Sea Snakes Need Fresh Water For Drinking
Cindy Spence, University of Florida
Although they spend their lives surrounded by water, sea snakes dehydrate for months at a time, waiting to quench their thirst with fresh water from rainfall, a University of Florida biologist has found.
The finding contradicts the accepted belief that marine vertebrates have evolved to use salt water to meet their water requirements, said professor Harvey Lillywhite, whose research appears [Wednesday/today] in the Proceedings of the Royal Society B, the flagship biological research journal of the Royal Society.
“These snakes refuse to drink salt water, even when dehydrated,” Lillywhite said. “They need fresh water to survive.”
Current physiology textbooks state that marine reptiles drink sea water, distilling the water by excreting excess salt via salt glands. While it is true that they excrete salt, Lillywhite said, “no sea snake we have tested drinks sea water.”
The findings are the result of three years of field studies of the Yellow-bellied Sea Snake, the most widely distributed pelagic sea snake, which inhabits tropical oceans. Lillywhite and his colleagues also studied the sea snakes in the laboratory.
Both in the field and in the lab, sea snakes shunned salt water, taking a drink only when fresh water was available. At sea, sips of fresh water depend on rainfall.
Rainfall is less dense than sea water, so it floats on the surface, forming a “lens” of fresh water. Such layers of water may persist as fresh water or dilute brackish water at the ocean surface for days, but snakes probably drink the rainwater during or shortly after a rainstorm. In lagoons, where sea snakes often are abundant, a fresh water lens might persist longer than one in the open ocean.
The snakes appear to sense rainfall.
“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 said.
During or after a rain, the snakes, which spend most of their time deeper in the water, surface to take a sip of fresh water. The Yellow-bellied Sea Snake, Lillywhite said, consumes varying quantities, from small amounts up to 25 percent of its body mass when fresh water is available.
The open ocean is a virtual desert, especially during the dry season, which can last six or seven months at Guanacaste, Costa Rica, where the snakes were studied, Lillywhite said. During that time, sea snakes slowly dehydrate, and lose possibly up to 25 percent of their body mass, a level that would be “way past lethal for a human,” Lillywhite said.
Lillywhite said diminishing rainfall might be the cause of declining populations of sea snakes in some areas, such as drought-stricken Northern Australia, where sea snake populations have declined for 10 years and two local species are thought now to be extinct.
If global climate change causes drought conditions to worsen, sea snakes and other marine vertebrates that depend on rainfall for fresh water could be hurt, Lillywhite said. There are more than 60 species of entirely marine sea snakes and eight species of sea kraits that live in the sea but spend some time on shore. Lillywhite’s study also raises the question of whether other marine species might depend more on fresh water than previously thought.
An interesting follow-up project, he said, would be to study sea turtles – which live in salt water and have a broad distribution similar to the pelagic species of sea snake – to determine if they depend, even partially, on fresh water.
“Understanding the water requirements and drinking behaviors of marine vertebrates could help with conservation efforts,” Lillywhite said. “In areas of intensifying drought, they will need to move or die out.”