The Thirsty Little Snake That Swam Across the World
It’s always dehydrated, and it's not a great swimmer, but it can somehow cross oceans.
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.