Members of the Phylum Echinodermata are often highly mobile components of the deep-sea. They include the Ophiuroidea (brittle stars and basket stars), Asteroidea (sea stars), Echinoidea (sea urchins) and Holothuroidea (sea cucumbers), but also the sessile stalked Crinoidea (crinoids). All echinoderms share a unique adult radial symmetry, with body structures often repeated in multiples of five, a common example being the arms of a star fish.
Echinodermata have a tough external leathery skin which encloses an internal skeleton comprised of interlocking calcium carbonate plates called ossicles. These calcareous plates can become strongly fused, such as in sea urchins to form an enclosed ball with openings for the mouth and other structures. Many species of Echinoidea are covered in spines and the outer skin is covered with pinching pedicellariae to ward off predators. The ossicles may also remain unfused in Echinodermata allowing the animal to be very flexible such as in sea stars and brittle stars.
Feeding behaviour in the Echinodermata is diverse, but many deep-sea species are omnivorous, feeding on a wide variety of different foods. In the Rockall Trough Ophiuroidea make up 27% of the echinoderm species collected, but numerically they far outnumber the remaining megafauna collected (63%). On cold-water coral reefs, Asteroidea have been observed predating upon coral polyps, and the reef habitat may also provide high quality habitat for sessile echinoderms such as crinoids.