triangular-shaped spleen is located near the posterior end of the stomach. Although a part the lymphatic system, the spleen is closely associated with the digestive organs in all vertebrates. The small intestine is composed of, anteriorly, a duodenum, and posteriorly, an ileum (valvular intestine). The duodenum is a short "U"-shaped and connects the stomach to the intestine. The bile duct from the gall bladder enters the duodenum. The valvular intestine is the second and much larger portion of ileum, which contains a scroll valve. The scroll valve is the longitudinal fold hanging from roof of ileum which is coiled within the valvular intestine. It adds surface area for digestion and absorption to an otherwise relatively short intestine. The large intestine, rectum or colon, a shorter section than the small intestine has a rectal gland entering it. The rectum is the narrowed continuation of the valvular intestine. It is located at the posterior end of the 6
  • 7. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod body cavity. The rectum, the most posterior portion, ends in the anus which projects into the cloaca, a common opening with the urogenital ducts. The rectal gland is a slender, blind-ended, finger-like structure that leads into the rectum by means of a duct. It has been shown to excrete salt (NaCI) in concentrations higher than that of the shark's body fluids or sea water. It is thus an organ of osmoregulation, regulating the shark's salt balance. The cloaca is the last portion of the alimentary canal. It collects the products of the colon as well as the urogenital ducts. It is a catch-all basin leading to the outside by means of the cloacal opening. Respiratory system In Scoliodon respiration takes place through 5 pairs of gill slits or gill pouches. They are present in a series on the wall of pharynx on either lateral side, behind the hyoid arch. Each gill pouch opens into the pharynx by a large branchial aperture and to outside through an external branchial aperture or gill slit. Two adjacent gill pouches are completely separated by a vertical fibro-muscular partition, the interbranchial or gill septum. The inner or pharyngeal border of each gill septum is supported by a cartilaginous visceral arch or gill arch with its slender branchial rays. The septum is covered by epithelium and contains blood vessels, nerves etc. Gill chamber of shark Respiratory system of shark The mucus membrane of a septum is raised into numerous horizontal leaf like folds called gill lamellae or gill filaments. These constitute the gill proper and are richly supplied with blood capillaries. Each septum bears two sets of gill-lamellae, one on its anterior face and the other on its posterior face. Each set makes a half gill called hemibranch. A gill pouch thus contains two hemibranchs belonging to two different adjacent gills. These two hemibranchs with their interbranchial septum and the visceral arch constitute a complete gill or holobranch. The posterior hemibranch of each holobranch is larger than the anterior one. In Scoliodon, the hyoid arch bears only a hemibranch posteriorly. The first four branchial arches bear a holobranch each. Thus scoliodon has total nine hemibranchs while the fifth branchial arch is devoid of gills and is called abranch. The inner part of each interbrachial septum has a supporting visceral arch from which cartilaginous gill rays arise in a single row and project into the interbranchial septum for further support. Visceral arches also give out rigid comb-like gill rackers, which project the internal branchial apertures from food. 7
  • 8. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod Blood vascular system Sharks have a two-chambered heart, with an atrium (also called the auricle) and a ventricle. The heart is an S-shaped tube that is located in the head region of the shark. It is located at ventrally in thoracic pericardial cavity lined by a thin pericardium. The blood is pumped by the heart through the afferent branchial arteries (ventral aorta) to capillaries in the gills (where the blood is oxygenated). The blood then flows through efferent branchial arteries (paired dorsal aorta), then through the tissues of the body, and then back to heart in veins. 1. Heart of shark The shark heart is two chambered heart having single atrium and a single ventricle. The heart of a shark contains four important parts, the atrium and ventricle, the sinus venosus, and the conus arteriosus. The sinus venosus is a small sac with thin walls which collects deoxygenated blood from the fish’s veins, after which the blood flows into the atrium, a large muscular chamber. The atrium is a one-way compartment for blood to flow into the ventricle (which does the pumping for the heart). The bulbus arteriosus is a large tube into which the blood is pumped from the ventricle. The Conus arteriosus then leads the blood to the ventral aorta, through which blood flows to the shark’s gills 2. The Aortic arches The anterior end of the conus arteriosus continues forward as the ventral aorta. It gives off five pairs of afferent branchial arteries which carry deoxygenated blood from the heart to the gills. Each arterial collector loop is formed from the union of a pre-trematic artery from the anterior side of a gill slit with a post-trematic artery from the posterior side of the gill slit. The pre and posttrematic arteries receive tiny branches from the adjacent gill lamellae and both join together to give rise to an efferent branchial artery. The four pairs of efferent branchial arteries collect oxygenated blood from the gills. The four pairs of efferent branchial arteries join at the mid-dorsal line to form the large dorsal aorta. 3. Arterial system: Internal carotid artery Supplies brain; Dorsal aorta supplies entire trunk; Hyoidian artery supplies carotid arteries; Pharyngoesophageal artery supplies esophagus and pharynx; Subclavian artery supplies pectoral fin; Coeliac artery supplies gonads and entire digestive tract; Gastric artery supplies stomach; Hepatic artery supplies liver; Ovarian artery/testicular artery supplies gonads; Pancreaticomesenteric artery supplies pancreas and intestine; Anterior intestinal artery supplies intestine; Anterior mesenteric artery supplies intestine; Gastrosplenic artery supplies spleen, stomach, and pancreas; Iliac artery supplies pelvic fins. 8
  • 9. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod 1. Venous system: Hepatic portal vein drains entire nutrient rich blood from digestive tract to liver which break into capillaries; Gastric vein drains stomach; Pancreaticomesenteric vein drains intestine, spleen, and pancreas; Anterior intestinal vein drains intestine; Anterior lienogastric vein drains spleen and stomach; Leinomesenteric vein drains pancreas; Posterior intestinal vein drains intestine; Posterior cardinal vein drains kidneys; Posterior cardinal sinus drains entire trunk. Nervous system 1. Brain Prosencephalon (forebrain) - subsequently divides into the telencephalon (cerebrum) & Diencephalon (epithalamus, thalamus, & hypothalamus) Mesencephalon (midbrain) - develops without further subdivision & forms the tectum Rhombencephalon (hindbrain) - subdivides into the metencephalon (pons & cerebellum) and myelencephalon (medulla oblongata) a. Forebrain: Sharks have very well developed olfactory lobes, a mid-brain with a thalamus, hypothalamus, pituitary gland, amygdala and a large pallium (forerunner of the cortex) with well-defined basal ganglia, although the hippocampus has not much differentiated. b. Midbrain: There are two large optic lobes situated on top of the midbrain. c. Hindbrain: The hind-brain has a well developed brainstem with a large, elongated cerebellum. Posteriorly it gives out a pair of auricles of cerebellum and medulla oblongata which encloses fourth ventricle; and finally gives out a dorsal tubular spinal cord. 9
  • 10. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod 2. Cranial nerves Terms Olfactory nerve Optic nerve Origin Definitions Olfactory lobe Cranial nerve I, terminates in the olfactory bulb Optic lobe Oculomotor nerve Optic lobe Cranial nerve II, arises in the retina and extends to the diencephalon and then to the optic lobes Cranial nerve III, innervates 4 eye muscles (inferior oblique, superior, inferior and medial recti) Betn. Optic & Cranial nerve IV, innervates the superior oblique muscle cerebellum Internal Cranial nerve V, (3 branches): opthamalic, maxillary, Trigeminal nerve surface of mandibular medulla Ventral surface Abducens nerve Cranial nerve VI, innervates the lateral rectus of medulla Common with Cranial nerve VII, (3 branches) arises from the medulla and Facial nerve trigeminal base: superficial ophthalmic, buccal nerve, hyomandibular Cranial nerve VIII innervates the inner ear by means of a Medulla Statoacoustic vestibular branch to the anterior part of the ear and a saccular (Auditory) branch to the posterior part Cranial nerve IX, has multiple branches, innervates the the Glossopharyngeal Medulla third visceral arch nerve Cranial nerve X, gives off lateral line trunk and four branchial Vagus nerve Medulla rani to each of the remaining gills Medulla Cranial nerve XI, accessory to vagus….Motor Accessory Trochlear nerve Medulla Hypoglossal Cranial nerve XII, tongue muscles ………….Motor 10
  • 11. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod Receptor organs of shark 1. Skin (general cutaneous sense organs): Skin has numerous free nerve endings enabling shark for the reception of sense of touch, pain and temperature in the exterior. There are some touch corpuscles present at the bases of the fins. 2. Lateral line system (LL) and neuromasts: LL system includes a network of canals and sense organ called as neuromast. Two longitudinal canals run laterally along the flanks of the body from the tip of the caudal fin towards the anterior of the shark. These lateral longitudinal canals divide into cephalic and orbital canals in the head region. Orbital canals re-divide into supra-orbital and infra-orbital canals. Both supra orbital and infra-orbital canals divide further and from a network on the head. Longitudinal canals also unite by a transverse commissural occipital canal at the junction of head with trunk. Neuromasts are special sense organs present in aquatic organisms. They are arranged in a series along the LL canals. Each neuromast is a spherical or flask like hollow body located in a fibrocartilage. Internally it has a layer of connective tissue except its base. At the base it contains a lower layer of sensory cells (hair cells) embedded in supporting cells and an upper layer made of vacuolated gelatinous cells called as cupula. Cupula remains in close contact with the sensory cells. Each sensory cell has sensory projections, a kinocilia and few stereocilia. When water enters in canal system cupula displaces and bends kinocilia sending the impulses to axons attached to sensory cells. 11
  • 12. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod 3. Electroreceptors (ampullae of Lorenzini) The ampullae of Lorenzini are small vesicles that form part of an extensive subcutaneous sensory network system. These vesicles are found around the head of the shark. These are modified LL organs and respond to detect weak magnetic fields produced by other fish at short ranges. This enables a shark to locate prey buried in the sand or to orient to nearby movement. Each ampulla is a bundle of sensory cells innervated by several nerve fibers. These fibers are enclosed in a jelly-filled tubule that has a direct opening to the surface through a pore. These pores on the head of the shark are visible to the naked eye, and appear as dark spots. These are two type ampullary and tuberous receptors. Ampulla of Lorenzini has a flask like ampullary sac with a long tube and is filled with gelatinous substance. There are few chambers in the ampullary sac containing sensory cells which are connected to neurons ventrally. A series of ampullae of Lorenzini are embedded in connective tissue are mostly found in the head region. Tuberous organs lack the ampullary sac. In the marine water a weak electric field is generated naturally in number of ways are received by shark to differentiate between the objects event in the dark. 4. Internal ear Only internal ear is present in shark. It is also called as membranous labyrinth. Labyrinth is enclosed in auditory capsule of the cranium floating in perilymph. It is made of a vertical flattened utriculus and ventral sacculus. Utriculus has outgrowths forming three semicircular canals lying perpendicular to each other namely anterior, posterior and horizontal canal. Sacculus gives out a posterior outgrowth called lagena. The cavity of entire 12
  • 13. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod labyrinth is filled with fluid called as endolymph. Membranous labyrinth has four functions i.e. controls musculature; change in direction during swimming; to regulate depth and sound reception. 5. Olfactory sac There are two olfactory organs/sacs/chambers. Each olfactory organ is located in olfactory capsule of skull which is guarded by a nasal fold. There is an incurrent opening placed anteriorly and an excurrent canal posteriorly. Olfactory chamber has about 40 parallel, vertically arranged olfactory folds or ridges internally. Each olfactory ridge gives out numerous lateral alternating leaflets like outgrowths lined by olfactory epithelium. These outgrowths increase the surface area. The epithelium is lined by olfactory chemoreceptor cells. Sense of smell is acute in shark at a capacity to detect upto 1 ppm dilution (1 part in million parts of sea water). Taste buds are also similar to the olfactory receptors but they are not distant field chemoreceptores. 6. Eye: In Scoliodon laticaudus have laterally positioned eyes. Scoliodon laticaudus is considered to have generally small eyes compared to body size. The outer layer of the shark eye comprises a thick cartilaginous sclera. Shark eyes need to handle wide water pressure variations, as they range from shallow to deep water looking for food. Shark eyes have a large, spherical lens, a cornea, a retina (with both rods and cones), an iris, and a pupil. The retina has a greater proportion of rods (light intensity sensors) than cones (color sensors), so Scoliodon laticaudus very sensitive to small differences in light intensity (dark versus light). They even have good vision in dim light. Scoliodon laticaudus has a mirrorlike layer in the back of the eye, the tapetum lucidum. This layer doubles the intensity of incoming light, enhancing light sensitivity. Unlike other fish, shark's pupils can dilate and contract to control the amount of incoming light. Scoliodon has a nictitating membrane, a type of eyelid that protects the eye during hunting. External eyelids are immovable. 13
  • 14. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod Urogenital system 1. Male urogenital system: The kidneys are flattened; ribbonlike, dark colored structures lying dorsally on either side of the midline, along the entire length of pleuroperitoneal cavity and deeply placed dorsally. The upper portion of kidney is rounded known as head of kidney is an active part of reproductive system. The male reproductive system begins in the testes. These sex organs are located in the dorsal side of the shark next to the liver; however, the testes are noticeably larger than the ovaries. The size of the testes, however, varies in size depending on the month of the year (they are larger during the breeding months of January and February). The function of the testes is to produce sperm that will later be used to fertilize the female ova. The sperm is taken from the testes to the Archinephric Duct by few small tubes known as Vasa efferentia. The vasa efferentia dilate to form a vide vas defferens transports sperm to the Seminal Vesicle. The Seminal Vesicle is directly attached to the spiralled Archinephric Duct and is noticeably larger and straight. The Seminal Vesicle leads to the Sperm Sac. The Sperm Sac is the last stop before excretion in the male reproductive system. The Sperm Sac contains seminal fluid that will allow the sperm to pass safely to the female and fertilize the ova. The entire embryonic developmental stage occurs in the female. For this reason, males must have developed certain structures that allow them to fertilize the female egg (equivalent to the male penis). This structure is known as the Clasper. The Clasper allows the male shark to hold on to the female shark during sexual reproduction. Each Clasper posses a Clasper Groove, allowing the Clasper to bend at possible awkward angles. Also part of the Clasper structure is the Siphon Sac. All of these structures are essential during sexual reproduction. To start off sexual reproduction, the Siphon Sac absorbs some seawater. This seawater will help to propel the seamen from the male into the female. 2. Female urogenital system: The kidneys are flattened; ribbonlike, dark colored structures lying dorsally on either side of the midline, along the entire length of pleuroperitoneal cavity. In female the upper portion of kidney is non-functional. The female reproductive system begins in the ovaries. The ovaries are where the egg or ova is produced and are located on the dorsal side of the shark, just next to the liver. Every egg yellow colored and is enveloped in a follicle. The color is a result of the large amount of yolk in each ovum. From here, the ovum leaves the 14
  • 15. Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod ovary, heading for the oviduct. The oviduct is a tube-like structure where the ovum is fertilized by the male sperm. It is clearly visible in mature females due to its large size. The oviduct is connected directly to the cloaca, allowing for sperm to pass directly to the ovum. Different areas of the oviduct have been shown to perform different functions. One area of the oviduct that has been enlarged (distal to the ovaries) is knows as the nidamental or shell gland. This gland has been shown to store sperm for great lengths of time to allow for fertilization later. The oviduct thins out again, only to enlarge once more in a more caudal location to form what is known as the uterus. The uterus is the location of embryonic development. The opening is located towards the dorsal portion of the cloaca. Copulation, fertilization and development This shark is placentally viviparous, with arguably the most advanced reproductive mode of the elasmobranchs. Male dogfish reach maturity at 11 years while female dogfish reach maturity at 19-20 years. Eggs are ovulated at only 1 mm in diameter and the stalked placenta forms when the embryos are only a few millimeters in length. The young are born at a length of 12-15 cm. Males mature at 24-36 cm and females at 33-35 cm. Breeding occurs throughout the year and females probably mate at least once each year. Viviparous, with an unusual columnar placenta; litter size varies from 1 to 14. [Litter sizes range from 618, with a mean of 13.] The young are born throughout the year, after a gestation period of five or six months. Economic importance Although dogfish liver oil was once valued for its vitamin A content and dogfish supported a thriving fishery off Washington state and British Columbia, they are now rarely landed by North American fisheries. However, substantial quantities are marketed for human food in Europe and Asia. When abundant, dogfish may become a serious pest and inflict severe economic losses on fisheries by damaging fishing gear and the catches of the more desirable fish. Furthermore, huge schools of dogfish sweeping over the sea bottom devour and all but eliminate the smaller fishes and invertebrates in their paths. Despite its commercial importance, overall fishery statistics for the spadenose shark are lacking. A 1996 report found it to be the most common coastal shark on Chinese markets, and it is also one of the most common sharks caught off northern Australia. Substantial numbers are caught by Indian and Pakistani fisheries; from 1979 to 1981, an average of 823 tons were caught annually off Verval, India. The spadenose shark is also caught as bycatch, particularly in gillnet fisheries off Kalimantan. Made by Mr. S. D. Rathod Associate Professor Department of Zoology B. N. Bandodkar College of Science Thanr-400605 15

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