7. echinoderms and ascidians

Phylum Echinodermata

• “spiny skin”• 7000 living species• All are marine and

none are parasitic• Morphologically very

diverse• 5 rayed symmetry,

mostly radial• Pelago-benthic

lifecycle


• Ossicles - Calcite endoskeleton that grow with the animal

• Three forms:– Small articulating plates

e.g. Sea stars– Large closely joined plates

– test e.g. sea urchins– Widely separated

microscopic plates e.g. sea cucumbers


• Water Vascular System• Functions in:

– Locomotion– Respiration– Feeding– Sensory

• a network of radial canals, which extend through each of the five extensions of the animal. Each canal has a lateral connection which leads to a tube foot. Internally is the ampulla and externally is the podia

Phylum EchinodermataWater Vascular System


• Mutable collagenous tissue• ossicles connected by

collagenous ligaments• Ligaments are normally

"locked" (rigid), but can be temporarily "unlocked" (loosened)

• mechanical advantage– maintain a variety of postures

with no muscular effort• Under neuronal control


Other Characteristics• Pentaradial body organization

in adults but all larval forms are biradial

• Deuterostome embryology (same as humans)

• No excretory organs• No brain

– Decentralized nervous system consisting of a central nerve ring surrounding the gut, composed primarily of fiber tracks connecting the radial nerves


Stelleroidea Echinoidea Holothuroidea

Asteroidea Ophiuroidea Sea Stars Brittle Stars Sea Urchins Sea Cucumbers

Phylum EchinodermataStelleroidea – Ophiuroidea

Brittle Stars e.g. Ophiopholis sp• ~ 2000 living species• 5 very flexible arms and

central body disk• Arms are for movement, tube

feet (without suckers) are for food gathering

• Filter feeders, carnivores, scavengers, deposit feeders

• Arms break off easily (for defense?) - regenerative


• The disk contains all of the internal organs for digestion (mouth and stomach no intestine or anus) and reproduction both tissues never enter the arms, as they do in the Sea stars.


• The underside of the disc contains the mouth. The madreporite is usually located within one of the jaw plates, and not on the upper side of the animal as it is in sea stars


• Nervous system consists of a nerve ring in the disc that sends out a radial nerve to each arm.

• Lack eyes but the epidermis is sensitive to light and other stimuli.


• Gonochoristic• Indirect development with a

planktotrophic, ophiopluteus larva• A few brood embryos by retaining

eggs in the bursa. Sperm enter the bursa and fertilization occurs. Embryos undergo direct development in the bursae and become tiny brittle stars before they are released.

Phylum EchinodermataStelleroidea – Asteroidea

Sea Stars e.g. Henricia sp.• ~ 1500 living species• Range from 2 cm to 1 m

in size• Can live up to 35 years• Most have five arms (but

as many as 40) with a central disc.

• Scavengers and carnivores


• Movement via tube feet (~2000 usually).

• Internal muscle contractions squeeze fluid to the tube feet, which then elongate

• The end of the tube feet have suckers, which chemically adhere to or release from the substrate.


• Aboral surface covered by a thin epidermis covering short spines, pedicellaria and papulae.

• Papulae and Tube feet have respiratory and excretory functions.


• e.g. Pycnopodia sp.• The non-centralized

nervous system allows echinoderms to sense their environment from all sides. Sensory cells on the epidermis sense light, contact, chemicals and water currents.


• Several long, narrow sensory tube feet extend from the tip of each arm have chemo- and mechanoreceptors.

• At the tip of the arm is a small circle of short, blunt movable spines that surround a small, pale red or yellow eyespot


• They feed by grasping the prey, then everting their stomach and secreting primary enzymes on the prey. The digestive juices break down the tissue of the prey, which the asteroids then suck up.


• The hemal system– the least understood of

echinoderm organ systems.

• It is a blood vascular system as a space in the connective tissue. But: – blood vessels end blindly– there is no continuous

circulation of blood.


e.g. Pisaster sp• Reproduction• Gonochoristic• Fertilization is external.

– Each individual has a pair of gonads in each arm

– Every gonad connects to its own gonopore (aboral, base of arm) via an inconspicuous gonoduct


• two larval forms:– Bipinnaria– Brachiolaria

• The larvae are planktotrophic, ciliated, bilaterally symmetrical and feed on diatoms.


• Adult pheromones– may attract larvae,

which tend to settle near conspecific adults.

– May trigger Metamorphosis.

• A few hermaphroditic species brood their eggs.

e.g. Leptasterias sp.

Phylum EchinodermataEchinoidea

Sea Urchins, Sand Dollars• ~ 950 living species• Fused ossicles forming

thin plates• Pentamerously radially

symmetrical• Movable spines vary in

size and shape. • Tube feet are the major

respiratory organs.


• Classified as Regulare.g. Strongylocentrotus– radial symmetry,– nearly spherical

bodies, – long spines– tube feet are longer

than the spines– Mouth oral, anus

aboral– Most are epifaunal.


• Classified as Irregular– e.g. sand dollars,

Dendraster sp.– usually infaunal in soft

sediments – body is usually

flattened– spines short– Mouth is oral, anus at

the edge of the test– Reduced aristotle’s

lantern


• Aristotle's lantern, equipped with five strong teeth, used for scraping food from hard substrates.

• 40 skeletal ossicles and 60 muscles that support and operate the teeth.


• The tube feet are used for locomotion and respiration, and some use them to hold bits of shell or vegetation above the body, presumably for camouflage or protection from UV radiation in shallow water


• Pedicellariae– have three tiny jaws at

the end of a pedicle– have a skeleton

consisting of three ossicles in the jaws and a long slender ossicle in the pedicle


Reproduction• Gonochoristic• Gametogenesis

– regulated by photoperiod so that spawning of most or all members of a population occurs during the same time.


• Reproduction• Some brood their

young externally, within the protection of their spines or tube feet.

• In species with indirect development, an echinopluteus larva is produced.

Phylum EchinodermataHolothuroidea

• Sea Cucumber• 1100 living species• 1 cm – 1 m long• Live for 5 to 10 years• Closely related to

echinoidea except– have tiny ossicles,– they are elongated– lye on the side of the body– madreporite is internal


e.g. Cucumaria sp.• Cucumbers are

radially symmetrical but also possess a superficial bilateral symmetry in which the oral end is also the anterior end. The aboral end is also the posterior end


• The anterior (oral) end bears mouth and a circle of ten branched tentacles, the buccal podia. It is an introvert that can be fully retracted into the body by a set of powerful retractor muscles.


• Chemosensory and mechanosensory receptors are present in the epidermis and they exhibit a general sensitivity to light but have no special photoreceptive organs.

e.g. Eupentacta sp.


• Podia, which are the external portion of the tube feet, may, be suckered, reduced, or lost. Podia are more randomly scattered along the body than in other echinoderms

e.g. Psolus sp.


• Respiratory trees, which branch out near the rectum of the animal are used for gas exchange as water is pumped through the anus. The respiratory trees are part of the organs that are expelled occasionally by the sea cucumber


• Reproduction– Gonochoristic– Broadcast spawners– The larvae are

planktotrophic

• Two larval forms– Auricularia – Doliolaria

• Adult pheromones may attract larvae to settle near conspecific adults.


• ~ 30 brooding species. Some capture eggs with tentacles, placing the eggs at the sole or dorsal body surface for incubation. A few have internal fertilization and development, where hatched young are released


• Chemical stimulation– changes the

mechanical properties of the dermal portion of the sea cucumbers

allows the animal to– become flexible and

squeeze through narrow passages

– become so rigid that it cannot be dislodged.

e.g. Parastichopus sp


Eviscerate• expell their organs,

which are later regenerated. This is a seasonal event, but is also thought to be an anti-predator defense.


All are filter feeders or deposit feeders moping up sediments from the ocean floor.

7. echinoderms and ascidians

Documents

narrow sensory tube

sea urchinswidely

sea starslarge

tube foot

blood vascular system

central disc

base of arm

small articulating plates