Echinoderms CHAPTER 14

Phylum Echinodermata: Diversity and Characteristics All members have a calcareous skeleton Spiny endoskeleton consists of plates Unique water-vascular system radial symmetry in adults

Phylum Echinodermata: Diversity and Characteristics Likely descended from bilateral ancestors Larvae are bilateral Perhaps evolved radiality as an adaptation to sessile existence Body plan is derived from crinoid-like ancestors transformed into free-moving descendants Lack ability to osmoregulate Restricts them to marine environments

Phylum Echinodermata: Diversity and Characteristics None are parasitic Asteroids or sea stars Mostly predators Ophiuroids or brittle stars Move by bending their jointed muscular arms May be scavengers, browsers, or commensal Holothurians or sea cucumbers Mostly suspension or deposit feeders

Phylum Echinodermata: Diversity and Characteristics Echinoids or sea urchins Found on hard bottoms while sand dollars prefer sand substrate Feed on detritus Crinoids Sessile and flower-like as young and detach as adults Suspension feeders

Phylum Echinodermata: Diversity and Characteristics Ecology, Economics, and Research Due to spiny structure, echinoderms are not often preyed upon A few fish and otters are adapted to feed on sea urchins Sea stars feed on molluscs, crustaceans, and other invertebrates May damage oyster beds

Class Asteroidea Form and Function External Features Have a central disc with tapering arms extending outward Body is flattened and flexible, with a pigmented and ciliated epidermis Mouth is on the underside or oral surface Usually there are 5 arms but there may be more rows of tube feet

Class Asteroidea Water-Vascular System This system is another coelomic compartment and is unique to echinoderms Functions in locomotion, food-gathering, respiration, and excretion Opens to outside at madreporite on aboral side Madreporite leads to stone canal, which joins ring (circular) canal that encircles the mouth Radial canals diverge from ring canal and extend into each ray

Class Asteroidea Feeding and Digestive System Mouth leads through a short esophagus to large central stomach Lower part of stomach can be everted through the mouth during feeding Anus is inconspicuous Consume a wide range of food Sea urchins Molluscs Sea stars pull valves apart and evert stomach through crack Small particles carried by tube feet to mouth

Class Asteroidea Reproductive System, Regeneration, and Autonomy Sexes separate in most sexes Pair of gonads Fertilization is external Eggs and sperm are shed into the water in early summer Regenerate lost parts Cast off injured arms and regenerate new ones An arm can regenerate a new sea star if at least one-fifth of central disc is present

Class Asteroidea Sea Daisies Diversity and Characteristics Small, disc-shaped animals discovered in deep water off New Zealand Described in 1986, only two species are known

Class Ophiuroidea Form and Function Largest in number of species Over 2000 extant species Arms of brittle stars are slender and distinct from the central disc Tube feet lack suckers Biology Brittle stars are secretive and live on hard or sandy bottoms where little light penetrates Often under rocks or in kelp holdfasts Browse on food or suspension feed Basket stars perch on corals and extend branched arms to capture plankton

Class Echinoidea Diversity Reproduction Approximately 950 species of living echinoids Sea urchins lack arms but their tests show five-part symmetry Most sea urchins have radial symmetry and long spines Sand dollars and heart urchins (irregular echinoids) have become bilateral with short spines Echinoids occur from intertidal regions to deep ocean Reproduction Sexes separate Gametes are shed into sea for external fertilization

Class Holothuroidea Diversity Approximately 1150 species of holothuroids As common name suggests, these animals resemble cucumbers Greatly elongated in the oral-aboral axis Ossicles are greatly reduced and body is soft Some species crawl on the ocean bottom, others are found under rocks or burrow

Class Holothuroidea Reproduction Sexes are separate in most Fertilization is external A few brood their young inside the body or on the body surface Biology Sea cucumbers use ventral tube feet and muscular body waves to move Some trap particles on the mucus of tentacles, ingesting food particles in pharynx Others graze sea bottom with tentacles Cast out part of viscera when irritated Must regenerate these tissues Organs of Cuvier are expelled in direction of an enemy Sticky and have toxins One small fish, Carapus, uses the cloaca and respiratory tree of a sea cucumber for shelter

Class Crinoidea Diversity Crinoids include both sea lilies and feather stars Far more numerous in fossil record Unique in being attached for most of their life Sea lilies have a flower-shaped body at tip of a stalk Feather stars have long, many-branched arms Adults are free-moving but may be sessile Many crinoids are deep-water species Feather stars are found in more shallow water