Definitions

sea snake

sea snake

sea snake, name for any of the venomous marine snakes of the family Hydrophidae, found in tropical waters of the Indian and Pacific oceans. The sea snake's body is flattened laterally and its oarlike tail is used as a scull. A specialized lung and nostrils with valves enable it to remain submerged for periods of up to 8 hr. Most species are dark above and lighter below, or ringed with black and grayish green. They feed on small fish and are preyed upon by sea birds, sharks, and larger fish. Their potent venom quickly immobilizes their prey; however, they are not aggressive and rarely strike at humans when caught. Most sea snakes are completely marine and lack the enlarged ventral scales that enable land snakes to grip the ground. These snakes bear live young at sea. Most inhabit the shallow waters of the Indonesian region, but the common sea snake, Pelamis platurus, ranges from Madagascar E to Mexico and is sometimes found swarming by the thousands in open ocean. It is black or dark brown above and yellow below and grows up to 3 ft (90 cm) long. A few sea snake species leave the water to lay eggs on coral reefs. These snakes have ventral plates like those of land snakes. Sea snakes are classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Squamata, family Hydrophidae.

For sea snakes in mythology and cryptozoology, see Sea serpent.

Sea snakes, or "seasnakes", are venomous elapid snakes that inhabit marine environments for most or all of their lives. Though they evolved from terrestrial ancestors, and some such as Laticauda sp. retain ancestral characteristics which allow limited movement on land, most are extensively adapted to a fully aquatic life and are unable to even move on land. They are found in warm coastal waters from the Indian Ocean to the Pacific. All have paddle-like tails and many have laterally compressed bodies that give them an eel-like appearance. However, unlike fish, they do not have gills and must come to the surface regularly to breathe. Nevertheless, they are among the most completely aquatic of all air-breathing vertebrates. Among this group are species with some of the most potent venoms of all snakes. Some have gentle dispositions and bite only when provoked, while others are much more aggressive. Currently, 17 genera are described as sea snakes, comprising 62 species.

Description

Adults of most species grow to between 120-150 cm (4-5 ft) in length, with the largest, Hydrophis spiralis, reaching a maximum of 3 m (10 ft). Their eyes are relatively small with a round pupil and most have nostrils that are located dorsally. The skulls do not differ significantly from 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 sea snakes are completely aquatic and have adapted to their environment in many ways, the most characteristic of which is a paddle-like tail that has increased 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, even difficult to distinguish from the adjoining scales. Their lack of ventral scales means that they have become virtually helpless on land, but since they live out their entire life cycle at sea, they never have any need to come out of the water.

The only species that have retained their enlarged ventral scales are the sea kraits, represented by the genus Laticauda, with only five species. This is considered to be a more primitive group, as they still spend much of their time on land where their ventral scales afford them the necessary grip. They are also the only sea snakes with internasal scales, i.e. their nostrils are not located dorsally.

As 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 that consist of a specialized spongy tissue to keep water out, and the windpipe can be drawn up to where the short nasal passage opens into the roof of the mouth: an important adaptation for an animal that must still come to the surface to breathe air, 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 it seems likely that the rear portion developed to aid buoyancy, rather than to exchange gas. The extended lung may also serve as a means of storing air for dives.

Sea snakes in general are able to respire through their skin. This is unusual for reptiles, because their skin is thick and scaly, but experiments with Pelamis have shown that this species can satisfy about 20% of its oxygen requirements in this manner, which allows for prolonged dives.

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 diet and when sea water is inadvertently swallowed. This meant that they had to evolve a more effective means of regulating the salt concentration of their blood. Mammals have the advantage of being able to pass salt in solution, mostly in the urine, but kidney function in birds and reptiles is too weak to remove salt in sufficient amounts. In birds, such as penguins, salt is removed through nasal glands, just as with the marine iguanas of the Galapagos Islands. Sea turtles have lacrimal glands that allow them to produce very salty tears. But in sea snakes, the posterior sublingual 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 snakes, species that have imbricate scales to protect against abrasion, the scales of most pelagic sea snakes do not overlap. Reef dwelling species, such as Aypisurus, 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. The black-and-yellow sea snake, Pelamis platurus, a pelagic species, 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 aiding it in remaining hidden inside of coral holes during the day. While other species have not been tested, it is possible that A. laevis is not unique among sea snakes in this respect.

Distribution and habitat

These snakes are mostly confined to the warm tropical waters of the Indian Ocean and the western Pacific Ocean, with a few species found well out into Oceania. The geographic range of one species, Pelamis platurus, is wider than that of any other reptile species, save for a few species of sea turtles. It extends from the east coast of Africa, from Djibouti in the north to Cape Town in the south, across the Indian Ocean, the Pacific, south as far as New Zealand, all the way to the western coast of the Americas, where it occurs from northern Peru in the south (including the Galápagos Islands) to the Gulf of California in the north. One source claims that Pelamis occurs as far north as Orange County, California in the United States.

Sea snakes do not occur in the Atlantic Ocean, although Pelamis would doubtless be found there were it not for the cold currents off Namibia and western South America that keep them from crossing into the eastern South Atlantic, or south of 5° latitude along the South American west coast. Sea snakes do not occur in the Red Sea, possibly because of its increased salinity, so there is no danger of them crossing through the Suez Canal, and it is possibly also due to salinity, or rather a lack thereof, that Pelamis has not managed to cross into the Caribbean via the Panama Canal. On the other hand, it has been speculated that Pelamis will enter the Atlantic if global warming eventually causes the aforementioned cold currents to become warm enough.

Despite their marine adaptations, most species prefer shallow waters not far from land, around islands, especially waters that are somewhat sheltered, as well as near estuaries. They may swim up rivers and have been reported as much as 160 km from the sea. Others, such as Pelamis platurus, are pelagic and are found in drift lines; slicks of floating debris brought together by surface currents. Some species inhabit mangrove swamps and similar brackish water habitats and there are even two landlocked fresh water forms: Hydrophis semperi occurs in Lake Taal in the Philippines, and Laticauda crockeri in Lake Te Nggano on Rennell Island in the Solomon Islands.

Behavior

Much has been said regarding their temperament. Stidworthy (1974) describes all sea snake species as being reluctant to bite. Fichter (1982) states that they are quite docile and even reluctant to bite. Spawls and Branch (1994) claims that they are mainly docile and non-aggressive. The U.S. Navy describes them as generally mild tempered, although there is variation among species and individuals. Mehrtens (1987) suggests that species such as Pelamis platurus, that feed by simply gulping down their prey, are more likely to bite when provoked because they seem to use their venom more for defense. This is in contrast to others, such as Laticauda, that use their venom for prey immobilization; these snakes are frequently handled with impunity by local fisherman. Species that have been reported as much more aggressive include Aipysurus laevis, Astrotia stokesii, Enhydrina schistosa and Hydrophis ornatus.

Ditmars (1933) mentions that when they are taken out of the water, their movements become very erratic. They crawl awkwardly in these situations and can become quite aggressive, striking wildly at anything that moves. Yet, they are also frequently caught in nets by fishermen who unravel and throw them back barehanded, usually suffering no harm. On land, sea snakes are not able to coil and strike like terrestrial snakes.

It seems they are active both during the day and at night. In the morning, and sometimes late in the afternoon, they can be seen at the surface basking in the sunlight. When disturbed, they dive down below. Sea snakes have been reported swimming at depths of over 90 m (295 ft). They can remain submerged for as much as a few hours, possibly depending on temperature and degree of activity.

Huge aggregations of sea snakes are occasionally reported. In 1932, millions of Astrotia stokesii, a relative of Pelamis, were seen from a steamer in the Strait of Malacca, off the coast of Malaysia, and formed a line of snakes 3 m (10 ft) wide and 100 km (62 mi) long. The cause of this phenomenon is unknown, although it likely has something to do with reproduction. Ditmars (1933) mentions that, in that same area, sea snakes can sometimes be seen swimming in schools of several dozen, and that after typhoons many dead specimens can be found on the beaches.

Feeding

Most species prey on fish, especially eels. The latter, when bitten, stiffen and die within seconds. One species prefers molluscs and crustaceans, such as prawns, while a few others feed only on fish eggs, which is unusual for a venomous snake. Some reef dwelling species have small heads and thin necks, making it possible for them to extract small eels from the soft bottom that they hide in. Stidworthy (1974) states that sea snakes will sometimes take bait from a fishing line.

Reproduction

Except for a single genus, all sea snakes are ovoviviparous; the young are born alive in the water where they live out their entire life cycle. In some species, the young are quite large: up to half as long as the mother. The one exception is the genus Laticauda, which is oviparous; its five species all lay their eggs on land.

Venom

Like their cousins in the Elapidae family, the majority of sea snakes are highly venomous, however when bites occur it is rare for much venom to be injected, so that envenomation symptoms usually seem non-existent or trivial. For example, Pelamis platurus has more potent venom than any other terrestrial snake species in Costa Rica, but despite being abundant in the waters off its western coast, few human fatalities have been reported. Nevertheless, all sea snakes should be handled with great caution.

Bites in which envenomation does occur are usually painless and may not even be noticed when contact is made. Teeth may be left in the wound. There is usually little or no swelling involved and it is rare for any nearby lymph nodes to be 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. Symptoms that can occur after 30 minutes to several hours post 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 elapid envenomations: a progressive flaccid paralysis, starting with ptosis and paralysis of voluntary muscles. Paralysis of muscles involved in swallowing and respiration can be fatal. 3-8 hours post bite, myoglobin as a result of muscle breakdown may start to show up in the blood plasma, can cause the urine to turn a dark reddish, brown or black color, and eventually lead to acute renal failure. 6-12 hours post bite, severe hyperkalemia, also the result of muscle breakdown, can lead to cardiac arrest.

Taxonomy

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 16 genera with 57 species), and the more primitive Laticaudinae, or sea kraits (1 genus, Laticauda, with 5 species). Eventually, as it became clear just how closely related the sea snakes are to the elapids, the taxonomic situation became less well-defined. Some taxonomists responded by moving the sea snakes to the Elapidae, thereby creating the subfamilies Elapinae, Hydrophiinae and Laticaudinae, although the latter may be omitted if Laticauda is included in the Hydrophiinae. Unfortunately, no one has yet been able to convincingly work out the phylogenetic relationships between the various elapid subgroups, meaning that the situation is still unclear. Therefore, others opted to either continue to work with the older traditional arrangements, if only for practical reasons, or to lump all of the genera together in the Elapidae, with no taxonomic subdivisions, to reflect the work that remains to be done.

Genus Taxon author Species Subsp.* Common name Geographic range
Acalyptophis Boulenger, 1869 1 0 Spiny-headed seasnake Gulf of Thailand, South China sea, the Strait of Taiwan, and the coasts of Guangdong, Indonesia, Philippines, New Guinea, New Caledonia, Australia (Northern Territory, Queensland, Western Australia)
Aipysurus Lacépède, 1804 7 1 Olive sea snakes Timor Sea, South China Sea, Gulf of Thailand, and coasts of Australia (North Territory, Queensland, West Australia), New Caledonia, Loyalty Islands, southern New Guinea, Indonesia, western Malaysia and Vietnam.
Astrotia Fischer, 1855 1 0 Stoke's sea snake Coastal areas from west India and Sri Lanka through Gulf of Thailand to China Sea, west Malaysia, Indonesia east to New Guinea, north and east coasts of Australia, Philippines
Emydocephalus Krefft, 1869 2 0 Turtlehead sea snakes The coasts of Timor (Indonesian sea), New Caledonia, Australia (North Territory, Queensland, West Australia), and in the Southeast Asian Sea along the coasts of China, Taiwan, Japan, and the Ryukyu Island.
Enhydrina Gray, 1849 2 0 Beaked sea snakes In the Persian Gulf (Oman, United Arab Emirates, etc.), south to the Seychelles and Madagascar, SE Asian Sea (Pakistan, India, Bangladesh, Myanmar, Thailand, Vietnam), Australia (North Territory, Queensland), New Guinea and Papua New Guinea.
Ephalophis M.A. Smith, 1931 1 0 Grey's mudsnake North-western Australia
Hydrelaps Boulenger, 1896 1 0 Port Darwin mudsnake Northern Australia, southern New Guinea
Hydrophis Latreille In Sonnini & Latreille, 1801 34 3 Sea snakes Indoaustralian and Southeast Asian waters.
Kerilia Gray, 1849 1 0 Jerdon's sea snake Southeast Asian waters.
Kolpophis M.A. Smith, 1926 1 0 Bighead sea snake Indian Ocean.
Lapemis Gray, 1835 1 1 Shaw's sea snake Persian Gulf to Indian Ocean, South China Sea, Indo-Australian archipelago and the western Pacific.
Laticauda Laurenti, 1768 5 0 Sea kraits Southeast Asian and Indoaustralian waters.
Parahydrophis Burger & Natsuno, 1974 1 0 Northern mangrove sea snake Northern Australia, southern New Guinea
Parapistocalamus Roux, 1934 1 0 Hediger's snake Bougainville Island, Solomons
Pelamis Daudin, 1803 1 0 Yellow-bellied sea snake Indian and Pacific Oceans
Praescutata Wall, 1921 1 0 From the Persian Gulf to the Indian Ocean, the South Chinese Sea, and northeast to the coastal region of Fujian and Strait of Taiwan.
Thalassophis P. Schmidt, 1852 1 0 Anomalous sea snake South Chinese Sea (Malaysia, Gulf of Thailand), Indian Ocean (Sumatra, Java, Borneo)
*) Not including the nominate subspecies.

Captivity

At best, these snakes make difficult captives. Ditmars (1933) described them as nervous and delicate captives that usually refuse to eat while preferring only to hide in the darkest corner of the tank. Over fifty years later, Mehrtens (1987) wrote that while they were rarely displayed in western zoological parks, some species were regularly on display in Japanese aquariums. Available food supply is one factor that limits the number of species that can be kept in captivity, since some have diets that are too specialized. Another is that some species appear too intolerant to handling, or even being removed from the water. For any exhibit, the fish-eating species are the most logical choice. Regarding their facilities, the Laticauda species need to be able to exit the water somewhere and bask, while the other strictly aquatic genera do not, basically requiring only a tank of filtered (synthetic) sea water maintained at about 29°C, along with a submerged shelter. Species that have done relatively well in captivity include the ringed sea snake, Hydrophis cyanocinctus, which feed on fish and eels in particular. Pelamis platurus has done especially well in captivity, accepting small fish, including goldfish. However, care should be taken to house them in round or oval tanks, or in rectangular tanks with corners that are well-rounded, to prevent the snakes from damaging their snouts by swimming into the sides.

Conservation status

Sea snakes are exploited for their meat, skin and internal organs, but despite great numbers being killed every year, CITES does not afford them any protection. Only one species, Laticauda crockeri, is classified as vulnerable (VU) according to the IUCN Red List of Threatened Species.

See also

References

Further reading

  • Graham JB, Lowell WR, Rubinoff I, Motta J. 1987. Surface and subsurface swimming of the sea snake Pelamis Platurus. J. exp. Biol. 127, 27-44. PDF at the [Journal of Experimental Biology]. Accessed 7 August 2007.
  • Rasmussen AR. 1997. Systematics of sea snakes; a critical review. Symp. Zool. Soc. London 70, 15-30.
  • Smith MA. 1926. Monograph of the sea snakes (Hydrophiidae). British Museum of Natural History, London.
  • Voris HK. 1977. A phylogeny of the sea snakes (Hydrophiidae). Fieldiana Zool. 70, 79-169.
  • Whitaker R. 1978. Common Indian Snakes: A Field Guide. Macmillan India Limited.

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