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''Velella'' is a cosmopolitan genus of free-floating hydrozoans that live on the surface of the open ocean. There is only one known species, ''Velella velella'', in the genus. ''Velella velella'' is commonly known by the names sea raft, by-the-wind sailor, purple sail, little sail, or simply ''Velella''.〔Harrington Wells (1937). Seashore Life. Wagner Publishing Company, USA (see pages 138 and 144 in the 1942 edition)〕 These small cnidarians are part of a specialised ocean surface community that includes the better-known cnidarian siphonophore, the Portuguese man o' war. Specialized predatory gastropod mollusks prey on these cnidarians. Such predators include nudibranchs (sea slugs) in the genus ''Glaucus''〔Gosliner, T.M. (1987). ''Nudibranchs of Southern Africa'' page 127, ISBN 0-930118-13-8〕 and purple snails in the genus ''Janthina''.〔Branch, G.M., Branch, M.L, Griffiths, C.L. and Beckley, L.E. (2010). ''Two Oceans: a guide to the marine life of southern Africa.'' Cape Town:Struik Nature. page 188. ISBN 9781770077720.〕 Each apparent individual ''Velella velella'' is in fact a hydroid colony, and most are less than about 7 cm long. They are usually deep blue in colour, but their most obvious feature is a small stiff sail that catches the wind and propels them over the surface of the sea. Under certain wind conditions, they may be stranded by the thousand on beaches. Like other Cnidaria, ''Velella velella'' are carnivorous. They catch their prey, generally plankton, by means of tentacles that hang down in the water and bear cnidocysts (also called nematocysts). Though the toxins in their nematocysts are effective against their prey, ''V. velella'' is essentially harmless to humans. While cnidarians all possess nematocysts, in some species the nematocysts and toxins therein are more powerful than other species. ''V. velella'' has nematocysts that are relatively benign to humans, although each person may respond differently to contact with the nematocyst toxin. It is wise to avoid touching one's face or eyes after handling ''V. velella'', and itching may develop on parts of the skin that have been exposed to ''V. velella'' nematocysts. ==Distribution and habitat== ''Velella velella'' lives in warm and temperate waters in all the world's oceans. They live at the water/air interface, with the float above the water, and polyps hanging down about a centimetre below. Organisms that live partly in and partly out of the water like this are known as pleuston. Offshore boaters sometimes encounter thousands of ''V. velella'' on the water surface. The small rigid sail projects into the air and catches the wind . However ''Velella'' sails always align along the direction of the wind where the sail may act as an aerofoil so that the animals tend to sail downwind at a small angle to the wind. Having no means of locomotion other than its sail, ''V. velella'' is at the mercy of prevailing winds for moving around the seas, and are thereby also subject to mass-strandings on beaches throughout the world. For example, most years in the spring, there is a mass stranding that occurs along the West Coast of North America, from British Columbia to California, beginning in the north and moving south over several weeks' time. In some years, so many animals are left at the tide line by receding waves, that the line of dying (and subsequently rotting) animals may be many centimetres deep, along hundreds of kilometres of beaches. Mass strandings have been reported also on the west coast of Ireland. 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「Velella」の詳細全文を読む スポンサード リンク
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