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Sea cucumbers are echinoderms from the class Holothuroidea. They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothurian species worldwide is about 1,717 with the greatest number being in the Asia Pacific region. Many of these are gathered for human consumption and some species are cultivated in aquaculture systems. The harvested product is variously referred to as ''trepang'', ''bêche-de-mer'' or ''balate''. 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 degradation process.〔 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 some species these can sometimes be enlarged to flattened plates, forming an armour. In pelagic species such as ''Pelagothuria natatrix'' (Order Elasipodida, family Pelagothuriidae), the skeleton is absent and there is no calcareous ring. The sea cucumbers are named after their resemblance to the vegetable cucumber. == Overview == Most sea cucumbers, as their name suggests, have a soft and cylindrical body, more or less lengthened, 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. Holothurians measure generally between 10 and 30 centimeters long, with extremes of some millimeters for ''Rhabdomolgus ruber'' and up to more than 3 meters 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 ,〔For an example, in Charles Edward Lincoln, "Variation development and growth in ''Holothuria floridana''" ''Biometrika'' 6.2–3 July 1908:279.〕 and long. Most possess five rows of tube feet (called "podia"), but Apodida lacks these and moves by crawling; the podia can be of smooth aspect or provided with fleshy appendages (like ''Thelenota ananas''). The podia on the dorsal surface generally have no locomotive role, and are transformed into papillae. 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. Holothurians don't look like other echinoderms at first glance, because of their tubular body, without visible skeleton nor hard appendixes. Furthermore, the fivefold symmetry, classical for echinoderms, although preserved structurally, is doubled here by a bilateral symmetry which makes them look like chordates. 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. There is thus no "oral" or "aboral" face as for sea stars and other echinoderms, but the animal stands on one of its sides, and this face is called ''trivium'' (with three rows of tube feet), while the dorsal face is named ''bivium''. A remarkable feature of these animals is the "catch" collagen that forms their body wall.〔"Catch" collagen has two states, soft and stiff, that are under neurological control.(Jose del Castillo and David S. Smith. (1996) "We Still Invoke Friction and Occam's Razor to Explain Catch in the Spines of ''Eucidaris Tribuloides''." ''Biological Bulletin'' 190:243-244 )〕 This can be loosened and tightened at will, and if the animal wants to squeeze through a small gap, it can essentially liquefy its body and pour into the space. To keep itself safe in these crevices and cracks, the sea cucumber will hook up all its collagen fibers to make its body firm again. The most common way to separate the subclasses is by looking at their oral tentacles. Order Apodida have a slender and elongate body lacking tube feet, with up to 25 simple or pinnate oral tentacles. Aspidochirotida are the most common sea cucumbers encountered, with a strong body and 10–30 leaf like or shield like oral tentacles. Dendrochirotida are filter-feeders, with plump bodies and 8–30branched oral tentacles (which can be extremely long and complex). Image:Holoturia.004 - Aquarium Finisterrae.JPG|Details of the mouth with its tentacles. Image:Synaptula lamberti.JPG|''Synaptula lamperti'' lives on sponges (here in Indonesia). Image:Reef1118 - Flickr - NOAA Photo Library.jpg|''Synapta maculata'', the longest known sea cucumber (Apodida). Image:Thelenota anax géante.JPG|The king sea cucumber (''Thelenota anax'', Stichopodidae family) is one of the heaviest known holothurians. Image:Chiridota heheva.jpg|''Chiridota heheva'', abyssal species. Image:Cucumaria big.jpg|''Cucumaria miniata'', a filter-feeding sea cucumber. Image:Pseudocolochirus axiologus.jpg|''Pseudocolochirus'' ("sea apple"). Image:Holothuria leucospilota.jpg|''Holothuria leucospilota'' Image:Isostichopus badionotus.jpg|''Isostichopus badionotus'' Image:Treasure chest candycane sea cucumber 2.jpg|''Thelenota rubralineata'' Image:Holothuria fuscopunctata.jpg|''Holothuria fuscopunctata'' Image:Bohadschia argus.jpg|''Bohadschia argus'' 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sea cucumber」の詳細全文を読む スポンサード リンク
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