[2] Most sea snakes are venomous, except the genus Emydocephalus, which feeds almost exclusively on fish eggs.

They are found in warm coastal waters from the Indian Ocean to the Pacific and are closely related to venomous terrestrial snakes in Australia.

[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 of Emydocephalus) as many as 18 smaller teeth behind them on the maxilla.

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.

[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.

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.

[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.

[9] Vision, chemoreception (tongue-flicking), and hearing are important senses for terrestrial snakes, but these stimuli become distorted in water.

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.

Behavioural observations indicate that vision has a limited role for catching prey and mate selection, but sound vibrations and chemoreception may be important.

[17][18] One study identified small sensory organs on the head of Lapemis curtus[19] similar to the mechanoreceptors in alligators and aquatic snake Acrochodus that are used to sense the movement of fish prey.

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.

[10] Others, such as P. platurus, are pelagic 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 in Lake Taal in the Philippines, and Laticauda crockeri in Lake Tegano on Rennell Island in the Solomon Islands.

[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.

[10] For example, Hydrophis platurus has a venom more potent than any terrestrial snake species in Costa Rica based on LD50, 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.

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.

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.

Sitprija et al (1973) found evidence of tubular necrosis throughout all portions of the renal tubules in two patients severely envenomated by sea snakes.

Eventually, as just how closely related the sea snakes are to the elapids became clear, the taxonomic situation became less well-defined.

[34][2][8][9][4] Molecular data studies suggest all three monotypic semiaquatic genera (Ephalophis, Parahydrophis and Hydrelaps) are early diverging lineages.

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.

Species that have done relatively well in captivity include the ringed sea snake, Hydrophis cyanocinctus, which feeds on fish and eels in particular.