On the abyssal plain, the exclusively marine echinoderms dominate the mobile megafauna with sea stars, brittle stars, sea urchins and sea cucumbers (Smith et al. 2009). Sea lilies are sessile and contribute to the conscious sessile megafauna of the deep-sea floor (Herring, 2002).
All echinoderms of the deep posses a calcareous plate skeleton (ossicles) which can be strongly jointed (sea urchin tests), flexibly connected by tissue (sea stars) or very loosely connected (soft bodied sea cucumbers) (Gibson et al. 2001).
Consistent will shallow water counterparts, deep sea echinoderms display radial symmetry based on five rays (pentasymmetry) but are generally smaller in size due to exponential decline in food resources with increasing depth (Capezzuto et al. 2010).
Asteroidea (sea stars and starfish) have a distinct star shape consisting of broad, hollow arms radiating from a central disc on which the mouth is positioned on the underside (Figure 2; Hayward and Ryland, 1995). Ophiuroidea (brittle stars and basket stars) can have thin, flexible, single or branched arms and a shape similar to that of Asteroidea. Brittle and basket stars have an orally positioned mouth surrounded by five jaws (Ruppert et al. 2004).
Echinoids comprise of regular forms (sea urchins) and irregular forms (heart urchins). Sea urchins have a spherical shape bearing long spines whereas heart urchins have a secondary bilateral symmetry creating an oval shape bearing shorter spines (Ruppert et al. 2004). Out of the five orders of Echinoids, Cidaroidea (a regular Sea urchin) and Cassiduliodea (an irregular Sea urchin) are exclusively deep sea orders (Gibson et al. 2001).
Holothurians (sea cucumbers) have an elongate soft body with an anterior mouth surrounded by feeding tentacles (Figure 3).
Sea cucumbers have a long, looped gut to aid digestion of nutrient poor sediment which ends in an opening surrounded by the respiratory tree (Gibson et al. 2001).Crinoids have a pentamerous crown with an aboral mouth and are either free-living (feather stars) or attached by a stalk (sea lilies) (Hayward and Ryland, 1995).