sea cucumber, any of the flexible, elongated echinoderms belonging to the class Holothuroidea. Although sea cucumbers have the basic echinoderm radial symmetry (see Echinodermata), they do not have arms like starfish. Instead the oral-anal distance is greatly increased, resulting in the typical cucumber-shaped body.
Sea cucumbers live with one side facing permanently down. Like other echinoderms, sea cucumbers have a water-vascular system; the locomotor tube feet are concentrated in three areas on the ventral, or under, side, in some species forming a muscular, creeping sole. Some species burrow in sand or mud and have lost all tube feet. The leathery body wall contains minute, scattered skeletal ossicles, or bonelike plates; a few species have an armor of close-set plates.
Some species eat bottom material, while others use tube feet modified as branched oral tentacles to capture particles or plankton and transfer them to the mouth. Most sea cucumbers have highly branched tubes called respiratory trees attached to the intestine near the anus. Water is pumped in and out, facilitating respiratory exchange and excretion. In some species, branches called tubules of Cuvier, attached to or near the bases of the respiratory trees, are ejected when the organism is attacked; they swell and become sticky, entangling the pursuer. Many sea cucumbers eject most of the internal organs when sufficiently irritated, later regenerating a new set.
Sea cucumbers have a single, branched gonad. Eggs are usually expelled into the sea where, after fertilization, free-swimming larvae develop. After a second larval stage, metamorphosis occurs and the adult body shape appears.
Sea cucumbers occur in all seas and at all depths. Most do not exceed 1 ft (30.5 cm) in length, but Stichopus variegatus from the Philippines may reach 3 ft (91 cm) in length. Known as trepang or bêche-de-mer, a number of species are caught along warm coasts of Australia, the East Indies, and some Pacific island nations. They are dried and sold, mainly to markets in E Asia, for use as food or in traditional medicine. Sea cucumbers are classified in the phylum Echinodermata, class Holothuroidea.