Sea cucumbers are echinoderms from the class Holothuroidea. They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothurian /ˌhɒlɵˈθjʊəriən/ species worldwide is about 1250 with the greatest number being in the Asia Pacific region. Many of these are gathered for human consumption and some species are cultivated in aquaculture systems. The harvested product is variously referred to as trepang, bêche-de-mer or balate. Sea cucumbers serve a useful purpose in the marine ecosystem as they help recycle nutrients, breaking down detritus and other organic matter after which bacteria can continue the degradation process.
Echinoderms (Phylum Echinodermata) are a phylum of marine animals. The adults are recognizable by their (usually five-point) radial symmetry, and include such well-known animals as starfish, sea urchins, sand dollars, and sea cucumbers. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as birds, fishs, mammals, and reptiles). Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives.
Aside from the hard-to-classify Arkarua (a Precambrian animal with Echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian period. The word "echinoderm" is made up from Greek ἐχινόδερμα (echinóderma), "spiny skin", cf. ἐχῖνος (echínos), "hedgehog; sea-urchin" and δέρμα (dérma), "skin", echinodérmata being the Greek plural form.