A sea cucumber is an echinoderm in the class Holothuroidea which can be found in all of the oceans of the world at depths ranging from the inter-tidal zone to the bottom of oceanic trenches. There are over 1,000 species of Holothurians, although all of them have the long cucumber-shaped bodies which give the animals their common name. Like other echinoderms, sea cucumbers display five sided radial symmetry along the length of their bodies. The rather bizarre ocean creatures are also prized in some Asian culinary traditions.

From a distance, one could be forgiven for mistaking a sea cucumber for an actual cucumber. Most of the time, the animals rest on the ocean floor, filtering food from the water which flows around them. Their dark, knobbly bodies do strongly resemble cucumbers, although some exotic species give away their animal nature with warts in colors like orange and blue. Sea cucumbers eat plankton and other organic material, either through filter feeding or by sifting through the sediment on the ocean floor with the flexible tentacles which surround their mouths.

Like other ocean-going animals, the sea cucumber filters out oxygen from seawater to respire. In the case of the sea cucumber, an apparatus to extract oxygen called a breathing tree starts at the base of the anus and runs along the animal's body. The sea cucumber intakes small amounts of water through the anus and cloaca, and then expels the waste back through the cloaca. The simple breathing and digestive system of the sea cucumber suggests that the animals have been around for millions of years, and there are fossilized examples to support this hypothesis from the Silurian period, 400 million years ago.

The sea cucumber does have a trick up its sleeve when self defense is needed. Under moderate amounts of stress, a sea cucumber will squirt water from both ends. Under extreme stress, however, the sea cucumber will regurgitate its stomach and escape in the subsequent chaos. The stomach will regrow within a relatively brief period of time. Other echinoderms are also able to force their stomachs inside out, usually to feed, but the stomach usually retracts back into the body.

In Asian cuisine, sea cucumber appears both fresh and dried, and is commonly used in soups and stews. The flavor is bland, and the texture is rubbery and slightly gelatinous. Sea cucumber is also used in some traditional medicines, and Western pharmaceutical companies have begun to study the sea cucumber to see if it contains useful properties. Some species appear to have anticoagulant, anti-inflammatory, and antimicrobial compounds which may be usable in medical treatment.