Sea cucumbers are echinoderms, belonging to the class Holothuroidea of the phylum Echinodermata. About 1,000 species have been described, which vary in size from only 1.2 in (3 cm) to more than 3.3 ft (1 m) in length. Sea cucumbers occur in all of the oceans, being found in waters up to 655 ft (200 m) in depth, and perhaps deeper. In appearance, these animals range from an almost spherical to long and wormlike in shape. Most are colored black, brown, or olive-green, although tropical species may be reddish, orange, or violet.
Sea cucumbers are slow-moving, bottom-dwelling, marine invertebrates that are usually partially or completely immersed in the soft substrate. Some species have numerous small, foot like structures (pseudopods) that enable them to move slowly along the bottom, but the majority move by contracting the muscular wall of the body in a similar manner to that of earthworms. Their elongate form facilitates a burrowing lifestyle.
The body of sea cucumbers is a tube like arrangement. The outer body has a tough, leathery texture, although a few species have hardened calcareous patches for additional protection from predators. The head region is adorned with a cluster of tentacles (usually 10 to 30) surrounding a simple mouth. Sea cucumbers are deposit- or suspension-feeders, particularly on small invertebrates, algae, bacteria, and organic detritus. They feed by brushing their tentacles across the substrate to find food, or by extending the tentacles into the water column and trapping food directly. If they find food, the tentacles are bent inwards to reach the gullet, where the particles are removed for ingestion. At the same time, the tentacles are re-covered in a sticky mucus emitted from special glands that line the pharynx, preparing them once again for catching prey. Burrowing species ingest large amounts of sediment and absorb organic nutrition from that matrix.
Most sea cucumbers are either male or female, although a few species are hermaphroditic (i.e., each individual contains organs of both sexes). In most species the process of fertilization takes place outside of the body. The fertilized eggs develop into free-living larvae that are dispersed with water currents. A few species of cold-water sea cucumbers brood their larvae in special pouches on their body.
Being soft-bodied animals, sea cucumbers are prone to predation by a wide range of species, including crabs, lobsters, starfish, and fish. Remaining partly concealed in sediment provides the animal with some degree of security. In addition, when disturbed or threatened sea cucumbers are capable of emitting large quantities of sticky filaments from their anus, which may engulf the potential predator and incapacitate it long enough for the sea cucumber to escape, or on occasion, may even kill the predator.
In some parts of the world, particularly Southeast Asia and China, sea cucumbers are considered a delicacy and are widely harvested as food. Often preserved dried, this trepang or bêche de mer is an ingredient of some kinds of oriental cuisine. Recent increases in market demand have had a significant impact on local populations of sea cucumbers, and overharvesting has resulted in the virtual disappearance of these animals from some areas.
Kerr, A.M., and J. Kim. “Phylogeny of Holothuroidea (Echinodermata) Inferred from Morphology.” Zoological Journal of the Linnean Society 133 (2001): 63–81.