Phylum Echinodermata

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 water-vascular 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 star can regenerate from a piece of broken arm • Broken part must contain portion of central disk • Complete regeneration may take up to a year Click here for regeneration video

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 larvae bilaterally symmetric

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 Maintenance Capable of regeneration (autonomy for defense) Dioecious, males are smaller and often carried by females • Predators and scavengers • Use arms and tube feet to sweep in food and trap plankton

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 Reproduction & Development Dioecious Gametes shed into water External fertilization Larva undergoes metamorphosis • Feed on algae, coral polyps and dead animal remains • Specialized chewing apparatus: “Aristotle’s Lantern”

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 Response Lack a nerve ring but do have a “nerve mass” and radial nerves that extend through each arm Controls tube feet • Suspension feeding: Outstretched arms to trap plankton • Cilia carries to mouth • Water-vascular system used mostly for feeding (not locomotion)

Class Crinoidea Reproduction • Most dioecious • In monoecious species, male gametes develop first to allow for cross-fertilization • Regeneration

Phylum Echinodermata