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Sea snakes, or coral reef snakes, are Elapidae snakes that inhabit marine environments for most or all of their lives. They belong to two subfamilies, Hydrophiinae and Sea krait. Hydrophiinae also includes Australasian terrestrial snakes, whereas Laticaudinae only includes the sea kraits ( Laticauda), of which three species are found exclusively in freshwater. If these three freshwater species are excluded, there are 69 species of sea snake divided among seven Genus.
Most sea snakes are venomous snake, except the genus Emydocephalus, which feeds almost exclusively on fish eggs. Sea snakes are extensively adapted to a fully aquatic life and are unable to move on land, except for the sea kraits, 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 sea snakes have paddle-like tails and many have laterally compressed bodies that give them an eel-like appearance. Unlike fish, they do not have and must surface regularly to breathe. Along with , they are among the most completely aquatic of all extant air-breathing .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. (cloth), (paper). Among this group are species with some of the most potent venoms of all . Some have gentle dispositions and bite only when provoked, while others are much more aggressive.
Most sea snakes are completely aquatic and have adapted to sea environments in many ways, the most characteristic of which is a paddle-like tail that has improved their swimming ability. 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, and 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 lifecycles at sea, they do not 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; that is, their nostrils are not located dorsally.
Since a snake's tongue can fulfill its olfactory function more easily 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 sea snakes can 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, Hydrophis platurus (a pelagic species), have shown this species can satisfy about 30% 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. .
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, when seawater is inadvertently swallowed. Because of this, a more effective means of regulating the salt concentration of their blood is required. In sea snakes, the posterior sublingual glands, located under and around the tongue sheath, 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 scales, 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.
Relatively little is known about sea snake vision. A study of photoreceptors in the retina of spine-bellied, Lapemis curtus, and horned, Acalyptophis peronii, sea snakes found three classes of all from . Despite the absence of in sea snake eyes, Simeos et al. found the rhodopsin ( rh1), the opsin of the rods, still Expressed gene suggesting that in sea snakes some cones may be transmuted rods. Behavioural observations indicate that vision has a limited role for catching prey and mate selection, but sound and chemoreception may be important. One study identified small sensory organs on the head of Lapemis curtus similar to the mechanoreceptors in and aquatic snake Acrochordidae that are used to sense the movement of fish prey. Westhoff et al. recorded auditory brain responses to vibration underwater in Lapemis curtus, which are sensitive enough to detect movement in prey, but were not as sensitive as Lateral line systems. Similarly, vision appears to be of limited importance for finding mates. Shine experimented with applying skin secretions () to snake-like objects to see if male turtle-headed sea snakes, Emydocephalus annulatus, are attracted to female pheromones. Shine found that although vision may be useful over short distances (less than ), pheromones are more important once the male comes in physical contact with an object.
The olive sea snake, 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.
Other unique senses, such as Magnetoreception and Hearing detection, have been proposed for sea snakes, but scientific studies have yet to be performed to test these senses.
Sea snakes do not occur in the Atlantic Ocean. Pelamis possibly would be found there were it not for the cold currents off Namibia and western South Africa that keep it from crossing into the eastern South Atlantic, or south of 5°S latitude along the South American west coast. Sea snakes do not occur in the Red Sea, believed to be due to its increased salinity, so no danger exists of them crossing through the Suez Canal. A lack of salinity is also thought to be the reason why Pelamis has not crossed into the Caribbean via the Panama Canal.
Despite their marine adaptations, most sea snakes prefer shallow waters near land, around islands, and especially somewhat sheltered waters, as well as near estuaries. They may swim up rivers and have been reported as far as from the sea. Others, such as P. platurus, are pelagic and are found in drift lines, slicks of floating debris brought together by surface currents. The sea snakes are coming at NewScientist. Accessed 13 January 2009. Some sea snakes inhabit mangrove swamps and similar brackishwater habitats, and two landlocked freshwater forms are found: Hydrophis semperi occurs in Taal Lake in the Philippines, and Laticauda crockeri in Lake Tegano on Rennell Island in the Solomon Islands.
On land, their movements become very erratic. They crawl awkwardly in these situations and can become quite aggressive, striking wildly at anything that moves, although they are unable to coil and strike in the manner of terrestrial snakes.
Sea snakes appear to be active both day and night. In the morning, and sometimes late in the afternoon, they can be seen at the surface basking in the sunlight, and they dive when disturbed. They have been reported swimming at depths over , and can remain submerged for as long as a few hours, possibly depending on temperature and degree of activity.
Sea snakes have been sighted in huge numbers. For example, in 1932, a steamer in the Strait of Malacca, off the coast of Malaysia, reported sighting "millions" of Astrotia stokesii, a relative of Pelamis; these reportedly formed a line of snakes wide and long. The cause of this phenomenon is unknown, although it likely has to do with reproduction. They can sometimes be seen swimming in schools of several hundred, and many dead specimens have been found on beaches after typhoons.
Bites in which envenomation does occur are usually painless and may not even be noticed when contact is made. Teeth may remain in the wound. Usually, little or no swelling occurs, and rarely are any nearby affected. The most important symptoms are rhabdomyolysis (rapid breakdown of skeletal muscle tissue) and paralysis. Early symptoms include headache, a thick-feeling tongue, thirst, sweating, and vomiting. The venom is very slow acting and symptoms that happen from as little as 30 minutes to several hours after the bite include generalized aching, stiffness, and tenderness of muscles all over the body. Passive stretching of the muscles is also painful, and trismus, which is similar to tetanus, is common. This is followed later on by symptoms typical of other Elapidae envenomations, a progressive flaccid paralysis, starting with ptosis and paralysis of voluntary muscles. Paralysis of muscles involved in swallowing and respiration can be fatal.Warrell DA. 2004. Snakebites in Central and South America: Epidemiology, Clinical Features, and Clinical Management. In Campbell JA, Lamar WW. 2004. The Venomous Reptiles of the Western Hemisphere. Comstock Publishing Associates, Ithaca and London. 870 pp. 1500 plates. .
Vick et al (1975) estimated that the LD50 of three sea snake venoms ( H. platurus, L. semifasciata and L. laticaudata) for a 70 kg human range from 7.7 to 21 mg. Data from the only sea snake venom conducted in monkeys at that time suggested that primates were slightly more resistant to the venom effects on a dose response basis than mice. Ishikawa et al (1985) indicated a substantially lower binding affinity between sea snake neurotoxin and human and chimpanzee AChR's compared to that in other animals. In humans, the venom targets appear mainly to be the cell walls of voluntary (skeletal) muscles and distal tubular portions of the kidney including the loop of Henle, the second convoluted tubule and the collecting tubules. Sitprija et al (1973) found evidence of tubular necrosis throughout all portions of the renal tubules in two patients severely envenomated by sea snakes. Sea snake venoms in humans are thus more often myotoxic and/or nephrotoxic rather than neurotoxic.
| Aipysurus | Lacépède, 1804 | 9 | 1 | olive sea snakes | Timor Sea, South China Sea, Gulf of Thailand, and coasts of Australia (Northern Territory, Queensland, Western Australia), New Caledonia, Loyalty Islands, southern New Guinea, Indonesia, western Malaysia and Vietnam |
| Emydocephalus | Gerard Krefft, 1869 | 3 | 0 | turtlehead sea snakes | the coasts of Timor (Indonesian Sea), New Caledonia, Australia (Northern Territory, Queensland, Western Australia), and in the sea along the coasts of China, Taiwan, Japan, and the Ryukyu Islands |
| Ephalophis | M.A. Smith, 1931 | 1 | 0 | Grey's mudsnake | northwestern Australia |
| Hydrelaps | Boulenger, 1896 | 1 | 0 | Port Darwin mudsnake | northern Australia, southern New Guinea |
| Hydrophis | Latreille in Sonnini & Latreille, 1801 | 49 | 3 | sea snakes | Indo-Australian and waters. The Hydrophiidae at Cyberlizard's home pages. Accessed 12 August 2007. |
| Laticauda | Laurenti, 1768 | 8 | 0 | sea kraits | Southeast Asian and Indo-Australian waters |
| Parahydrophis | Burger & Natsuno, 1974 | 1 | 0 | northern mangrove sea snake | northern Australia, southern New Guinea |
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