Pelagic yellow-bellied sea snakes spend their entire lives in the ocean, but cannot quench their thirst with sea water. Instead, they wait until it rains to drink fresh water, sometimes staying dehydrated for 6 or 7 months.
Some marine reptiles, like sea turtles, can drink sea water to meet their needs, distilling water by excreting excess salt through salt glands. “These snakes refuse to drink salt water, even when dehydrated,” says Harvey Lillywhite from the University of Florida. “They need fresh water to survive.” At sea, sips of fresh water depend on seasonal rainfall.
Lillywhite and colleagues spent three years studying yellow-bellied sea snakes (Hydrophis platurus) living in open Pacific waters off the coast of Guanacaste, Costa Rica. They collected over 500 live snakes during the course of 10 trips and found that, both in the field and in the lab, sea snakes shunned salt water, taking a drink only when fresh water was available.
But the open ocean is a virtual desert during the dry season from December through May or June. During that time, sea snakes could lose up to 25 percent of their body mass, levels that would be way past lethal for a human. If they were captured during a dry spell, they were light and dehydrated, so they drank heavily; if they were caught in a wet month, they were plump from water and they’d hardly drink.
During the heavy rainfall in the wet season, they rehydrate by coming up to drink from the “freshwater lens” that forms temporarily on the ocean surface. Since rainfall is less dense than sea water, it floats on the surface, forming a layer that may persist for days as fresh water or dilute brackish water. In lagoons, where lots of sea snakes hang out, a freshwater lens might persist longer than in the open ocean. The snakes consume varying quantities: from small amounts up to 25 percent of their body mass. In case you're wondering, they don’t lap water. “The snake simply gulps water and then closes the mouth while water is forced into the stomach,” Lillywhite tells Discovery.
The snakes appear to sense rainfall. After all, without gills, they must regularly surface for air. “We think they almost certainly know that it rains because their behavior changes during the approach of a tropical storm as the atmospheric pressure changes,” Lillywhite explains in a press release.
They also confirmed that the snakes do, in fact, excrete salt. Small glands around their tongues eliminate salt from the fish they eat or from the water gulped while eating. But the glands are nowhere near efficient enough to handle the massive amounts of salt ingested if they were to actually drink sea water.
To survive severe dehydration, they have skin that's impermeable to sodium, and they lose water slowly. They also cope by starting with an unusually high amount of water in their bodies to begin with: The body water content of hydrated snakes was around 80 percent. Freshwater snakes and sea turtles measured about 70 and 65 percent, respectively.
However, “in areas of intensifying drought, they will need to move or die out,” Lillywhite says. If global climate change causes drought conditions to worsen, sea snakes will continue to decline.
The work was published in Proceedings of the Royal Society B: Biological Sciences this week.