The Thirsty Little Snake That Swam Across the World

If someone asked you to think about a global animal that has spread over much of the earth, you’ll probably think of something like the brown rat, the rock pigeon, or us humans. You probably won’t think about the yellow-bellied sea snake.

It’s a striking animal—two to three feet in length, with a black back and yellow belly. And it is extraordinarily far-ranging for a snake. It lives throughout the Pacific Ocean, which is already more area than all the continents combined, and the Indian Ocean too. Of all the tetrapods—the animal group that includes mammals, birds, reptiles, and amphibians—this little-known snake is one of the most abundant and widespread.

“Off the coast of Costa Rica, there are some good days where you can observe hundreds or thousands of them over a couple of hours,” says the French scientist Francois Brischoux, who has been studying this species for a decade.

The 62 species of sea snakes are all wonderfully adapted to life in the oceans, but they almost always come ashore to lay eggs. But not the yellow-bellied one; it is the only member of the group that lives full-time in the open ocean. It eats at sea, mates at sea, and gives birth to live young at sea. It has special valves in its nose to stop water from getting in, and can even partially breathe through its skin. It hunts by sitting amid flotsam and picking off small fish that gather beneath it. And it swims by propelling itself with a flattened, paddle-like tail.

And yet, in some ways, it is so ill-suited to life in the ocean that its existence borders on poetic tragedy. For example, a few years ago, Brischoux and his colleague Harvey Lillywhite from the University of Florida showed that the yellow-bellied sea snake is almost constantly thirsty and dehydrated.

If you tried to swallow water in the ocean, your kidneys would remove the extra salt by diluting it in urine. In doing so, you’d actually get rid of more water than you ingested. This is why, when humans drink seawater, they get dehydrated. Some marine animals cope with this problem using special salt-removing glands, but Lillywhite showed that—contrary to what scientists previously believed—sea snakes do not. They live most of their lives in the oceans, but they never swallow seawater. Instead, they try quench their thirst with fresh water.

Some species stick close to coastal sites with nearby sources of fresh water, like springs or streams that empty into the sea. But the yellow-bellied sea snake has no such option. Instead, it drinks from the thin layers of freshwater that briefly form on the surface of the ocean when it rains. That seems precarious, and it is. For much of the year, from November to May, these snakes are almost constantly dehydrated.

The yellow-bellied sea snake isn’t a great swimmer either. “It is really small,” says Brischoux. “It can move in the water, but not for a very long period of time and not against really strong currents—unlike, say, a seal.” So how could it possibly occupy such a large range? The only other tetrapods that are so widespread are either powerful swimmers like the giant whales or strong fliers like seabirds. The yellow-bellied sea snake is neither, and yet it has spread over two-thirds of the Earth’s surface.

Brischoux thinks that it did so passively. He teamed up with oceanographer Philippe Gaspar to create an accurate simulation of the world’s oceans, including currents and temperatures. He then dropped 10,000 virtual sea snakes into 28 sites within this artificial world, and allowed them to drift for ten years. “We assumed that the snakes were inert particles and let them move in the current,” he says.

And yet, the simulations did a pretty good job of emulating the actual biogeography of the snakes. As a population, they spread all over the Pacific and Indian Oceans, but were especially common around Indonesia—the very site where their species originated.

As individuals, the virtual snakes didn’t stick to their sites of release, but drifted far and wide. Those that survived the full decade traveled 30,000 kilometers on average, and 100,000 kilometers at most. On their aimless travels, they often encountered other sea snakes that had come from very different parts of the globe, which explains why real populations from opposite ends of the Pacific are so genetically similar.

It’s an innovative, well-planned, and interesting study, says Harold Heatwole, a sea snake expert at North Carolina State University. It helps to explain not just how these hard-to-study animals get to where they live, but also why they don’t exist in places like the west coast of South America—perhaps the currents there are too cold, or flowing in the wrong direction.

As technology improves, it may eventually be possible to fix GPS loggers to these snakes to find out exactly where they go. But for now, Brischoux and Gaspar’s simulations show that they can survive and thrive by simply following oceanic surface currents, acting as little more than long bits of plankton. Through minimal effort, these ironic snakes that thirst constantly and swim poorly have conquered the oceans.