Sea Snakes Conquered by Salt

by Elizabeth Pennisi

PHOENIX, ARIZONA--Shipwrecked sailors shouldn't drink ocean water no matter how thirsty they get. And neither should sea snakes. Contrary to the current dogma, at least some of these serpentine mariners must have freshwater to survive. Research shows that without it, at least one group of sea snakes--and likely others--will gradually waste away, researchers reported here yesterday at the annual meeting of the Society for Integrative and Comparative Biology. The need for access to fresh water may limit where these snakes can live, explaining their patchy distribution along certain coastlines.

All organisms must work to keep dehydration in check. Kidneys concentrate urine to conserve water, and many marine animals have special adaptations for getting rid of the excess salt taken in from the surrounding environment. Sea snakes--dozens of species of which live in the open ocean, while a few others hang out inshore--have a gland under their tongues for this purpose. Researchers have long assumed that this gland worked so well that the snakes could get away with sipping salt water whenever they needed a drink.

But Harvey Lillywhite, an ecological physiologist at the University of Florida in Gainesville, began to suspect otherwise when he had trouble keeping file snakes, which live almost fulltime in the ocean, alive in his lab. He discovered the snakes did fine once he put them in fresh water and began to wonder if the same was true of other marine snakes.

With the help of Ming Tu from the National Taiwan Normal University in Taipei, Lillywhite and his colleagues collected three species of sea kraits, snakes that live in the coastal waters of islands off Taiwan but, at the very least, come ashore to lay their eggs, usually in rocky caves close to the intertidal zone. Two of the species also visit land occasionally. All have the brine-secreting gland, suggesting they are well adapted to constant immersion in salt water.

For their experiments, the researchers first took the snakes out of water long enough to allow them to dehydrate. They then put the snakes in different concentrations of seawater. None of the dehydrated snakes tried to drink anything that was 50% or more salt water (They live in full-strength seawater.) But they did gulp down water fresh water and imbibed 25% saltwater concentrations, Lillywhite reported.

In a second study, Lillywhite's team tracked the weight of the snakes for 10 days. For the experiment, they kept the snakes in the seawater without food. The researchers placed half of the snakes in fresh water every other day for an hour. All the snakes experienced dehydration and lost weight, but the ones exposed to fresh water lost significantly less, says Lillywhite.

The results help explain the demographics of these Taiwanese snakes, Lillywhite says. They tend to be most plentiful along the shore, where there are springs or other sources of fresh water nearby. Furthermore, there are more sea snake species in areas with higher mean annual rainfalls, notes Lillywhite. Under calm conditions, thin layers of rain will float on top of the salt water, apparently providing ample supplies for the snakes.

It's a "major finding," says Harold Heatwole, an ecologist at North Carolina State University in Raleigh. Physiologist Lisa Hazard from Montclair State University in Upper Montclair, New Jersey, agrees. "He shows pretty clearly that [sea snakes] have to have access to fresh water," she says.

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