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Phylum Echinodermata Chapter 16 Phylum:Echinodermata Means “spiny-skinned” Approximately 7,000 species All are found in marine habitats – all depths Echinoderm Characteristics Calcareous endoskeleton in the form of ossicles Adults with pentaradial symmetry and larvae with bilateral symmetry Water-vascular system used in locomotion, attachment, and/or feeding Complete digestive tract Hemal system derived from coelomic cavities Nervous system: nerve net, nerve ring, radial nerves ( no “brain” ) Echinoderm Evolution Thought to share a common ancestor with hemichordates and chordates. Evidence: Deuterostome development Bilaterally symmetrical larval stages Extinct forms were not radially symmetric Pentaradial Symmetry Body parts are arranged in fives, or multiples of fives, around an oral-aboral axis No distinct head region Adaptive for sedentary or slow moving animals (allows a uniform distribution of sensory, feeding, and other structures) Echinoderm Endoskeleton Pentaradial symmetry may be a result of the evolution of the echinoderm endoskeleton Consists of a series of ossicles – calcium carbonate plates Frequently modified into spines that project from body surface X-Ray of endoskeleton Water-Vascular System Unique to echinoderms Series of water-filled canals with extensions called tube feet Originates as a modification of the coelom Ring canal surrounds the mouth, opens to the outside through a stone canal and an opening called the madreporite 5 (or multiples of) radial canals branch from the ring canal; one in each arm of sea stars Water-Vascular System Water-Vascular System Tube feet are extensions of the canal system and emerge through skeletal ossicles Internally: tube feet end in a bulblike, muscular ampulla Ampulla contracts and forces water into the tube foot which then extends Suction cup at opposite ends of tube feet contract and create a vacuum Water- Vascular System Used in locomotion but has additional functions: Original purpose: feeding Soft membranes of tube feet allow for gas exchange and diffusion of nitrogenous wastes across the body wall Hemal System Consists of strands of tissue that encircle an echinoderm near the ring canal of the watervascular system and run into the radial canals of the arms Derived from the coelom Circulates fluid using cilia that line channels Function largely unknown but may aid in transport of large molecules, hormones, or coelomocytes (engulf and transport waste) Class Asteroidea Includes Sea Stars ~ 1500 species Most live on hard substrates Most brightly colored Class Asteroidea Characteristics Usually have five arms that radiate from center Has movable and fixed spines Oral and aboral surface Class Asteroidea Characteristics Dermal branchiae or papulae: thin folds of the body wall which function in gas exchange Ambulacral groove: series of ossicles in arm which run the length of the oral surface Houses the radial canal Paired rows of tube feet on either side Class Asteroidea Movement Sea Star tube feet move in a stepping motion coordinated by the nervous system Alternate extension, attachment and contraction. All tube feet move in the same direction, but not in unison. Provide attachment during wave action Class Asteroidea Feeding Feed on snails, bivalves, crustaceans, corals, detritus, etc. Mouth opens to a short esophagus and then to stomach area. Two stomachs: Cardiac stomach – large oral stomach which receives ingested food Pyloric stomach – smaller aboral stomach absorbs digested food Class Asteroidea Feeding How sea stars feed on bivalves: Wraps around bivalve’s opening Tube feet attach to outside of shell and forces valves apart Cardiac stomach lowered into bivalve Digestive enzymes released into shell for partial digestion Continued digestion occurs in pyloric stomach Stomach retracts into sea stars Sea Star Eating Squid Class Asteroidea Regeneration Capable of regeneration In some cases, an entire sea Click here for star can regenerate from a piece of broken arm regeneration video Broken part must contain portion of central disk Complete regeneration may take up to a year Class Asteroidea Reproduction Most are dioecious Two gonads present in each arm External fertilization Spawning coordinated by environmental factors Length of light/dark Water temperature Bipinnaria symmetric larvae bilaterally Class Ophiuroidea “Snake Tail” Over 2,000 species Most diverse group Includes brittle stars and basket stars Arms are long and sharply set off from central disk Central disk has pentagonal shape Class Ophiuroidea Water vascular system is NOT used for locomotion Ossicles (calcium carbonate plates of skeleton) are modified to permit a unique form of grasping and movement Result is snake-like locomotion Tube feet do not have suction disks (unlike sea stars) Brittle Star "Crawling" Class Ophiuroidea Feeding Predators and scavengers Use arms and tube feet to sweep in food and trap plankton Maintenance Capable of regeneration (autonomy for defense) Dioecious, males are smaller and often carried by females Class Echinoidea ~1,000 species “Spiny” Includes sea urchins, sand dollars and heart urchins Attach to hard substrates or burrow in sand Class Echinoidea Skeleton is called a “test” Made of 10 sets of closely fitting plates Move by using spines for pushing against substrate and tube feet for pulling Some sea urchins have sharp spines and venom Class Echinoidea Feeding Feed on algae, coral polyps and dead animal remains Specialized chewing apparatus: “Aristotle’s Lantern” Reproduction & Development Dioecious Gametes shed into water External fertilization Larva undergoes metamorphosis Class Holothuroidea “Sea Cucumber” ~1,500 species Includes the sea cucumbers Lack arms Have elongate bodies Class Holothuroidea Tube feet enlarged and highly modified Surround mouth and are called “tentacles” Mostly sluggish burrowers and creepers Locomotion using tube feet is inefficient Contraction of body-wall muscles produce wormlike movements Class Holothuroidea Feeding Ingest particles using tentacles Food is trapped by mucus on tentacles Thrust tentacles into mouth and “wipe off” trapped food Stomach, long, looped intestine, a rectum, and an anus Class Holothuroidea Respiration “Respiratory Trees” Pair of tubes which attach at rectum and branch throughout body Pumping action of rectum circulates water into tubes Gases and nitrogenous wastes exchanged between water and coelom through respiratory trees Class Holothuroidea Defense Defenseless against predators? No! Many produce toxins in body walls Some evert tubules of respiratory trees through anus (turn inside out) Tubules have toxins and are sticky Can entangle and immobilize predators Evisceration of respiratory trees and/or gonads a result of chemical and physical stress Regeneration of lost parts Class Holothuroidea Reproduction Most are dioecious Have a single gonad External fertilization Tentacles may trap eggs and bring to body surface for brooding Embryos develop into planktonic larvae Can also reproduce by transverse fission followed by regeneration Class Crinoidea Only ~630 living species Includes Sea Lilies and Feather Stars Most primitive of all living echinoderms Extensive fossil records indicates high numbers during Paleozoic era (200 to 600 mya) Class Crinoidea Very different from other echinoderms Sea Lilies attach permanently to substrate by a stalk (sessile) Feather Stars swim by raising and lowering the arms and crawl by pulling with the tips of their arms. Class Crinoidea Feeding Suspension feeding: Outstretched arms to trap plankton Cilia carries to mouth Water-vascular system used mostly for feeding (not locomotion) Response Lack a nerve ring but do have a “nerve mass” and radial nerves that extend through each arm Controls tube feet Class Crinoidea Reproduction Most dioecious In monoecious species, male gametes develop first to allow for cross-fertilization Regeneration
