Sea snake ecology, evolutionary history and classification

Species diversity and distribution
There are approximately 70 species of sea snakes living in our modern oceans. They account for 86% of marine reptile species alive today. Other marine reptiles include seven species of sea turtles, the salt water crocodile and the marine iguana.

Sea snakes are cold blooded reptiles and are found primarily in warm tropical waters of the Indo-West Pacific. They are not found in the Atlantic Ocean or Caribbean Sea.

The area of highest species diversity is Indonesia and northwest Australia. Each of these regions has more than 20 species, however, many species that occur in Indonesia and other parts of south east Asia do not occur in Australia and vice versa. Most of Australia’s tropical coastal waters have a high species diversity, which ranges from 17 to 21 species. The Great Barrier Reef has 14 species of sea snakes. Although sea snakes need warm tropical waters to survive, they are occasionally blown south by storms and have been recorded in Sydney harbour.

Evolutionary history
Snakes are the most recently evolved group of reptiles. They evolved from lizards, probably in the late Jurassic around 135 million years ago. These first snakes probably occurred in the ocean, however the sea snakes that we see today have evolved much more recently. Indeed, the snakes inhabiting our modern oceans have arisen as the result of invasions into the marine environment by five different groups of terrestrial snakes, giving rise to five evolutionary distinct radiations of marine snakes.

Five radiations of sea snakes

Marine hydrophiines - ‘true’ sea snakes
The largest group of sea snakes are the marine hydrophiines or ‘true’ sea snakes. There are ~60 species of hydrophiine sea snakes and they evolved from Australian terrestrial elapids. Potentially rather confusingly, Australian and Melanesian terrestrial elapids also belong to the Subfamily Hydrophiinae. Thus, the subfamily Hydrophiinae includes all Australian and Melanesian terrestrial elapids, as well as all 'true' sea snakes. The subfamily Hydrophiinae also includes the genus Laticauda or sea kraits (see next section). The remaining 'old world' elapids found in Asia and Africa, such as cobras and mambas, belong to the subfamily Elapinae. Together the Hydrophiinae and Elapinae make up the family Elapidae.

All elapid snakes have fixed front fangs (proteroglyphs) and most have highly toxic venom. The less venomous species originally evolved from venomous ancestors but have lost their toxic venom for one reason or another. For example, turtleheaded sea snakes (Genus Emydocephalus) feed exclusively on fish eggs, thus, do not need venom to subdue or immobilise their prey. The venom of turtleheaded sea snakes is far less toxic than the venom of other species, including sea snake species closely related to turtleheaded sea snakes.

Hydrophiine sea snakes are ovoviviparous - which means they give birth to live young. They evolved from a group of Australian live-bearing terrestrial snakes, which includes the tiger snakes, death adders, swamp snakes and whip snakes. Hydrophiine sea snakes have exploited the viviparity of their terrestrial ancestors and freed themselves entirely from the need to return to land to breed.

Hydrophiine sea snakes are by far the youngest radiation of sea snakes. The exact dates when this lineage evolved is still uncertain, however, they almost certainly evolved less than 15 million years ago, and maybe as recently as 10 million years ago. One reason it is difficult to determine the dates when this lineage evolved is because few fossils have been found of snakes that were closely related to the marine hydrophiines.

Laticauda - sea kraits

The genus Laticauda, or sea kraits, comprise approximately seven species, six of which are marine. They are strongly banded and commonly seen in large numbers on beaches in south east Asia and some Pacific Islands.

Sea kraits also evolved from terrestrial elapids, however, they evolved separately from the marine hydrophiines. In fact, the genus Laticauda evolved much earlier than the marine hydrophiines. The exact date when this lineage evolved is also uncertain, however, it is likely that they evolved 15-20 million years ago.

As members of the family Elapidae, Laticauda are also proteroglyphs and have highly toxic venom. However sea kraits are generally very placid and unlikely to bite unless provoked.

Sea kraits are the only group of sea snakes that are oviparous (egg laying) and must return to land to breed.

Acrochordidae - file snakes

The damily Acrochordidae or file snakes is the oldest sea snake lineage, having evolved ~40 million years ago. There are three extant species in this family and two of these species are marine.

File snakes are not venomous and they give birth to live young.

Homalopsidae - mangrove or mud snakes

The family Homalopsidae comprises ~34 species of aquatic or semi-aquatic snakes. Approximately 10 species occur in marine habitats such as mudflats, mangroves and estuaries across southeast Asia and into northern Australia, however only three species are fully marine. This family belongs to the superfamily Colubroidae and evolved 15-25 million years ago.

Homalopsids are venomous but they are rear-fanged. They also give birth to live young.

Natricinae - salt marsh snakes

The Natricinae also belong to the superfamily Colubroidea and are confined almost entirely to salt marsh environments. Three species occur in marine habitats and they are confined to temperate and subtropical north America.

Natricines also give birth to live young. They are not venomous. It is thought that the marine natricine species might be in the early stages of evolving adaptations to the marine environment.
Further Reading:
General
Heatwole H (1999) Sea snakes. University of New South Wales Press. Sydney.
Murphy J (2007) Homalopsid Snakes. Evolution in the mud. Krieger Publishing, Malabar.
Scientific Literature
Alfaro ME and others (2008) Phylogeny, evolutionary history, and biogeography of Oriental-Australian rear-fanged water snakes (Colubroidea: Homalopsidae) inferred from mitochondrial and nuclear sequences. Molecular Phylogenetics and Evolution 46: 576-593
Lukoschek V & Keogh JS (2006) Molecular phylogeny of sea snakes reveals a rapidly diverged adaptive radiation. Biological Journal of the Linnean Society 89: 523-539