Cnidaria: General BiologyThe colorful cnidarians are the flowers of the sea.Mostly marine (10,000 species); a few fresh water (20 species), but none live on land.Cnidarians can be solitary but can be big (like jellyfish), or colonial (like stony corals).Most cnidarians are carnivores that catch and disable prey with stinging tubules fired from uniquely specialized cells called cnidocytes.To humans, cnidarian stings create a burning sensation like that of a nettle plant, after which Cnidaria is named ( cnid- = nettle).
Form and Symmetry of Solitary Individuals• General In basic form, the saccate cnidarian body resembles a gastrula, which consists of a gut like cavity enclosed by a solid body wall. The cavity, called coelenteron, opens the exterior via a mouth surrounded by one or more whorls of tentacles. Sensory organs, when present, are distributed at or near the bases of the tentacles.
• The cnidarian body exhibits radial symmetry around one axis, the oral-aboral axis, that extends from mouth to base. The oral end of the body, which bears the mouth, is at one end of the axis, and opposite it is the aboral end. Radial symmetry may be useful when an abundant but diffuse resource plankton, light- or danger has an equal probability of arriving from any or all directions. This applies to most cnidarians because they do not stalk their food, but instead feed opportunistically on prey as it inadvertently swims or drifts into contact with them.
Cnidaria: 2 body forms Polyps Medusae• Resembles flower • umbrella or and stem bell-shaped• Pedal disk & • manubrium : oral disk “elephant trunk”• Sessile & benthic; • “mouth-down” “mouth-up”
Cnidaria: solitary body formsRadially symmetric Radially symmetric medusa polyp
Tissues and Body Compartments The cnidarian body wall is composed ofthree tissue layers: an outer epithelium,the epidermis; an inner epithelium, thegastrodermis, which lines the gut-likecoelenteron and joins the epidermis at themouth; and between these two epithelia,a gelatinous extracellular matrix calledmesoglea.
A stolonate colony, like a strawberry runner,consists of a prostate colon, single or branched in atwo-dimensional network, that attaches to thesurface of the substrantum and bears upright,unbranched zooids. The stolon is a tubular body walloutgrowth that includes the coelenteron, from whichzooids bud at intervals along its length. Colonies of several taxa consist of zooids that arisefrom the surface of a continous sheet of tissue calleda stolonal mat or coenosarc that unites the zooidsand attaches the colony to the substantrum.
The colonies described so far are more thanless simple, consisting of zooids borne eitherdirectly on stolons like bulbs on a string ofChristmas lights or individually on acoenosarc. More complex colonies arise notonly by the budding of zooids from stolons orcoenosarc, but also from other zooids. With afew exceptions, zooidal budding produces anupright, plantlike, fruticose colony that maybe grassy, bushy, shrubby, or feathery inappearance.
Cnidaria: fruticose (shrubby) colony A fruticose colony A fruticose colony canresulting from also grow from an axial polyp (stem-like),fixed-length budding. which buds a succession of lateral zooids.
Skeleton• The startling variety of cnidarian skeletons, especially evident in polyps, exceeds even that of sponges. Exoskeletons may be the thin cuticles of chitinous periderm, as found in many small-bodied solitaty polyps and colonies, but not Hydra or most anemones, which are naked. The polyps of stony corals secrete a hard calcareous exoskeleton that in some reef species can, over many generations, reach a diameter of 1m or more and weigh several tons.
Medusa: locomotion by swimming (jet propulsion )
Muscular and Movement• The principal muscle of medusae is the halo- shaped, circular coronal muscle on the subumbrelar surface. The cross-strained coronal muscle is antagonized by the elastic mesoglea.• Cnidarian muscles are primarily epidermal and gastrodermal epitheliomuscular cells, but in anthozoans and scyphozoans, some of these cells have abandoned the epithelia, entered the mesoglea, and transformed into myocytes, or “true” muscle cells.
Hydra: locomotion by crawling or somersaultingHydras and some sea anemones can detach and shift locations by creepingon their bases, by inchworm-like crawling, by somersaulting, or by floating.
Cnidaria: nervous systemNerve net of a polyp Nervous system of medusa
• The cnidarian nervous system, like that of all eumetazoans, consists of superficial sensory neurons that monitor the environment, motorneurons that activate effectors such as muscles or cnidocytes, and interneurons that join the sensory receptors to the motorneurons.• In Cnidaria, interconnected neurons form a pair of complex, two-dimensional nerve nets.• Diffuse conduction is a characteristic of nerve nets.
A ganglion is a concentration of neuronsthat serves as a brainlike integration center.As such, it receives sensory input, integratesthat information with other inputs, andgenerates motor output.
Cnidocytes And Cnidae• A unique and defining feature of cnidarians is the cnidocyte: a combined sensory-effector cell that plays a central role in prey capture and defense. Each cnidocyte houses a cnida, a fluid-filled membranous capsule containing a long tubular invagination of the capsule wall.• Three general types of cnidae- nematocytes, spirocytes, and ptychocytes.
Cnidaria: circulatory systemPattern of ciliary circulation in the radial canals, ring canal, &tentacles of a medusa in side view (A) and from above (B).In (C) shows a pattern of circulation in a sea anemone,including flows in the pharynx.
Coelenteron: Nutrition and Internal Transport• The coelenteron is a blind, saclike cavity lined by gastrodermis and opening to the exterior via the mouth. In large polyps, the cavity may be more or less partitioned by septa, which increase the surface area of the gastrodermis. Among medusae, it is often regionally specialized into a centeral stomach from which radial canals extend to join a marginal ring canal.
Regeneration and Clonal Production• Cnidarians have an impressive ability to heal and regenerate missing parts after injury. In Hydra, for example, loss of the oral end is followed by regrowth and differentiation of a new mouth and tentacles. Living specimens of the common anemone Aiptasia pallida can be dissected, pinned open for observation, and then returned to an aquarium, where they will heal after a few days. Medusae also heal and regenerate missing parts after damage, as do planula larvae.
Sexual Reproduction and Life Cycles• Cnidarian adults are generally gonochoric, but certain taxa such as stony corals, have many hermaphroditic species. Germ cells originate in the endoderm and generally grow and differentiate in the gastrodermis, except in some hydrozoans, in which they migrate into the epidermis.• Further development leads to a planktonic larva known as a planula. It’s epidermis is monociliated and its gastrocdermis is often yolky.
Cnidaria: development, larva & life cycle View A-B cleavage starts at the animal pole C-D bastula stages E-F gastrulation establishes the 2 germ layers, ectoderm and endoderm G the planula larva H the presumed primitive life cycle is: polyp planula polyp.
Anthozoa: anatomycutaway view of a sea anemone
Polyp Form• The body consists of a tubular column surmounted by a wide plateau, the oral disc. The oval or slitlike mouth is in the center of the oral disc and a whorl of tentacles occurs at its margin.• In solitary individuals, the base of the column is often expanded into a pedal disc that adheres to the substratum. The mouth leads into a tubular but laterally compressed pharynx that descends below the oral disc and opens into the coelenteron via an internal opening.
• Because of the pharyngeal compression, the mouth and internal pharynx opening are elongated slits. The two opposite corners of the mouth exted inward along the length of the pharynx as ciliated grooves called siphonoglyphs.
Anthozoa: anatomycross section of body at level of pharynx