Sea cucumber, (class Holothuroidea), any of 1,200 species of marine invertebrates that constitute a class within the phylum Echinodermata. The soft cylindrical body, 2 to 200 cm (about 0.75 inch to 6.5 feet) long and 1 to 20 cm (0.4–8 inches) thick, is usually a dull, dark colour and often warty, thus resembling a cucumber. The internal skeleton is reduced to numerous distinctively shaped, tiny calcareous structures (ossicles) in the skin. Most species have five rows of tube feet extending from mouth to anus. The anal opening is used for both respiration and discharging wastes. The 10 or more retractile tentacles surrounding the mouth are used for taking food (mud containing nutrients or small aquatic animals) or burrowing. Locomotion is usually sluglike, although some deepwater species can swim.

Sea cucumber (Cucumaria frondosa) Grant Heilman/Encyclopædia Britannica, Inc.

Many sea cucumbers can expel their internal organs through the anus and grow new ones. This may be a device for escape from a predator, or it may occur for physiological reasons. Some species also expel specialized sticky filaments that ensnare or confuse an enemy. Sea cucumbers shelter pearlfish (Carapus species) in the anal cavity, with the head of the fish extruding. A number of sea cucumber species exude a toxin that is lethal to small animals but not to humans; South Sea islanders place sea cucumber juices in water to kill or stupefy fish.

Sea cucumber (Kolga hyalina) from the deep seafloor of the Arctic Ocean's Canadian Basin. Bodil Bluhm—NOAA/Census of Marine Life

Sea cucumbers are found in all oceans, mostly in shallow water but sometimes at depths of many thousands of metres. They are best represented in the Indian Ocean and the western Pacific. The 80 to 100 species of large, warty sea cucumbers of the genus Holothuria are especially abundant on coral reefs. Most species of Holothuria are deposit feeders similar to earthworms: they ingest sediment to extract the organic constituents.

Sea cucumbers are harvested to produce numerous products, including medicines and dietary supplements, shampoo, and toothpaste. However, bêche-de-mer, the dried outer body wall, is considered to be the most valuable part of the animal. It is a delicacy throughout Asia, especially in China, and the voracious demand for bêche-de-mer has reduced the stocks of many sea cucumber species across the world.

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The populations of numerous high-value species, including H. fuscogilva, H. whitmaei, H. scabra, and Isostichopus fuscus, have been decimated by overfishing that has occurred since the 1980s. Harvesting operations subsequently targeted dozens of lower-value species, such as Cucumaria frondosa, an abundant species from temperate and polar waters. Population studies have been performed on relatively few commercial species, and, thus, little is known about the conservation status of these forms. Although attempts to raise some species, such as I. fuscus and H. scabra, in aquaculture ponds have been moderately successful, many ecologists worry that several species harvested for bêche-de-mer have become extinct in the rush to supply sea cucumbers to commercial markets.