Reptiles
Yellow-bellied Sea Snake
Hydrophis platurus · also called yellow-bellied sea snake, pelagic sea snake, yellow-and-black sea snake
The yellow-bellied sea snake is a snake that has fully committed to the ocean. It never touches land, never lays an egg on a beach, and cannot crawl if it ends up on one. It drifts at the surface of half the world’s tropical seas — the single most widely distributed snake species on Earth — waiting for fish to gather in its shadow.
Overview
The yellow-bellied sea snake lives in the surface waters of the tropical Indian and Pacific Oceans, from East Africa to the Pacific coast of the Americas — everywhere warm except the Atlantic. It is small (65–90 cm), jet-black above and bright yellow below, with a flattened, paddle-shaped tail. It is the only sea snake that is fully pelagic, living entirely in open water with no need of reef or shore, and it turns up thousands of kilometres from any coast.
An ocean animal
It has a compressed body and oar tail for swimming, one greatly elongated lung plus skin breathing for dives up to a couple of hours, a salt-excreting gland under the tongue, and it gives live birth in the water. It hunts by floating motionless along drift lines — the calm ribbons where currents meet and small fish shelter — and striking sideways or backward.
Drinking rain
It cannot drink seawater. It rehydrates from a thin lens of freshwater that floats on the sea surface right after heavy rain, and it can go months dehydrated — losing a quarter of its body water — between storms.
Venom, and people
Its venom is potently neurotoxic, but it is rarely dangerous — small fangs, small mouth, mild in the water, and no reason to attack a person. Almost all bites come from handling stranded snakes or removing them from nets; antivenom exists.
Strandings
It cannot steer against currents or crawl off a beach. Onshore storms carry drifting snakes ashore, where they lie immobilised and die — mass strandings of dozens or hundreds after a storm are documented on Pacific coasts.
Status
Least Concern, abundant across an enormous range.
Yellow-bellied Sea Snake: common questions
Is the yellow-bellied sea snake dangerous to swimmers?
It has a highly potent neurotoxic venom, comparable to that of its cobra relatives, but it is very rarely a danger to people. It has small fangs, a small mouth, and a mild temperament in the water, it does not defend a territory or a nest, and it strikes to catch fish, not to attack large animals. Recorded bites on humans are almost all from people handling stranded snakes on beaches or fishers removing them from nets, and even then many bites are "dry" (no venom injected). There is antivenom. A yellow-bellied sea snake seen swimming or drifting at the surface should simply be left alone; it will not pursue a person.
How does a snake live its whole life in the open ocean?
The yellow-bellied sea snake is completely adapted to a pelagic life. It has a flattened, oar-like tail for swimming and a compressed body; it can absorb some oxygen through its skin and has a single greatly elongated lung, letting it stay submerged for up to a couple of hours; it gives birth to live young in the water, so it never needs to come ashore to lay eggs; and it excretes excess salt through a gland under the tongue. It spends much of its time drifting passively along drift lines — the ribbons of calm water, foam, and floating debris where currents meet — where small fish congregate and where it can ambush them.
How does it get fresh water if it lives in the sea?
This was a long-standing puzzle, because sea snakes were assumed to drink seawater and desalinate it with their salt gland. Research showed instead that the salt gland is not efficient enough for that, and that the yellow-bellied sea snake becomes progressively dehydrated during dry periods, losing up to a quarter of its body mass in water. It rehydrates by drinking from a thin layer of freshwater — a "lens" a few centimetres thick — that floats, unmixed, on top of the denser seawater immediately after heavy rain. Snakes will gather at the surface to drink during and just after a storm. In regions with a long dry season, this ties their distribution to rainfall patterns.
Why does it wash up on beaches, sometimes in huge numbers?
Because it cannot control where the currents take it, and it cannot get itself off a beach once it is there. The yellow-bellied sea snake drifts at the surface, and when a strong onshore wind or storm pushes the surface water toward a coast, snakes are carried in and stranded on the sand along with other floating life. On land they are essentially immobilised — their bodies are built for swimming, not crawling — so they lie helplessly at the tide line, where they overheat, dehydrate, or are taken by gulls and crabs. Mass strandings of dozens or hundreds after a storm are well documented, for example on the Pacific coasts of Costa Rica and, rarely, California.