|
The Hydrophiinae, also known as coral reef snakes or sea snakes, are a subfamily of venomous elapid snakes that inhabit marine environments for most or all of their lives. Most are extensively adapted to a fully aquatic life and are unable to move on land, except for the genus ''Laticauda'', which have limited land movement. They are found in warm coastal waters from the Indian Ocean to the Pacific and are closely related to venomous terrestrial snakes in Australia. All have paddle-like tails and many have laterally compressed bodies that give them an eel-like appearance. Unlike fish, they do not have gills and must surface regularly to breathe. They are among the most completely aquatic of all air-breathing vertebrates.〔Parker HW, Grandison AGC. 1977. Snakes – a natural history. Second Edition. British Museum (Natural History) and Cornell University Press. 108 pp. 16 plates. LCCCN 76-54625. ISBN 0-8014-1095-9 (cloth), ISBN 0-8014-9164-9 (paper).〕 Among this group are species with some of the most potent venoms of all snakes. Some have gentle dispositions and bite only when provoked, but others are much more aggressive. Currently, 17 genera are described as sea snakes, comprising 62 species.〔〔 ==Description== The majority of adult Hydrophiinae species grow to between in length,〔Stidworthy J. 1974. Snakes of the World. Grosset & Dunlap Inc. 160 pp. ISBN 0-448-11856-4.〕 with the largest, ''Hydrophis spiralis'', reaching a maximum of .〔Fichter GS. 1982. Poisonous Snakes. A First Book. Franklin Watts. 66 pp. ISBN 0-531-04349-5.〕 Their eyes are relatively small with a round pupil〔Ditmars RL. 1933. Reptiles of the World. Revised Edition. The MacMillan Company. 329 pp. 89 plates.〕 and most have nostrils that are located dorsally.〔Mehrtens JM. 1987. Living Snakes of the World in Color. New York: Sterling Publishers. 480 pp. ISBN 0-8069-6460-X.〕 The skulls do not differ significantly from those of terrestrial elapids, although the dentition is relatively primitive with short fangs and (with the exception of ''Emydocephalus'') as many as 18 smaller teeth behind them on the maxilla.〔 Most Hydrophiinae are completely aquatic and have adapted to their environments in many ways, the most characteristic of which is a paddle-like tail that has increased their swimming ability.〔(Sea snakes ) at (Food and Agriculture Organization of the United Nations ). Accessed 7 August 2007.〕 To a varying degree, the bodies of many species are laterally compressed, especially in the pelagic species. This has often caused the ventral scales to become reduced in size, even difficult to distinguish from the adjoining scales. Their lack of ventral scales means they have become virtually helpless on land, but as they live out their entire life cycles at sea, they have no need to leave the water.〔〔 The only genus that has retained the enlarged ventral scales is the sea kraits, ''Laticauda'', with only five species. These snakes are considered to be more primitive, as they still spend much of their time on land, where their ventral scales afford them the necessary grip.〔〔 ''Laticauda'' species are also the only sea snakes with internasal scales, i.e., their nostrils are not located dorsally.〔 Since it is easier for a snake's tongue to fulfill its olfactory function under water, its action is short compared to that of terrestrial snake species. Only the forked tips protrude from the mouth through a divided notch in the middle of the rostral scale.〔 The nostrils have valves consisting of a specialized spongy tissue to exclude water, and the windpipe can be drawn up to where the short nasal passage opens into the roof of the mouth. This is an important adaptation for an animal that must surface to breathe, but may have its head partially submerged when doing so. The lung has become very large and extends almost the entire length of the body, although the rear portion is thought to have developed to aid buoyancy rather than to exchange gases. The extended lung possibly also serves to store air for dives.〔〔 Most species of Hydrophiinae are able to respire through the top of their skin. This is unusual for reptiles, because their skin is thick and scaly, but experiments with the black-and-yellow sea snake, ''Pelamis platurus'' (a pelagic species), have shown this species can satisfy about 25% of its oxygen requirements in this manner, which allows for prolonged dives.〔Campbell JA, Lamar WW. 2004. The Venomous Reptiles of the Western Hemisphere. Comstock Publishing Associates, Ithaca and London. 870 pp. 1500 plates. ISBN 0-8014-4141-2.〕 Like other land animals that have adapted to life in a marine environment, sea snakes ingest considerably more salt than their terrestrial relatives through their diets, and when seawater is inadvertently swallowed. This meant they had to evolve a more effective means of regulating the salt concentration of their blood. In sea snakes, the posterior sub lingual glands, located under and around the tongue sheath, evolved to allow them to expel salt with their tongue action.〔〔 Scalation among sea snakes is highly variable. As opposed to terrestrial snake species that have imbricate scales to protect against abrasion, the scales of most pelagic sea snakes do not overlap. Reef-dwelling species, such as ''Aipysurus'', do have imbricate scales to protect against the sharp coral. The scales themselves may be smooth, keeled, spiny or granular, the latter often looking like warts. ''Pelamis'' has body scales that are "peg-like", while those on its tail are juxtaposed hexagonal plates.〔 ''Aipysurus laevis'' has been found to have photoreceptors in the skin of its tail, allowing it to detect light and presumably ensuring it is completely hidden, including its tail, inside coral holes during the day. While other species have not been tested, ''A. laevis'' possibly is not unique among sea snakes in this respect.〔Zimmerman K, Heatwole H. 1990. Cutaneous Photoreception: A New Sensory Mechanism for Reptiles. Copeia, vol. 1990, no. 3 (September 19), pp. 860-862〕 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「Hydrophiinae」の詳細全文を読む スポンサード リンク
|