Sea snake

Subfamily of reptiles
sea snakes in mythology and cryptozoology

Sea snakes, or coral reef snakes, are elapid snakes that inhabit marine environments for most or all of their lives. They belong to two subfamilies, Hydrophiinae and Laticaudinae. 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 snakes divided among seven genera. Most sea snakes are venomous, 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 gills and must surface regularly to breathe. Along with Cetaceans, they are among the most completely aquatic of all extant 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.

The majority of adult sea snakes species grow to between in length, with the largest, Hydrophis spiralis, reaching a maximum of Their eyes are relatively small with a round pupil and most have nostrils located dorsally. The skulls do not differ significantly from those of terrestrial elapids, although their dentition is relatively primitive with short fangs and (with the exception of Emydocephalus) as many as 18 smaller teeth behind them on the maxilla. 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 lifecycles at sea, they have no need to leave the water. 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. 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. 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. 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 opsins all from cone cells. Despite the absence of rod cells in sea snake eyes, Simeos et al. found the rhodopsin (rh1), the opsin of the rods, still expressed 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 vibrations and chemoreception may be important. One study identified small sensory organs on the head of Lapemis curtus similar to the mechanoreceptors in alligators and aquatic snake Acrochodus 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 fish 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 ), 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 electromagnetic reception and pressure detection, have been proposed for sea snakes, but scientific studies have yet to be performed to test these senses. 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. across the Indian Ocean, the Pacific, south as far as the northern coast of 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. Isolated specimens have been found as far north as San Diego and Oxnard in the United States. Sea snakes do not occur in the Atlantic Ocean. Some sea snakes inhabit mangrove swamps and similar brackishwater habitats, and two landlocked freshwater forms are found: Hydrophis semperi occurs in Lake Taal in the Philippines, and Laticauda crockeri in Lake Te Nggano on Rennell Island in the Solomon Islands.

Except for a single genus, all sea snakes are ovoviviparous; the young are born alive in the water where they live their entire lives. 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 lymph nodes 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 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 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. 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.

Molecular data studies suggest all three monotypic semiaquatic genera (Ephalophis, Parahydrophis and Hydrelaps) are early diverging lineages.

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. One species, Laticauda crockeri, is classified as vulnerable. Several species of Aipysurus are listed with conservation status of greater concern, the Timor species A. fuscus is known to be endangered, and two others found in seas north of Australia, the leaf scaled A. foliosquama and short nosed A. apraefrontalis, are classified as critically endangered according to the IUCN Red List of Threatened Species.