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.
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.
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.
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.
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.
|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)|
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.