Like all echinoderms, sea cucumbers have an endoskeleton just below the skin, calcified structures that are usually reduced to isolated microscopic ossicles joined by connective tissue. These can sometimes be enlarged to flattened plates, forming an armour. In pelagic species , the skeleton is often absent.
Sea cucumbers are generally scavengers, feeding on debris in the benthic zone of the ocean. Exceptions include cucumbers and the species ''Rynkatropa pawsoni'', which has a commensal relationship with deep-sea anglerfish. The of most cucumbers consists of plankton and decaying organic matter found in the sea. Some sea cucumbers position themselves in and catch food that flows by with their open tentacles. They also sift through the bottom sediments using their tentacles.
Some species of coral-reef sea cucumbers within the order Aspidochirotida can defend themselves by expelling their sticky cuvierian tubules to entangle potential predators. When startled, these cucumbers may expel some of them through a tear in the wall of the cloaca in an process known as . Replacement tubules grow back in one-and-a-half to five weeks, depending on the species.
They can be found in great numbers on the deep sea floor, where they often make up the majority of the animal biomass. At depths deeper than 5.5 mi , sea cucumbers comprise 90% of the total mass of the macrofauna. The body of deep water holothurians 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 float over it to move to new locations with a minimum of energy.
In more shallow waters, sea cucumbers can form dense populations. The strawberry sea cucumber of New Zealand lives on rocky walls around the southern coast of the South Island where populations sometimes reach densities of 1,000 animals per square metre. For this reason, one such area in Fiordland is simply called the strawberry fields.
Sea cucumbers extract oxygen from water in a pair of 'respiratory trees' that branch off the cloaca just inside the anus, so that they 'breathe' by drawing water in through the anus and then expelling it. A variety of fish, most commonly pearl fish, have evolved a symbiotic relationship with sea cucumbers in which the pearl fish will live in sea cucumber's cloaca using it for protection from predation, a source of food , and to develop into their adult stage of life. Many polychaete worms and crabs have also specialized to use the cloacal respiratory trees for protection by living inside the sea cucumber.
Ten percent of the blood cell pigment of the sea cucumber is vanadium. Just as the horseshoe crab has blue blood rather than because of copper in the hemocyanin pigment, the blood of the sea cucumber is yellow because of the vanadium in the pigment. Nonetheless, there is no evidence that vanabins carry oxygen, in contrast to hemoglobin and hemocyanin.
Sea cucumbers reproduce by releasing sperm and ova into the ocean water. Depending on conditions, one organism can produce thousands of gametes.
The largest species is ''Holothuria floridana'', which abounds just below low-water mark on the Florida reefs.
The most common way to separate the subclasses is by looking at their oral tentacles. Subclass Dendrochirotacea has 8-30 oral tentacles, subclass Aspidochirotacea has 10-30 leaflike or shieldlike oral tentacles, while subclass Apodacea may have up to 25 simple or pinnate oral tentacles and is also characterized by reduced or absent tube feet, as in the order Apodida.
Sea cucumbers as food and medicine
"To supply the markets of Southern China, trepangers traded with the Indigenous Australians of Arnhem Land. This Macassan contact with Australia is the first recorded example of trade between the inhabitants of the Australian continent and their Asian neighbours."
Some varieties of sea cucumber are said to have excellent healing properties. There are pharmaceutical companies being built based on this gamat product. Extracts are prepared and made into oil, cream or cosmetics. Some products are intended to be taken internally. The effectiveness of sea cucumber extract in tissue repair has been the subject of serious study. It is believed that the sea cucumber contains all the fatty acids necessary to play an active role in tissue repair..
Sea cucumbers are believed to be endowed with aphrodisiac powers in the Far East. The reason for this belief is the peculiar reaction of the creature on being kneaded or disturbed slightly with fingers. It swells and stiffens and a jet of water is released from one end. This behaviour is similar to the erection and subsequent ejaculation of the male human penis. After releasing the jet, which is a defensive mechanism and contains irritants, the creature loses its stiffness and reverts to its original state.
On December 21, 2007, a study published in PLoS Pathogens found that a lectin from '''' impaired the development of the malaria parasite when produced by transgenic ..
In recent years the sea cucumber industry in Alaska has gained strength due to increased export of the skins and muscles to China.
In China, many commercial sea cucumbers are farmed in artificial ponds. These ponds can be as large as 1,000 acres, and satisfy much of the local demand. Wild sea cucumbers are caught by divers and these wild Alaskan sea cucumbers have higher nutritional value and are larger than farmed Chinese sea cucumbers. Larger size and higher nutritional value has allowed the Alaskan fisheries to continue to compete for market shares, despite the increase in local, Chinese sea cucumber farming.
Sea cucumbers in art and literature
Sea cucumbers have inspired musical composition: in the first of his ''Embryons desséchés'' for piano solo, Erik Satie presents the " of a Holothurian" and inserts a description of the animal in the score:
:The Holothurian crawls across boulders and rocky surfaces.
:This sea-animal purrs like a cat; also, it produces disgusting silky threads.
:Light appears to have an incommodating effect on it.
Nonetheless it is the sea cucumber's closest relative that gets the most attention from scientists, both as an embryo and as a fossil.
Sea cucumbers have also inspired thousands of haiku in Japan, where they are called ''namako'' , written with characters that can be translated "sea mice". In English translations of these haiku, they are usually called "sea slugs"; according to the Oxford English Dictionary, the term "sea slug" originally referred to holothurians , though biologists now use the name only for the nudibranch molluscs, marine relatives of land slugs. Almost 1,000 Japanese holothurian haiku translated into English appear in the book ''Rise, Ye Sea Slugs!'' by Robin D. Gill .
Sea Cumbers in Captivity
Sea cucumbers are very common in marine reef aquaria, particularly in reef tanks, where they are prized for their unusual appearance and behavior. Care of sea cucumbers is not complex, but these unusual creatures have unusual requirements. In the hobby, the term sea cucumber refers to only the detrivore sea cucumbers, that is, those that subsist by consuming the detritus that accumulates on the substrate .
In particular, these creatures have the remarkable ability to live for months, often up to half a year, without feeding. It is very common for these creatures to be introduced into a system that can't support them, and for the owners to have no idea that they are slowly starving to death. When this happens, the sea cucumber will slowly shrink as it digests its own body mass to survive.
In order to be sure a cucumber is feeding one must watch it at work. It will use the feeding tentacles around its mouth to pick up and swallow sand from the bottom of the aquarium. Particles too big will be of no use to the cucumber, so it is important to watch it to make sure it's feeding, and that it's regularly producing castings of excreted substrate.
In addition to the unusual feeding requirements of sea cucumbers, they release highly toxic compounds when injured. In particular, the filter feeding sea cucumbers, known as "Sea Apples" in the aquarium trade, are exceedingly lethal to the other tank inhabitants should they be injured. All powerheads and pumps should be covered as the cucumbers can squeeze into spaces much smaller than their body. Should a sea apple become injured it must be immediately removed from the aquarium, a major water change needs to be performed, and fresh activated carbon will need to be added if there is to be any hope of saving the other inhabitants.