Sea cucumbers serve a useful role in the marine ecosystem as they help recycle nutrients, breaking down detritus and other organic matter, after which bacteria can continue the decomposition process.

[4] Like all echinoderms, sea cucumbers have an endoskeleton just below the skin, calcified structures that are usually reduced to isolated microscopic ossicles (or sclerietes) joined by connective tissue.

In pelagic species such as Pelagothuria natatrix (order Elasipodida, family Pelagothuriidae), the skeleton is absent and there is no calcareous ring.

Most sea cucumbers have a soft and cylindrical body, rounded off and occasionally fat in the extremities, and generally without solid appendages.

Their shape ranges from almost spherical for "sea apples" (genus Pseudocolochirus) to serpent-like for Apodida or the classic sausage-shape, while others resemble caterpillars.

[6] Holothurians measure generally between 10 and 30 centimetres (3.9 and 11.8 in) long, with extremes of some millimetres for Rhabdomolgus ruber and up to more than 3 metres (9.8 ft) for Synapta maculata.

The largest American species, Holothuria floridana, which abounds just below low-water mark on the Florida reefs, has a volume of well over 500 cubic centimeters (31 cu in),[7] and 25–30 cm (10–12 in) long.

At one of the extremities opens a rounded mouth, generally surrounded with a crown of tentacles which can be very complex in some species (they are in fact modified podia); the anus is postero-dorsal.

However, a central symmetry is still visible in some species through five 'radii', which extend from the mouth to the anus (just like for sea urchins), on which the tube feet are attached.

Sea cucumbers are typically 10 to 30 cm (4 to 12 in) in length, although the smallest known species are just 3 mm (0.12 in) long, and the largest can reach 3 meters (10 ft).

The ambulacral grooves bear four rows of tube feet but these are diminished in size or absent in some holothurians, especially on the dorsal surface.

These are called the primary tentacles and were present in the common ancestor of echinoderms, but have been lost in all the other classes of the phylum,[11] and may be simple, branched or arborescent.

[10] The body wall consists of an epidermis and a dermis and contains smaller calcareous ossicles, the types of which are characteristics which help to identify different species.

The animal is, however, quite capable of functioning and moving about if the nerve ring is surgically removed, demonstrating that it does not have a central role in nervous coordination.

[9] Most sea cucumbers have no distinct sensory organs, although there are various nerve endings scattered through the skin, giving the animal a sense of touch and a sensitivity to the presence of light.

In the larger species, additional vessels run above and below the intestine and are connected by over a hundred small muscular ampullae, acting as miniature hearts to pump blood around the haemal system.

[9] Indeed, the blood itself is essentially identical with the coelomic fluid that bathes the organs directly, and also fills the water vascular system.

Phagocytic coelomocytes, somewhat similar in function to the white blood cells of vertebrates, are formed within the haemal vessels, and travel throughout the body cavity as well as both circulatory systems.

[15] Like all echinoderms, sea cucumbers possess pentaradial symmetry, with their bodies divided into five nearly identical parts around a central axis.

[9] Many sea cucumbers have papillae, conical fleshy projections of the body wall with sensory tube feet at their apices.

The body of some deep water holothurians, such as Enypniastes eximia, Peniagone leander and Paelopatides confundens,[19] is made of a tough gelatinous tissue with unique properties that makes the animals able to control their own buoyancy, making it possible for them to either live on the ocean floor or to actively swim [20] or float over it in order to move to new locations,[21] in a manner similar to how the group Torquaratoridae floats through water.

The strawberry sea cucumber (Squamocnus brevidentis) of New Zealand lives on rocky walls around the southern coast of the South Island where populations sometimes reach densities of 1,000 animals/m2 (93 animals/sq ft).

Many polychaete worms (family Polynoidae[29]) and crabs (like Lissocarcinus orbicularis) have also specialized to use the mouth or the cloacal respiratory trees for protection by living inside the sea cucumber.

Some species of coral-reef sea cucumbers within the order Aspidochirotida can defend themselves by expelling their sticky cuvierian tubules (enlargements of the respiratory tree that float freely in the coelom) to entangle potential predators.

[39] Such tissues can rapidly change their passive mechanical properties from soft to stiff under the control of the nervous system and coordinated with muscle activity.

Taxonomic classification according to World Register of Marine Species: To supply the markets of Southern China, Makassar trepangers traded with the Indigenous Australians of Arnhem Land from at least the 18th century and probably earlier.

[45] Another study suggested that sea cucumbers contain all the fatty acids necessary to play a potentially active role in tissue repair.

[47] Surgical probes made of nanocomposite material based on the sea cucumber have been shown to reduce brain scarring.

[54] As of 2013, a thriving black market was driven by demand in China where 450 grams (1 lb) at its peak might have sold for the equivalent of US$300[50] and a single sea cucumber for about US$160.

[56] In India, the commercial harvest and transportation of sea cucumbers has been strictly banned under Schedule I of the Wild Life (Protection) Act, 1972 (WLPA) since 2001.