Reptiles
Sea Snake
Hydrophiinae · also called coral reef snake
A sea snake is a cobra that moved into the ocean. It kept the potent neurotoxic venom of its land relatives and added a paddle-shaped tail, a body-length lung that doubles as an air tank, skin that breathes, and a gland to spit out salt. Most species now cannot survive on land at all.
Overview
Sea snakes are the subfamily Hydrophiinae, around 70 species, in the warm coastal waters of the Indian and Pacific oceans — there are none in the Atlantic. They are elapids, close relatives of cobras and mambas, and evolved from Australian land snakes only a few million years ago. All are carnivores, eating mostly fish, and all are venomous.
Built for the water
The tail is flattened into an oar, and the snake swims by undulating like an eel. Its single lung runs almost the whole body, storing air and controlling buoyancy for dives of 30 minutes to two hours, helped by taking up to a third of its oxygen through the skin. A salt gland under the tongue flicks out excess brine. Fully marine species give birth at sea and have belly scales too reduced to crawl.
Sea kraits
The sea kraits are the amphibious cousins — they hunt eels in the sea but come ashore to rest, drink fresh water, and lay eggs, and they keep the broad belly scales of a land snake.
Deadly but harmless in practice
Sea snake venom is among the most toxic of any snake, but the animals are placid, often bite dry, and have short fangs that struggle through a wetsuit, so they cause very few deaths — mostly among fishers handling nets.
Status
Sea snakes as a group are Not Evaluated. Many are common on healthy reefs, but several Australian species vanished from Ashmore Reef, and sea snakes generally decline where reefs are degraded, where they are taken as trawl bycatch, and where they are harvested for skins and meat.
Sea Snake: common questions
How dangerous are sea snakes to people?
Their venom is among the most toxic of any snake, being strongly neurotoxic and often also attacking muscle, so a fully envenomated bite without treatment can be fatal. In practice, though, sea snakes cause remarkably few deaths. They are not aggressive, most bites happen when a fisher grabs one in a net or a wader steps on one, a large fraction of bites deliver little or no venom, and the fangs are short and often fail to get through clothing or a wetsuit. Divers who leave sea snakes alone are almost never bitten. Where deaths do occur they are mostly among fishing communities in South and Southeast Asia.
How can a sea snake breathe underwater?
It cannot extract oxygen from water the way a fish does, but it has several adaptations that let it stay submerged for a very long time between breaths. Its single lung extends through most of the body and holds a large air reserve. It can absorb a meaningful share of its oxygen, roughly a fifth to a third, directly through its skin from the surrounding water, and offload carbon dioxide the same way, and its metabolism slows while diving. Together these let a foraging sea snake remain underwater for 30 minutes to two hours before it must surface to refill the lung.
What is the difference between a sea snake and a sea krait?
They are closely related but represent two stages of adapting to the ocean. True sea snakes are fully marine; they mate, feed, and give birth to live young entirely at sea, have strongly reduced belly scales, and are almost helpless on land. Sea kraits are amphibious; they hunt in the sea, mostly for eels, but return to land regularly to rest, digest, drink fresh water, shed, and lay eggs, and they keep the broad belly scales of a land snake and move well ashore. Sea kraits are banded black and pale and are a common sight on tropical beaches and rocky islets.
Are there sea snakes in the Atlantic Ocean or the Caribbean?
No. Sea snakes are found only in the Indian and Pacific oceans. They evolved relatively recently in the Australasian region and spread west to Africa and east across the Pacific, but they have never colonised the Atlantic. The yellow-bellied sea snake, the most widespread species and the only truly open-ocean one, reaches the Pacific coast of the Americas, but the cold water around the southern tip of South America and Africa has blocked it from rounding into the Atlantic. So swimmers and divers in the Caribbean, the Mediterranean, and along both American Atlantic coasts will never meet one.