Photo by Leslie Babonis/UF Department of Zoology
Some species of sea snake need freshwater to survive, a University of Florida zoologist has discovered.
Harvey Lillywhite says it has been the “long-standing dogma” that the roughly 60 species of venomous sea snakes worldwide slake their thirst by drinking seawater, with internal salt glands filtering and excreting the salt.
“Experiments with three species of captive sea kraits captured near Taiwan, however, found that the snakes refused to drink saltwater even if thirsty — and then would drink only freshwater or heavily diluted saltwater,” he says.
“Our experiments demonstrate they actually dehydrate in sea water, and they’ll only drink freshwater, or highly diluted brackish water with small concentrations of saltwater,” Lilywhite says.
The research may help explain why sea snakes tend to have patchy distributions and are most common in regions with abundant rainfall, Lillywhite said.
The findings are published in the online edition of the November/December issue of the journal Physiological and Biochemical Zoology.
Because global climate change tends to accentuate droughts in tropical regions, the findings also suggest that at least some species of sea snakes could be threatened now or in the future, he added.
“There may be places where sea snakes are barely getting enough water now,” he said. “If the rainfall is reduced just a bit, they’ll either die out or have to move.”
In lab studies, Lilywhite’s team kept snakes caught in the wild near Orchid Island, Taiwan, away from freshwater for two weeks. At the end of that period, dimpling of the snakes’ scales indicated they were dehydrated.
The researchers weighed the snakes, freed them in saltwater tanks for up to 20 hours, then weighed them again. None gained appreciably, indicating they didn’t drink, despite their thirst.
But when the researchers freed the snakes to swim in freshwater tanks, most immediately drank significant amounts. More experiments revealed the snakes would drink only freshwater or highly diluted saltwater.
The lab findings were consistent with field observations performed by Lillywhite’s team at Orchid Island. “Snakes were more numerous at all sites having a known source of fresh water,” Lillywhite said. Populations were also found to fluctuate over time as rainfall amounts changed.
Photo by James P. Blair/NGS
Lillywhite believes the sea snakes that spend their lives in the open ocean drink water from the “lens” of freshwater that sits atop saltwater during and after rainfall, before the two have had a chance to mix. “That would explain why some seawater lagoons, where the waters are calmer due to protection from reefs, are home to dense populations of sea snakes — the freshwater lens persists for longer periods before mixing into saltwater,” he says.
If rainfall patterns change as expected with global warming, sea snakes in certain parts of coastal regions and open oceans could find themselves without enough drinkable water.
These findings could be applicable to other types of marine reptiles, Lillywhite says, but more research is needed.
“Understanding the water requirements of all sea snakes could prove to be very important to their conservation,” the researchers write.
Sea snakes are members of the elapid family of snakes that also includes cobras, mambas and coral snakes. They are thought to have originated as land-dwelling snakes that later evolved to live in oceans. Most spend all, or nearly all, of their lives in seawater, including giving birth to live young while swimming. A minority, including the kraits that Lillywhite studied, lay eggs and spend at least a small part of their lives on land.
- Harvey Lillywhite’s research was funded in part by a grant from the National Geographic Society’s Committee for Research and Exploration.
More from National Geographic:
Relocated Sea Snakes Cross Seas to Go Home (National Geographic News)