Sea cucumbers are echinoderms from the class Holothuroidea (/ˌhɒləˌθjʊəˈrɔɪdiə, ˌhoʊlə-/ HOL-ə-thyuu-ROY-dee-ə, HOH-lə-). They are benthic marine animals found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786, with the greatest number being in the Asia–Pacific region. Sea cucumbers serve a useful role in the marine ecosystem as detritivores who help recycle nutrients, breaking down detritus and other organic matter, after which microbes can continue the decomposition process.
Sea cucumbers have a leathery skin and an elongated body containing a single, branched gonad; they are named for their overall resemblance to the fruit of the cucumber plant. Like all echinoderms, sea cucumbers have a calcified dermal endoskeleton, which is usually reduced to isolated microscopic ossicles (or sclerites) joined by connective tissue. In some species these can sometimes be enlarged to flattened plates, forming an armoured cuticle. In some abyssal or pelagic species such as Pelagothuria natatrix (order Elasipodida, family Pelagothuriidae), the skeleton is absent and there is no calcareous ring.