Sea Snakes in Australian Waters
Sea snakes are found primarily in tropical waters. They are marine reptiles and comprise about 86% of living marine reptiles species. Other marine reptiles include seven species of sea turtles, the saltwater crocodile (Crocodilus porosus) and the marine iguana (Amblyrinchus chrystatus).
The highest numbers of species of sea snakes are found between Singapore and Borneo, where 27 species are recorded. Australia's coastal waters also have high species diversity of sea snakes with between 17 and 21 species.
There are several features of the serpentine shape that have allowed sea snakes to adapt to the marine environment. Their elongate bodies are preadapted for efficient swimming. And most species have developed paddle shaped tails that further enhances their locomotory ability in water.
Sea snakes can spend long periods underwater because they have an elongate right lung that extends for almost the entire length of their body. They can also carry out cutaneous respiration (breathe through their skin).
Sea snakes avoid accumulating excess salt from seawater by using a salt excreting gland - the posterior sublingual gland that sits under the tongue.
There are five major groups of sea snakes. The two major groups, the Hydrophiidae and the Laticaudidae, have both evolved from the terrestrial Australian elapid snakes and comprise 80% of living species of sea snakes.
The Hydrophiidae, or true sea snakes, are the only species of sea snakes with breeding populations in Australian waters. There are a total of 54 species of true sea snakes, 32 of which are found in Australian waters, and 14 of which are found within the Great Barrier Reef Marine Park.
The Laticaudidae comprise four species, three of which are marine and two of which are found in Australian waters. They are thought to be transients from between Papua New Guinea and New Caledonia because resident populations of these species have not been found in Australian waters.
The other major groups of sea snakes, the Natricinae and the Homalopsinae, are subfamilies of the Colubridae and are confined almost entirely to saltmarsh and estuarine environments. The Homalopsinae are found primarily in tropical Asian waters, with three species occurring in north Australian waters, while the Natricinae are confined to temperate and subtropical north America.
The Laticaudidae are oviparous (egg laying) and therefore must return to land to breed. All species of the Hydrophiidae, the largest of the five groups of sea snakes, have exploited the viviparity (have live young) that exists in some of their terrestrial ancestors, and have thus freed themselves entirely from the need to return to land to breed. The Acrochoridae comprise of three species, two of which are found in Australian waters. One species is fully marine, Acrochordus granulatus, while the other species A. arafurae, inhabits fresh waters, estuaries and the sea. Both species give birth to live young.
Many advanced species have a well developed venom apparatus and highly toxic venom. The hydrophiids, or true sea snakes, are derived from proteroglypherous (front-fanged) Australian terrestrial elapids, and they share the same toxic venom and envenomation apparatus. The primary function of venom is to subdue prey, while defence is of secondary importance, and therefore most bites experienced by humans are blank. Minor envenomations do not cause painful wounds or severe symptoms, however severe envenomation can result in flaccid paralysis that may ultimately lead to paralysis of respiratory muscles and death, or muscular damage that results in muscle pain and may lead to kidney damage. Antivenom is manufactured from the venom of Enydrina schistosa, and has been an effective anti-venom against bites of all other sea snake species to date.
Very little is known about the status of populations of sea snakes in Australian waters, or about the basic ecology, movement patterns, life history strategies, reproductive biology and population genetics of most species of sea snakes.
The major threat to sea snake populations is probably the impact of the commercial trawl fishing industry. The trawl fishing industry across the north of Australia may inadvertently catch tens of thousands of sea snakes each year. It is likely that the trawl fishing industry in the Great Barrier Reef is having a similar effect on sea snake populations. The Great Barrier Reef Marine Park Authority (GBRMPA) is currently developing a management plan, the Representative Areas Program, that will hopefully protect sea snake species in the GBR, however this will need to be continuously evaluated as more information regarding sea snakes becomes available.
For information about CRC Reef research projects on sea snakes, click here.