Echinothuriidae

The order to which they belong, Echinothurioida, is defined by compound plates which cover the ambulacra and the peristome.^[2] The test is composed of thin and weakly calcified plates, not always continuous. The spines are attached to perforated and uncrenulated tubercles. Spines from the oral face end with a hyaline hoof, which allows walking on soft substrate. The jaw (Aristotle's lantern) has five aulodont teeth.^[3]

These sea urchins have a disc-like body, more or less bulging, structured by a flexible test, which is nearly unique among sea urchins. Most species can grow quite big and live in deep seas, though some genera contain shallow species (especially Asthenosoma).^[4]

This family, Echinothuriidae seems to have appeared at the end of the Cretaceous.^[3]

They are most often found in the Indo-Pacific, although they have been observed worldwide, and are among the most common urchins inhabiting deep marine environments.^[1] While they are most common at great depths (more than 200m below the surface) the genus Asthenosoma has been known to inhabit shallow zones.^[1]

Members of the Echinothuriidae have been observed to attached themselves to living and dead stony corals (Order: Scleractinia) allowing them a degree of protection from crustaceans and fish that feed on urchins. When the threat of predators is low they can be found assembling closely together which likely provides advantages for reproduction and feeding.^[5] The levels at which they seek refuge in the coral will also change based on predator presence as the tend to shift higher on the structures in the presence of decapod or crustacean predators, while an increase in fish will lead to an increase in urchins closer to the sediment.^[5]

Like other echinoids, members of the Echinothuriidae, namely those belonging to the genus Araeosoma, have been known to prey on or consume the remains of crinoids. Although the extent to which they are capable of successfully hunting the crinoids is unknown, experiments done in captivity present this predator-prey interaction as a definite possibility in nature.^[6] Gut contents have been found to also contain pieces of corals, crustacean exoskeleton, bryozoans, polychaetes, foraminifera, sponges, and a variety of other invertebrate elements. The high presence of sediment in the gut along with these elements indicates a scavenger like feeding strategy as they pick up deceased material from the sediment.^[6]