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General information on sea snakes


With 16 genera and around 64 species the sea snakes (Hydrophiinae) form a separate subfamily within the snakes (Serpentes) that have completely adapted to life in the ocean. Included herein are the six species of Sea kraits (Laticauda spp.), which are sometimes placed in the separate subfamily Laticaudinae. Together with the subfamily of the Elapinae, the Hydrophiinae are placed taxonomically in the Elapidae family. On the basis of their morphological characteristics and the composition of their venoms they appear to be closely related to the Australian elapids, which have likewise been included in the Hydrophiinae subfamily in recent years. Hydrophiinae and Australian Elapids are treated separately here in the VAPAGuide.

Morphological characteristics

The gross morphological characteristics of sea snakes are the same as for the terrestrial venomous snakes, although with the transition from a life on land to one in salt water, specific adaptations developed. Thus sea snakes possess a laterally flattened tail for efficient movement in the water. The left lung is greatly enlarged to allow long dives. In addition, as a static organ, it aids balance in the water. Salt is excreted via a special gland that is associated with osmoregulation, and this makes it possible for the snakes to remain permanently in salt water.

Sea snakes are easy to distinguish from eel-like fishes because they lack gills and fins, and the characteristic flat tail makes it impossible to mistake them for a terrestrial snake (Fig. 4.49). The broad ventral shields that are characteristic of terrestrial snakes are reduced to smaller scales in most sea snakes.

Fig. 4.49 Simple morphological characteristics to distinguish between snake-like fishes (a), sea snakes (b) and terrestrial snakes (c). In contrast to fishes, sea snakes have no fins or gills. The tail of sea snakes is laterally flattened, while that of terrestrial snakes is normally round.


Venom apparatus

Like the members of the Elapinae, the Hydrophiinae also have proteroglyphous dentition. However, their venomous fangs and the available amounts of venom are on average smaller. The teeth of certain spawn-eating species are so reduced that they are barely able to penetrate human skin. The fangs of most of the other species are, however, sufficiently long to cause envenoming in humans.

Range of venom effects

See Clinical flowchart: Sea snakes.

Way of life and distribution

The Sea kraits, Laticauda ssp.,  lead a semi-aquatic life, in that they regularly leave the water, in particular to lay their eggs on beaches, as do marine turtles. In contrast, the other genera of the Hydrophiinae have largely lost their ability to move on land and spend their whole life under water, where they bear live young.

Apart from Pelamis platurus (see below), sea snakes feed on eels and other bottom-dwelling fish on the sea bed in coastal waters. Consequently, and also because they constantly need to surface to breathe, they are only found in shallow coastal waters, where they barely dive deeper than 30 m. Some species are also found at river mouths or even further upstream. Two species have adapted completely to living in freshwater lakes, one in the Philippines and the other on the Solomon Islands. Starting from the probable centre of their development, the Indo-Australian region, sea snakes are now found in various coastal regions of the tropical to subtropical Indo-Pacific. Their area of distribution ranges from the Persian Gulf to Japan, southwards to Indonesia, New Guinea, Australia and Polynesia (Maps 9 & 10). Some species – of particular note Enhydrina schistosa, due to its prominent epidemiological significance – are very numerous in their habitats and fishermen in those regions encounter them daily in the course of their work. In the Philippines hundreds of thousands of individuals of the species Laticauda laticauda and L. semifasciata gather on certain islands during the breeding season and are caught in large numbers for commercial uses (especially for leather production).

The only species of sea snake that lives on the open sea, Pelamis platurus, has a distribution area that is far more extensive than that of the other Hydrophiinae. As a result of their planktonic way of life in the surface waters of the open sea, they drift over long distances in enormous colonies. In this way they are the only species to reach the Pacific Coast of Central America and northern South America as well as coastal regions of the Indian Ocean in East and South Africa (Map 10). Cold currents on the west coasts of South Africa and South America prevent them from entering the Atlantic, as they cannot tolerate water temperatures under 20°C for long. Thus there are no sea snakes in the Atlantic. Likewise the Red Sea, with its high salinity and water temperatures, prevents sea snakes from entering the Mediterranean.




Map 9 Hydrophiinae (excluding Pelamis platurus).




Map 10 Pelamis platurus.



Compared to their terrestrial venomous counterparts, the viperids, crotalids and elapids, sea snakes have a much lower impact on human health throughout the world. However, if only the actual group at risk is considered, namely fishing communities of the tropical to subtropical coastal regions of Asia, accidents with sea snakes are not negligible.

Although individual species of sea snakes are very numerous in certain areas and many of the Hydrophiinae venoms investigated in animal experiments to date have proved to be extremely toxic, the known cases of serious envenoming are actually relatively few. On the one hand this can be attributed to the fact that until now there have only been a very small number of very regional epidemiological investigations, and on the other to the fact that many sea snakes display hardly any defensive behaviour and do not even bite when greatly provoked. Nonetheless, some species are prepared to bite in defence. These include in particular those species that have been known to cause envenoming in humans, such as Enhydrina schistosa, Hydrophis cyanocinctus and Astrotia stokesii. Other species are also believed to bite in defence, such as Aipysurus laevis and A. fuscus or Lapemis hardwicki.


To date the only systematic investigations of the epidemiology and clinical course of sea snake bites have come from Penang, northwest Malaysia (Reid and Lim 1957, Reid 1975b, see Enydrina schistosa and Hydrophis spp.). Besides these there is a smaller, retrospective study of Hydrophis semperi bites from Lake Taal on Luzon, in the Philippines (Watt and Theakston 1985), as well as a few case reports from Australia and Japan.

In Reid's investigations, Enhydrina schistosa, and to a lesser extent Hydrophis cyanocinctus, played a prominent role as the cause of serious and fatal bites. On the basis of Reid's observations, the following points can be made with regard to the epidemiology of sea snake bites:


  • As a rule, sea snake envenoming is an occupational hazard. The probability of being bitten by a sea snake is highest among fishermen. 90% of observed patients were fisherman using simple fishing methods who were either unable to access appropriate medical facilities or did not use them for other reasons.
  • With regard to bites in humans, sea snakes only inject venom in a very small number of cases. Reid (1975b) found that in 68% of 101 bite victims there were no signs of envenoming at all. Moreover, this percentage does not reflect the real situation, as in a group of hospital patients there will always be a bias in favour of cases of clinically relevant envenoming.
  • The risks for fishermen depend on the methods used. In the 1950s and into the 1960s many of the inhabitants of Penang Island evidently still earned their living by fishing. They used a fishing method that was hazardous with regard to Hydrophiinae bites, in that they dragged their nets through shallow water while wading.


In more recent times fishing as an occupation has declined greatly on Penang, and nowadays nearly all fishing is done on a larger scale from fishing boats. In addition, there has been a clear decrease in the fish population along the coast of Penang Island, as well as an increase in marine pollution. There has been no investigation of the extent to which these factors have had a decimating impact on Hydrophiid populations. However, a visit to the Penang General Hospital nowadays makes it clear that sea snake bites are only a very rare reason for admission to the hospital. This despite the fact that there is undoubtedly greater acceptance of medical facilities than in Reid's days.


Accidents while bathing or diving in the ocean are extremely rare. Following the recommendations relevant for other ocean-dwelling venomous animals – swim rather than wade; wear a diving mask for better visibility under water; do not touch or otherwise disturb unknown animals or animals known to be venomous – reduces the risk of being bitten by a sea snake to practically zero.

As most accidents occur during fishing, it is particularly important to take care when emptying nets. In light of the fact that most sea snakes have only relatively short fangs, thick, sturdy gloves can offer a degree of protection against bites.


Smith 1926, Dunson 1975a, Reid 1975b, Heatwole 1987, 1999, Limpus 1987, Gasperetti 1988, O'Shea 2005


Venomous Snake Systematics Alert