 | It has been suggested that this article besplit out into a new article titledHydrophiini. (Discuss)(February 2025) |
| Sea snake | |
|---|---|
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| Yellow-bellied sea snake (Hydrophis platurus) on aCosta Rica beach | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Suborder: | Serpentes |
| Superfamily: | Elapoidea |
| Family: | Elapidae |
| Groups included | |
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| Range of sea snakes shown in lime green, except the widespread, pelagicyellow-bellied sea snake | |
Sea snakes, orcoral reef snakes, areelapid snakes that inhabitmarine environments for most or all of their lives. They belong to two subfamilies,Hydrophiinae andLaticaudinae. 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 69species of sea snake divided among seven genera.[2]
Most sea snakes arevenomous, except the genusEmydocephalus, which feeds almost exclusively on fish eggs.[3] 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 theIndian Ocean to thePacific and are closely related to venomous terrestrial snakes in Australia.[4]
All sea snakes have paddle-like tails and many have laterally compressed bodies that give them aneel-like appearance. Unlike fish, they do not havegills and must surface regularly to breathe. Along withcetaceans, they are among the most completely aquatic of all extant air-breathingvertebrates.[5] Among this group are species with some of the most potent venoms of allsnakes. Some have gentle dispositions and bite only when provoked, while others are much more aggressive.
The majority of adult sea snakes species grow to between 120 and 150 cm (4 and 5 ft) in length,[6] with the largest,Hydrophis spiralis, reaching a maximum of 3 m (10 ft).[7] Their eyes are relatively small with a round pupil[8] and most have nostrils located dorsally.[9] The skulls do not differ significantly from those of terrestrial elapids, although their dentition is relatively primitive with short fangs and (with the exception ofEmydocephalus) as many as 18 smaller teeth behind them on the maxilla.[5]
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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.[10] To a varying degree, the bodies of many species are laterally compressed, especially in thepelagic species. This has often caused theventral 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.[6][9]
The onlygenus 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.[6][9]Laticauda species are also the only sea snakes withinternasal scales; that is, their nostrils are not located dorsally.[10]
Since a snake's tongue can fulfill itsolfactory 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 therostral scale.[5] 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.[6][9]
Most species of sea snakes canrespire 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 (apelagic species), have shown this species can satisfy about 30% of its oxygen requirements in this manner, which allows for prolonged dives.[11]
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.[5][9]
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 asAipysurus, 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.[9]
Vision,chemoreception (tongue-flicking), andhearing are important senses for terrestrial snakes, but thesestimuli become distorted in water.[12][13] The poor visibility, chemical dilution, and limitation of ground-borne vibrations under water suggest that sea snakes and sea kraits may have unique sensory abilities to compensate for the relative lack of other sensory cues.[14]
Relatively little is known about sea snake vision. A study ofphotoreceptors in theretina of spine-bellied,Lapemis curtus, and horned,Acalyptophis peronii, sea snakes found three classes ofopsins all fromcone cells.[15] Despite the absence ofrod cells in sea snake eyes, Simeoset al. found therhodopsin (rh1), the opsin of the rods, stillexpressed[16] 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 soundvibrations and chemoreception may be important.[17][18] One study identified small sensory organs on the head ofLapemis curtus[19] similar to the mechanoreceptors inalligators and aquatic snakeAcrochodus that are used to sense the movement of fish prey.[20] Westhoffet al. recordedauditory brain responses tovibration underwater inLapemis curtus,[21] which are sensitive enough to detect movement in prey, but were not as sensitive asfish lateral line systems. Similarly, vision appears to be of limited importance for finding mates. Shine experimented with applying skin secretions (pheromones) 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 1 m [3 ft]), pheromones are more important once the male comes in physical contact with an object.[22]
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.[23]
Other unique senses, such aselectromagnetic reception andpressure detection,[24] have been proposed for sea snakes, but scientific studies have yet to be performed to test these senses.[14]
Sea snakes are mostly confined to the warmtropical waters of theIndian Ocean and the westernPacific Ocean,[6] with a few species found well out intoOceania.[25] The geographic range of one species,Pelamis platurus, is wider than that of any other reptile species, except for a few species of sea turtles.[5] It extends from the east coast ofAfrica, fromDjibouti in the north toCape Town in the south,[26] across the Indian Ocean, the Pacific, south as far as the northern coast ofNew Zealand,[25][27] all the way to the western coast of theAmericas, where it occurs from northernPeru in the south (including theGalápagos Islands) to theGulf of California in the north. Isolated specimens have been found as far north asSan Diego andOxnard in theUnited States.[28]
Sea snakes do not occur in theAtlantic Ocean.[9]Pelamis possibly would be found there were it not for the cold currents offNamibia and westernSouth 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 theRed Sea, believed to be due to its increased salinity, so no danger exists of them crossing through theSuez Canal. A lack of salinity is also thought to be the reason whyPelamis has not crossed into theCaribbean via thePanama Canal.[5]
Despite their marine adaptations, most sea snakes prefer shallow waters near land, around islands, and especially somewhat sheltered waters, as well as near estuaries.[6][10] They may swim up rivers and have been reported as far as 160 km (100 mi) from the sea.[10] Others, such asP. platurus, arepelagic and are found in drift lines, slicks of floating debris brought together by surface currents.[29] Some sea snakes inhabit mangrove swamps and similar brackishwater habitats, and two landlocked freshwater forms are found:Hydrophis semperi occurs inLake Taal in thePhilippines, andLaticauda crockeri in Lake Tegano onRennell Island in theSolomon Islands.[9]
Sea snakes are generally reluctant to bite,[6][7] and are usually considered to be mild-tempered, although variation is seen among species and individuals.[25] Some species, such asP. platurus, which feed by simply gulping down their prey, are more likely to bite when provoked because they seem to use their venom more for defense. Others, such asLaticauda spp., use their venom for prey immobilization. Sea snakes are often handled without concern by local fishermen who unravel and toss them back into the water barehanded, usually without getting bitten, when the snakes frequently become entangled in fishing nets.[6][9] Species reported as much more aggressive includeAipysurus laevis,Astrotia stokesii,Enhydrina schistosa,Enhydrina zweifeli, andHydrophis ornatus.[10]
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.[7][8]
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.[6] They have been reported swimming at depths over 90 m (300 ft), and can remain submerged for as long as a few hours, possibly depending on temperature and degree of activity.[7][25]
Sea snakes have been sighted in huge numbers. For example, in 1932, a steamer in theStrait of Malacca, off the coast ofMalaysia, reported sighting "millions" ofAstrotia stokesii, a relative ofPelamis; these reportedly formed a line of snakes 3 m (10 ft) wide and 100 km (60 mi) long.[29] The cause of this phenomenon is unknown, although it likely has to do with reproduction.[5] They can sometimes be seen swimming in schools of several hundred, and many dead specimens have been found on beaches after typhoons.[8]
They feed on small fish and occasionally young octopus. They are often associated with thesea snake barnacle (Platylepas ophiophila), which attaches to their skin.[30]
Except for a single genus, all sea snakes areovoviviparous; the young are born alive in the water where they live their entire lives.[9] In some species, the young are quite large, up to half as long as the mother.[7] The one exception is the genusLaticauda, which isoviparous; its five species all lay their eggs on land.[9]
Like their relatives in the family Elapidae, the majority of sea snakes are highly venomous. They rarely inject their venom when biting, so venomous bites to humans are rare.[10] For example,Hydrophis platurus has a venom more potent than any terrestrial snake species inCosta Rica based onLD50, but despite its abundance in the waters off its western coast, few human fatalities have been reported.[11] The death of a trawler fisherman in Australian waters during 2018 was reported to be the region's first sea snake fatality since a pearl diver was killed in 1935.[31]
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 nearbylymph nodes affected. The most important symptoms arerhabdomyolysis (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, andtrismus, which is similar totetanus, is common. This is followed later on by symptoms typical of otherelapid envenomations, a progressive flaccid paralysis, starting withptosis and paralysis of voluntary muscles. Paralysis of muscles involved in swallowing and respiration can be fatal.[32]
Vicket al (1975) estimated that the LD50 of three sea snake venoms (H. platurus,L. semifasciata andL. 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. Ishikawaet al (1985) indicated a substantially lower binding affinity between sea snake neurotoxin and human and chimpanzeeAChR'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 theloop of Henle, the second convoluted tubule and the collecting tubules. Sitprijaet al (1973) found evidence oftubular 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.[33]
| Cladogram |
| Cladogram showing the basic evolutionary relationships among sea snakes, sea kraits and other venomous terrestrial snakes. Sea kraits diverged earlier from the rest of Australasian elapids, in contrast other sea snakes are more closely related to Australasian elapids then they are to sea kraits. |
Sea snakes were at first regarded as a unified and separate family, the Hydrophiidae, that later came to comprise two subfamilies: the Hydrophiinae, or true/aquatic sea snakes (now 6 genera with 64 species), and the more primitive Laticaudinae, or sea kraits (one genus,Laticauda, with eight species). Eventually, as just how closely related the sea snakes are to the elapids became clear, the taxonomic situation became less well-defined. Some taxonomists responded by moving the sea snakes to the Elapidae. Most taxonomists now place the sea snakes in the elapid subfamilies Hydrophiinae and Laticaudinae, although the latter may be omitted ifLaticauda is included in the Hydrophiinae. Unlike the traditional Hydrophiinae, the Hydrophiinae as currently seen also includes Australasian terrestrial elapids.[34][2][8][9][4]
| Genus[2] | Taxon authority[34] | Species[2] | Subsp.[34][a] | Common name[35] | Geographic range[35] |
|---|---|---|---|---|---|
| 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, southernNew Guinea,Indonesia, westernMalaysia andVietnam |
| Emydocephalus | Krefft, 1869 | 3 | 0 | turtlehead sea snakes | the coasts of Timor (Indonesian Sea),New Caledonia, Australia (Northern Territory, Queensland, Western Australia), and in theSoutheast Asian sea along the coasts ofChina,Taiwan,Japan, and theRyukyu Islands |
| Ephalophis | M.A. Smith, 1931 | 1 | 0 | Grey's mudsnake | northwesternAustralia |
| Hydrelaps | Boulenger, 1896 | 1 | 0 | Port Darwin mudsnake | northern Australia, southern New Guinea |
| Hydrophis | LatreilleinSonnini &Latreille, 1801 | 49 | 3 | sea snakes | Indo-Australian andSoutheast Asian waters.[36] |
| 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 |
Molecular data studies suggest all three monotypic semiaquatic genera (Ephalophis,Parahydrophis andHydrelaps) are early diverging lineages.[37]
At best, sea snakes make difficult captives. Ditmars (1933) described them as nervous and delicate captives that usually refuse to eat, preferring only to hide in the darkest corner of the tank.[8] Over 50 years later, Mehrtens wrote in 1987 that although they were rarely displayed in Western zoological parks, some species were regularly on display in Japanese aquariums. The available food supply limits the number of species that can be kept in captivity, since some have diets that are too specialized. Also, some species appear intolerant of handling, or even being removed from the water. Regarding their requirements in captivity, theLaticauda species need to be able to exit the water somewhere at about 29 °C (84 °F), along with a submerged shelter. Species that have done relatively well in captivity include the ringed sea snake,Hydrophis cyanocinctus, which feeds onfish andeels in particular.Hydrophis platurus has done especially well in captivity, accepting small fish, includinggoldfish. Housing them in round tanks, or in rectangular tanks with well-rounded corners, prevents snakes from damaging their snouts on the sides.[9]
Most sea snakes are not on theCITES protection lists.[10][38] One species,Laticauda crockeri, is classified as vulnerable. Several species ofAipysurus are listed with conservation status of greater concern, the Timor speciesA. fuscus is known to be endangered, and two others found in seas north of Australia, the leaf-scaledA. foliosquama and short-nosedA. apraefrontalis, are classified ascritically endangered according to theIUCN Red List of Threatened Species.[39]
Sea snakes are true reptiles closely related to Australian venomous terrestrial snakes. Indeed, both groups are included in a single subfamily, Hydrophiinae, by most modern herpetologists.
