A deep dive into sea snakes, sea kraits and their aquatic adaptations

What is a sea snake?

‘Not all snakes living in the sea are sea snakes,’ explains Dr David Gower, Merit Researcher at the Museum and a specialist in snakes.

Biologists use the term ‘true sea snake’ for any of the 64 recognised species of snakes of the family Elapidae that spend the majority of their lives in the ocean. They even give birth to young in the water. This group otherwise contains land-based snakes such as cobras and mambas.

There are also eight species of sea krait, Laticauda, a semi-aquatic group of reptiles that return to land to lay eggs. Three species of sea krait are also found in freshwater habitats. 

‘The true sea snakes are more closely related to terrestrial elapids of Australasia than they are to sea kraits,’ says David.

Every species of sea snake has fangs at the front of their mouth which permanently stand up. This is a characteristic of members of Elapidae. They can deliver a powerful toxin via their fangs and because they inject it, marine snakes are considered venomous, rather than poisonous.  

The turtle-headed sea snakes, Emydocephalus, are an exception because they have a diet of fish eggs and lack venom. 

Master mariners

Sea snakes are generally found in shallow, tropical waters from the eastern Pacific Ocean to the Indian Ocean.

Most species favour coastal waters where they hunt in reefs, seagrass meadows or on the muddy sea floor. The yellow-bellied sea snake, Hydrophis platurus, can also be found far out to sea, preying upon fish near the surface of the water.

‘The yellow-bellied sea snake is exceptionally widely distributed, in almost all tropical waters other than the Atlantic, so it is perhaps one of the more likely species to be encountered, including individuals that might be washed up on beaches,’ says David.

To help them swim, true sea snakes have evolved tails shaped like paddles and narrow scales along their bellies so that they can maintain a laterally compressed shape to aid swimming. Sea kraits have paddle-like tails too but they have retained the larger belly scales found in their terrestrial ancestors so they can still move efficiently on land.   

Sea snakes have solved a major challenge of life in saltwater thanks to specialised glands in the floor of their mouth that allow them to excrete excess salt.

‘Sea snakes show various adaptations to marine life, including having specialised sensory organs on their scales to detect water-borne vibrations and having visual pigments in their retinas that are more tuned away from the wavelengths of light that don’t reach deeply into the water,’ explains David.

Are sea snakes dangerous?

As part of his research, David joined an expedition off the northwest coast of Australia, working with colleagues and world experts to take tissue samples from around 70 sea snakes, of seven species. Working at night to capture and clip the tail scales of these venomous snakes sounds highly perilous, but the snakes pose little threat to humans if handled with care.

‘In general, sea snakes are mellow and not interested in biting humans - unless they are grabbed or picked up aggressively. Most bites from sea snakes are to people trying to remove them from being accidentally tangled in their fishing nets,’ explains David.

Sea snakes have relatively small fangs and venom is not always injected with a bite. With bites often occurring in remote places, such as out at sea or in small fishing villages, it’s clear why records of sea snake bites are incomplete.

While sea snake bites are certainly far less numerous than terrestrial snake bites, the powerful neurotoxin in the venom causes paralysis, including in the respiratory muscles, which can prove fatal.

The title of ‘most venomous sea snake’ is a subject of debate, thanks in part to an incorrect record that named Belcher’s sea snake, Hydrophis belcheri, in a popular book of facts published in the 1990s. Scientists now agree Dubois’ sea snake, Aipysurus duboisii, found off the coasts of Papua New Guinea, New Caledonia and Australia, actually has a more potent venom among sea snakes.

Available data are very patchy but the deadliest sea snake in terms of human fatalities may be the beaked sea snake, Hydrophis schistosus. It’s described as aggressive and relatively frequently encountered by fishermen in the coastal waters of India.

Sea snake eyes

‘One of the aspects of snakes that I am fascinated by is their sensory systems, especially vision,’ says David.

‘There is an ongoing debate about how snakes evolved from lizards - two of the leading hypotheses are that they became snake-like by acquiring features that were adaptions to either burrowing in soils or to life in the sea, so I have been addressing aspects of this debate by carrying out research on the visual systems of these two groups of snakes.’

Snakes lost some parts of their ancestors’ colour vision abilities tens of millions of years ago as they adapted to low-light habitats, but recent research suggests sea snakes have ‘re-evolved’ some of their ability for colour sensitivity. Sea snakes experience highly variable light conditions as they dive for food. Underwater, blue light dominates, and yellow and red light is greatly reduced. They then return to the light of the surface in order to breathe.

Due to this diving behaviour, sea snakes are experts at holding their breath. In 2019 scientists reported the first evidence of snakes swimming to depths of more than 200 metres, into the cool waters of the ‘twilight zone’, possibly to hunt. Yet sea snakes don’t have gills to obtain oxygen from the water like fish.

As well as having lungs that they fill with air during visits to the surface, sea snakes are also able to absorb oxygen into their blood through their skin. Researchers have discovered that the annulated sea snake, Hydrophis cyanocinctus, has a dense network of veins on its forehead and snout that probably helps to get more oxygen to its brain during longer periods underwater.

What eats sea snakes?

Fossil evidence of the ancient giant marine snake Palaeophis colossaeus, which measured up to nine metres long, has been discovered in Morocco, firing the imagination with images of enormous sea serpents. But these extinct marine snakes are not very closely related to modern-day sea snakes and sea kraits.

Modern day species also fall short of this monstrous size. The yellow sea snake, Hydrophis spiralis, is recognised as perhaps the longest living sea snake in the world, recorded at nearly 3 metres long.

Most sea snakes feed on fish. Their predators include sharks, large eels, sea eagles and large bony fish such as groupers. There are even records of dolphins hunting the reptiles and of a sea snake being regurgitated by a leopard seal. 

Are sea snakes endangered?

Sea snakes and sea kraits are under threat from humans.

Historically, black-banded sea kraits, Laticauda semifasciata, were hunted for their meat, skin and organs in the Philippines and Japanese islands. Their numbers have not yet recovered following massive harvests in the 1970s and 1980s.

There is currently no CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora) protection for any sea snake species.

One of their greatest threats is as bycatch, when they accidentally become entangled in fishing equipment. Climate change also threatens these animals because they are finely adapted to their environment and will suffer the effects of rising sea temperatures and degradation of coral habitats.

A 2009 assessment indicated that 10% of sea snake species were at elevated risk of extinction, with the leaf-scaled sea snake, Aipysurus foliosquama, and short-nosed sea snake, Aipysurus apraefrontalis, considered critically endangered. Examples of both species were discovered off the western coast of Australia in 2015 however, suggesting more attention is needed for these unique marine reptiles.

David says a key way to protect these fascinating snakes is ‘by better studying their biology so that we fully understand the inventory of species, their natural histories, distributions, ecological tolerances and conservation threats.’