Echinoderms: Characteristics & Classification

Echinoderms Lecture Notes

• Echinoderms and chordates are deuterostomes.
• Echinoderms (phylum Echinodermata) include sea stars, sea urchins, and more.
• Chordates (phylum Chordata) include vertebrates.
• Echinoderms and chordates constitute the clade Deuterostomia.
• Deuterostomes share developmental characteristics like radial cleavage and anus formation from the blastopore.
• Deuterostomes are defined primarily by DNA similarities.
• Adult echinoderms have basic pentaradial symmetry, while larvae show bilateral symmetry.
• Echinoderms have a unique water-vascular system with tube feet for locomotion and feeding.
• Echinoderms are triploblastic.
• Echinoderms all live in marine environments and cannot osmoregulate
• Echinoderms are benthic creatures in all oceans.
• Echinoderms have a calcareous endoskeleton and possess pedicellariae and dermal branchiae (papulae).
• Most have separate sexes, and sexual reproduction is external.
• Asexual reproduction can occur through fragmentation in some species.
• Autotomy and regeneration are present in most echinoderms.
• Excretory organs are absent in echinoderms.
• Most have a complete digestive system.
• Echinoderms lack a brain but have nerves present.
• Sensory organs like photoreceptors and statocysts are present.
• Gas exchange occurs through papulae, tube feet, and other structures.
• They have a reduced blood vascular system.
• Larvae are bilaterally symmetrical and freely swimming.
• The larvae undergo metamorphosis to form young organisms.
• Each class has different larvae

Classes of Echinoderms

• Living echinoderms are divided into five clades/classes: Asteroidea, Ophiuroidea, Echinoidea, Crinoidea, and Holothuroidea.
• Asteroidea includes sea stars and sea daisies.
• Ophiuroidea includes brittle stars.
• Echinoidea includes sea urchins and sand dollars.
• Crinoidea includes sea lilies and feather stars.
• Holothuroidea includes sea cucumbers.

Asteroidea: Sea Stars

• Sea stars have multiple arms radiating from a central disk.
• The undersurface of each arm has tube feet, which grip the substrate with adhesive chemicals.
• The oral side contains the mouth, while the opposite side is the aboral side.
• An ambulacral groove runs from the mouth to the tip of each arm, contains rows of podia bordered by movable spines, and radiates out from the centrally located mouth.
• Ossicles or other dermal tissue cover the sea star's surface.
• The aboral surface is spiny with pincer-like pedicellariae at the base of the spines.
• They can feed on bivalves by prying them open and digesting their prey externally with digestive enzymes and can regrow lost arms.
• Usually have five arms, but there may be more.
• Arms merge gradually with the central disc
• A two-part stomach connects the mouth
• The lower cardiac part of the stomach can be everted through the mouth.
• The upper pyloric stomach is smaller and connected by ducts to a pair of pyloric ceca in each arm.
• A short intestine joins the stomach to the anus.
• Some lack an intestine and anus along with the anus being inconspicuous on the aboral side.
• They wrap themselves around mollusks applying constant pressure to open them
• Sea stars typically takes 30 minutes for adductor muscles of the bivalve prey to fatigue and relax
• Sea stars eat a wide range of prey like reguritating undigestible parts and sea urchins
• Some consume small particles that are carried up ambulacral grooves to the mouth.
• Pedicellariae keep the body surface of the sea star free of debris.
• Papulae (dermal branchiae or skin gills) are soft projections lined with peritoneum and serve in respiration.
• The water vascular system is a network of hydraulic canals branching into tube feet that function in locomotion and feeding.
• The aboral side features a circular madreporite, a sieve leading to the water-vascular system.
• This system is another coelomic compartment and is unique to echinoderms
• Polian vesicles may be attached; they serve for fluid storage.
• Small lateral canals, each with a one-way valve, connect the radial canal to the tube feet
• The inner end of each tube foot or podium is an ampulla that lies within the body coelom
• The outer end of each tube foot bears a sucker.
• The water-vascular system operates hydraulically; valves in lateral canals prevent backflow.
• Muscles contract and release to make to water vascular system work.
• Longitudinal muscles in the tube foot retract, forcing fluid back into the ampulla and enabling suction
• Mostly have separate sexes and external fertilization
• Echinoderms can cast off limbs or regenerate new ones
• An arm can regenerate a new sea star if at least one-fifth of the central disc is present.
• Common species of sea stars are pin cushion stars, crown-of-thorn star, necklace star, and granulated sea star

Asteroidea: Sea Daisies

• Sea daisies are a group of armless species, with only three known species.
• Sea daisies live on submerged wood and absorb nutrients through a membrane surrounding their body.

Ophiuroidea: Brittle Stars

• Brittle stars have a distinct central disk and long, flexible arms, which they use for movement.
• Some species are suspension feeders, while others are predators or scavengers.
• They are slender and distinct from the central disc and have ossicles
• Tube feet lack suckers and the madreporite is on the oral surface
• Five movable plates act as jaws and surround the mouth; there is no anus.
• Visceral organs are all in the central disc; the arms cannot accommodate them.
• Possess bursae which open to the oral surface to expel ripe sex cells

Echinoidea: Sea Urchins and Sand Dollars

• Sea urchins and sand dollars have no arms but have five rows of tube feet.
• Used spines for locomotion and protection
• They feed using a jaw-like structure called Aristotle's lantern on their underside.
• Lacking arms, their endoskeletal tests show the five-part symmetry.
• Dermal ossicles make up the test.
• The echinoid test has ten double rows of plates with movable, stiff spines;
• Tube feet extend along the five ambulacral rows
• Spines are moved by small muscles at the bases.
• Most have a hemispherical shape with radial symmetry and long spines.
• Sand dollars and heart urchins (irregular echinoids) have become secondarily bilateral with short spines.
• Regular urchins move by tube feet; irregular urchins move by their spines.

Crinoidea: Sea Lilies and Feather Stars

• Sea lilies live attached to the substrate by a stalk, while feather stars can crawl.
• Both are suspension feeders
• Have changed little over the course of evolution; they have primitive characteristics.
• They are unique in being deep water forms and attached to a substrate for most of their lives.
• Sea lilies have a flower-shaped body at the tip of a stalk
• Feather stars have long, many-branched arms
• Feather stars can be free-moving but may be sessile
• The body is covered with a leathery skin with calcareous plates.
• Have branched arms forming more arms, each with lateral pinnules
• Cirri that allows the organism to attach itself to various structures

Holothuroidea: Sea Cucumbers

• As their name suggests, these odd animals resemble cucumbers much more than echinoderms
• In the oral-aboral axis there is a great elongation and do not show cephalization
• Have a very reduced endoskeleton without spines
• Usually have five rows of tube feet; some develop as feeding tentacles
• Sluggish and utilize both ventral tube feet as well as muscular body waves to move.

Ecological Relationships

• Can be particulate feeders, predators, scavengers, browsers, deposit feeders, or filter feeders
• Active echinoderms like brittle stars can move by their arms
• Can be commensal in large sponges alongside sea stars who are generally particulate feeders; though can also be a predator in their own rate
• Many brittle stars scavenge, graze, browse food in the detritus
• There are few which can be commensal in the deep blue sea
• Sea cucumber: known filter feeders who are mostly sediment eaters and algae
• Sand dollars/ detrius consumers who spend their time among the sands and in the soil
• Filter feeders (sea lillies, feather stars) spend their days suspended up high and among the particles that drift down

Etymology

• Echinoderm: Gk hedgehog + skin
• Asteroidea: Gk star-like
• Holothuroidea: Gk sea-cucumber like
• Crinoidea: Gk lily-like
• Ambulacral: Lat avenue
• Autotomy: Gk self + cut
• Podia: Gk foot
• Ampullae: Lat bulb
• Pedicellaria: Lat little foot
• Test: Lat pot