Most yellow-bellied sea snakes spend their entire lives at sea. They rarely end up on land and are vulnerable there, since their paddle-shaped tails and keeled undersides make crawling difficult. Armed with potent venom, they drift in a vast territory that encompasses much of the world’s oceans, riding the currents and hunting fish near the sea surface.

Like other reptiles, these creatures need to drink water to survive. How does an animal surrounded by saltwater quench its thirst?

It used to be thought that these serpents drank from their salty surroundings. “Previous textbook dogma was that sea snakes drank seawater and excreted the excess salts using their sublingual salt glands,” explains Harvey Lillywhite, a biologist at the University of Florida.

Recent work has proven that false—and a new study suggests that yellow-bellied sea snakes (Hydrophis platurus) rehydrate at sea by drinking rainwater that collects on the ocean surface.

Water, water everywhere…

Work by Lillywhite and colleagues has shown that various sea snake species don’t drink pure saltwater—even when they’re dehydrated. And while sea snakes do have glands that secrete salt, they are proportionately small and secrete the mineral slowly, making them unable to give the animals all the freshwater they need.

A new study, published this month in the journal PLOS ONE, shows that the snakes are finding freshwater. In May 2017, Lillywhite and colleagues were in Costa Rica to study the snakes, during which time the six-month dry season suddenly ended with a deluge of rain. The researchers captured 99 yellow-bellied sea snakes over the course of the trip, before and after the rain’s arrival.

View Images Yellow-bellied sea snakes rarely come ashore. This mother was washed up on the beach and then gave birth to the baby. Somehow, they find fresh water out in the ocean. Photograph by Adrian Hepworth, Alamy

They brought the snakes into the lab and offered them freshwater, finding that 80 percent of those brought in before the rains drank—but over the next five days of rainy weather, that percentage continually dropped among newly captured snakes. Eventually, only about 10 percent of snakes brought in took the offer to rehydrate, a dramatic drop in only a few days.

“If a snake drinks, it is thirsty,” says Lillywhite. “If a snake is thirsty, it is dehydrated. If it is dehydrated when taken from the ocean—during the dry season—this means it is not drinking seawater as textbooks once stated.”

And not a drop to drink?

Since fewer sea snakes were thirsty as the dry season transitioned to wet, the snakes must have been quenching their thirst. As rain falls, the uppermost part of the water’s surface is diluted, creating a temporary freshwater “lens” that doesn’t immediately mix with saltwater. If the salinity falls enough, the sea snakes can drink from this surface layer and rehydrate after months without water.

The results help solidify earlier observations in the laboratory showing that the sea snakes were reliant on freshwater, despite their saltwater habitat. But by uncovering just how the reptiles exploit temporary weather conditions to access life-sustaining water, the researchers have identified where their coveted freshwater source comes from in the wild.

“I found this study very interesting,” says Vinay Udyawer, a marine ecologist at the Australian Institute of Marine Science not involved in this study. “It provides a fascinating insight into a rarely observed aspect of sea snake biology.”

For Udyawer, the results speak to the exceptional importance of freshwater to the lives of sea snakes as a group, even with a life spent in saltwater.

This reliance on freshwater “often limits their distributions, with sea snake populations often very patchy and close to large sources of freshwater like river mouths, estuaries or springs,” says Udyawer.

An animal completely at the whim of open ocean currents, like the yellow-bellied sea snake, says Udyawer, must have some way to hydrate far from any of these land-based water sources. The findings not only reveal how yellow-bellied sea snakes can survive long periods at sea, but also suggest a way that other marine vertebrates could be quenching their thirst.

But dependence on transient weather events like at-sea rainstorms could be even more risky in the face of ongoing global climate change, which is predicted to come with longer and more intense droughts. The animals could die of thirst if cut off from life-giving rainfall, Lillywhite says.

Watch: Sea Snake Swallows Eel Whole

A pivotal next step would be to directly observe sea snakes drinking from freshwater lenses in the ocean, Lillywhite says—an event which would, of course, be extremely difficult to capture.

Kanishka Ukuwela, an evolutionary biologist at Rajarata University in Sri Lanka also not involved in this study, says the paper has raised more fascinating questions.