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So Here's How Sea Snakes Find Fresh Water To Drink In A Salty Ocean

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SEA SNAKE
Yellow-bellied sea snake. | TOM MCHUGH via Getty Images

(Inside Science) -- Like most creatures, sea snakes need to hydrate from time to time, yet they live in a world of mostly undrinkable sea water. What’s a thirsty sea snake to do?

According to researchers at the University of Florida in Gainesville, they find places where it is raining heavily, wait for pools -- the scientists call them “lenses”-- of fresh water to form on the surface, and drink. They have the handy advantage of not needing to do that very often, sometimes going six or seven months without a drink.

The snakes studied by Harvey Lillywhite and colleagues are the yellow-bellied sea snake, a venomous animal that is the most widely distributed reptile in the world, and the only pelagic snake, meaning it lives in the open sea, often far from any land. Most of these snakes probably never leave the ocean.

Their study was published in the Proceedings of the Royal Society in England.

The yellow-bellied sea snake ranges from the coast of Southeastern Africa, across the Indo-Pacific to the shores of Central America. The Florida scientists caught their snakes using nets off the coast of Costa Rica.

The snakes can grow to be greater than a yard long, although the ones Lillywhite catches are generally a bit smaller. They have flattened tails that act as paddles. Like all snakes and marine mammals, they must breathe air to live even if they do spend their entire lives in the sea.

The yellow-bellied variety are called true sea snakes because they never go on land willingly, and those that do wind up ashore have trouble maneuvering, according to Jack Cover, general curator at the National Aquarium in Baltimore who has captured snakes off the Panamanian coast.

Other sea creatures rely on complex methods to hydrate. Dolphins gain water from the bodies of the fish they eat. They also invariably ingest saltwater but have physiological ways of excreting the salt, helped by special structures in their kidneys. Seals in polar climes eat snow.

Sea snakes rely mostly on lenses of fresh water that pool on the ocean surface.

“When you have a good rain storm, there’s a lot of fresh water falling on the ocean surface. It’s less dense than sea water so it tends to float” Lillywhite said. “How big the lens is, how pure it is, and how long it lasts, depends on how much rainfall there is and the nature of the mixing conditions at the time, driven by wind, and other factors.”

The water could be brackish, he said, but not very. If it is raining or just rained, it would be pure. The lenses can be thicker than 3 feet, and can persist for several days.

The snakes come up from below and drink from the collecting lenses.

How they find the lenses is another question. The snakes seem to know where it rains in the vicinity.

“I do not think these snakes go very far looking for the rain,” Lillywhite said. “Wherever they are, they come up to breathe air. If it is raining, they will detect it and take a drink.”

Some snakes can detect atmospheric pressure and can detect when a storm is approaching or if it is raining, research has shown.

Since the snakes rely on currents for much of their movements, opportunities for hydration are unpredictable. Months can go by with a snake encountering little or no rainfall, Lillywhite said. They would have to drift to where it’s raining.

But, these snakes are built for survival and are able to survive for months without a good drink.

Snakes lose body water slowly in sea water, but are able to retain a great amount of it for a long time. Their skins also are impermeable to the sea water.

A hydrated snake is up to 80 percent water, according to Lillywhite. Most animals, including humans, have around 60 percent.

The snakes can survive a body-water content low enough to kill a human, he said.

More important, thanks to efficient salt glands, they excrete the salt from sea water they ingest, Cover said.

The snakes are venomous, but Lillywhite said he has never been bitten. He has talked to herpetologists who have been bitten, but they report that nothing much happened; either the snakes did not inject venom -- dry bites -- or the venom isn’t as effective on humans as it is on fish. Many sea snakes, including a supposedly deadly variety known as sea kraits, just don’t bite.

“The snakes we study can be a little snappy. We just keep our hands far, far away from them,” said Lillywhite.

Cover said they are often very aggressive, dangerous and have to be handled carefully. Their venom, like cobra venom, is a neurotoxin and there have been fatalities. Fishermen in the Philippines often bring them up in fish nets and are bitten.

Reprinted with permission from Inside Science, an editorially independent news outlet of the American Institute of Physics, a nonprofit organization supporting the dissemination of science news and information to society.

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