Sea cucumbers are echinoderms, sea-dwelling animals related to sea urchins, starfish and brittle stars. Approximately 1250 species exist around the world, living everywhere from the ocean depths to very shallow water. Their biological classification is as follows: Kingdom: Animalia (animals); Phylum: Echinodermata (echinoderms); Subphylum: Asterozoa (sea cucumbers, sea urchins, sand stars, brittle stars, basket stars, sea stars (or starfish)), Class: Holothuroidea (sea cucumbers). Most belong to the genus Holothuria, although other genera include Pentacta, Stichopus and Parastichopus.
In appearance, they are sausage-shaped, like a vegetable cucumber, but covered in bumps, spines and bristles, more like a gherkin. Their colour ranges from green to red-brown to black, depending on the species, and they tend to be soft-fleshed, although some have a leathery skin. The size varies considerably depending on the species. Their bottom surface is covered in tube feet, which they use to cling on to rocks, rather than to walk on. Some live buried in sand, with just their tentacles poking out.
Around their mouths are modified tube feet, forming tentacles used to filter the water and pass interesting morsels into their mouths. Their diet is made up of decaying organic matter, plankton and algae. They have an important ecological role cleaning up small particles of dead and decayed matter in the water, and in churning up sediment and oxygenating water. Because of this action, they are sometimes called the "earthworms of the sea". Their eggs and larvae are food for a variety of sea creatures, from fish to molluscs and crustaceans. Adult sea cucumbers are fed on by fish, turtles and other sea animals.
When scared, some sea cucumbers can throw out some of their internal organs, which rupture the side of their body and burst out, in an attempt to distract or confuse, and possibly trap, predators. Luckily they are capable of growing new ones in a few weeks. Less drastically, they can also squirt jets of water at their attackers.
They reproduce either sexually or asexually. In sexual reproduction, eggs and sperm are released into water by the parents, where fertilization occurs. The young are planktonic, floating freely through the sea. If there are not many sea cucumbers around, asexual reproduction occurs instead.
Pearl fish sometimes live inside the sea cucumber in a commensal relationship. The fish gains shelter and a food source, and does no harm to the cucumber, although the sea cucumber doesn't get anything out of the relationship, so it is not a symbiotic relationship.
Some species of sea cucumbers, such as Stichopus fuscus, are eaten in Eastern Asia, including Korea and Japan, where they are cooked and dried, and their sexual organs are considered a delicacy. As a food, they are known as trepang or béche-de-mer. They are harvested by divers throughout the Pacific region, although in some locations stocks have been seriously depleted and fishing is no longer commercially viable.
In 1988 fishermen began harvesting the cucumbers around the Galapagos Islands off the West coast of South America. This was considered by the fishermen to be a great new source of the animal for the Asian market; however it had a very damaging effect on the sea cucumber population. They are slow-growing animals and were not able to replenish their stocks. The government of Ecuador, which owns the islands, introduced conservation measures, but at first these did not protect the population, which is still seriously depleted; this has had a bad effect on the marine ecology. Sea cucumber fishing is now banned around the Galapagos Islands.