Phylum Echinodermata
echinoderms

Echinodermata has approximately 7000 described living species and about 13,000 extinct species known from the fossil record. This phylum is the largest without any freshwater or terrestrial forms. (Brusca and Brusca, 2003; Waggoner, 1999)

Mainly a marine group, echinoderms are found in all the oceans. (Brusca and Brusca, 2003)

Except for a few species which inhabit brackish waters, all echinoderms are benthic organisms found in marine environments. Echinoderms inhabit depths ranging from shallow waters at tide lines to the deep sea. (Barnes, 1987; Brusca and Brusca, 2003; University of Alabama Center for Communication and Educational Technology, 2000; Waggoner, 1999)

Echinoderm means "spiny skin.". Because of its abundant fossil record, up to 25 classes have been recognized. A traditional hypothesis of the Echinodermata using only extant taxonomic groups places the Asteroidea and Ophiuroidea as sister groups based on the shared derived characteristic of a five-rayed body plan. An alternative hypothesis including fossil groups places Asteroidea as a more primitive group with the derived characteristic of five arms connected to a central disc. Ophiuroidea is grouped with Echinoidea and Holothuroidea based on the closed ambicular grooves. In both hypotheses, Echinoidea and Holothuroidea are considered sister groups, having two synapomorphies: 1) an extended ambiculacral groove along the sides of the body from the oral to aboral pole and 2) the reduced aboral surface.

Echinoderms are considered to be derived from sessile ancestors. Crinoidea, considered the more primitive forms, are mainly sessile.

Asteroidea and Ophiuroidea are sometimes considered classes, instead of subclasses. (Barnes, 1987; Brusca and Brusca, 2003; University of Paisley, 1998; Waggoner, 1999)

Larvae range from a few millimeters to a few decimeters, while adults can range from less than 1 cm to 2 m. While adult forms are radially symmetrical, larval forms are always bilateral. The radial symmetry is secondarily derived. The pentaradial form, whether it has arms or not, has a central disc.

An internal skeleton is present throughout members of the phylum. Ossicles, which make up the skeleton, are below an outer dermal layer. The skeletal and muscular arrangement varies among groups.

Pedicellariae produced by the skeleton, are pincer-like structures. Found mainly in echinoids and asteroids, their function is debatable. They may be used to capture prey, clean, or hold items to disguise from predators.

Echinoderms have a water vascular system consisting of a network of radial canals, which extend through each of the five extensions (arms or rays) of the animal. Each canal has a lateral connection which leads to a tube foot, which may be composed of three parts. Internally is the ampulla and externally is the podia. At the end of the podia is usually a sucker.

Grooves with rows of podia extending from the mouth are called the ambiculacra . Between each ambiculacra is the interambulacrum. For groups of animals with "arms" (sea stars, for example), the interambulacrum is just the space between the ambiculacra. For other animals without furrows (sea cucumbers, for example), the areas are like the ambiculacra, but usually lack holes for the tube feet.

The water vasuclar system opening, called a madreporite, lies on a particular interambulacrum. Letters are used to describe parts of echinoderms. The ambulacrum opposite the madreporite is section A. Moving clockwise, other parts are coded B through E. Sections C and D are termed the bivium while all the others are collectively termed the trivium. Interambulacrum sections are named using the letters of the ambulacra sections they are between (e. g. AB). (Barnes, 1987; Brusca and Brusca, 2003; Waggoner, 1999)

Echinoderms are deuterostomes. The larvae, which are planktotrophic or lecithotrophic, have 3-part paired coeloms. Embryonic coelomic structures have specific fates as the bilaterally symmetrical larvae metamorphose into radially symmetric adults. Adult pheromones may attract larvae, which tend to settle near conspecific adults. Metamorphosis in some species is triggered by adult pheromones. (Barnes, 1987; Brusca and Brusca, 2003)

Most radially symmetric animals are sessile, however, echinoderms are able to move. The water vascular system originally functioned for collection and transport of food, but evolved to function for locomotion as well. (Barnes, 1987; Brusca and Brusca, 2003)

The non-centralized nervous system allows echinoderms to sense their environment from all sides. Adult pheromones may attract larvae, which tend to settle near conspecific adults. Metamorphosis in some species is triggered by adult pheromones. (Brusca and Brusca, 2003)

Different groups have different feeding habits. Members of the Crinoidea sit with arms outstretched parallel to the currents and filter feed on passing particles. Most Asteroidea are predators or scavengers, everting their stomach (called a cardiac stomach), which secretes digestive enzymes on their prey. Some asteroids are also suspension feeders. Brittle stars of the Ophiuroidea are predators, deposit feeders, scavengers, and suspension feeders, which feed by outstretching their arms to capture prey. Ophiuroids lack an intestine and anus, and therefore have an incomplete digestive system. The members of Echinoidea are suspension feeders, herbivores, detritivores, and predators. Many have a group of hard plates which retract and grasp like teeth, commonly called Aristotle's lantern. This allows most sea urchins to graze on algae. Most Holothuroidea are suspension or deposit feeders. Holothurians may also eviserate their digestive and other organs in response to predation or seasonal events. (Brusca and Brusca, 2003; Waggoner, 1999)

Echinoderms in general are most vulnerable in their larval stage. As adults, asteroids have an anti-predator adaptation where they can lose an arm to a predator and the arm is later regenerated. Holothurians discharge sticky tubules, known as Cuvierian tubules , at a potential predator. Otters prey mainly on sea urchins. (Brusca and Brusca, 2003)

Echinoderms are usually intricate parts of their ecosystems. Many asteroids are keystone species. Sea urchins, if not controlled by predators, may overgraze their habitat. Asteroids have several commensals, including polychaetes that feed on leftovers from the sea star's prey items. (Barnes, 1987; Brusca and Brusca, 2003)

Research on echinoderms has contributed to the overall knowledge of animal fertilization and development. Many echinoderms are easy to culture and maintain in a lab setting, and produce a large amount of eggs. Sea urchin eggs are also edible and often served in sushi bars. (Brusca and Brusca, 2003; University of Alabama Center for Communication and Educational Technology, 2000)

The European edible sea urchin, Echinus esculentus, is listed as endangered by the IUCN. Isostichopus fuscus, a holothurian, is listed by CITES. It occurs on the coasts of Ecuador, Galapagos, Mexico and Peru. (UNEP-WCMC, 2005; World Conservation Monitoring Centre, 2004)