"Water, water everywhere, nor any drop to drink." Was the Rime of the Ancient Mariner actually about sea snakes? New research suggests that the serpents, unlike other marine reptiles like iguanas and turtles, can't quench their thirst by drinking sea water. The problem is that fresh water alleviates thirst (you knew this) but the briny, salty seawater has the opposite effect. It makes you more thirsty.
Above: A yellow-bellied sea snake, photographed on a beach in Costa Rica. (Source)
At National Geographic's Phenomena blog, Ed Yong explains:
If you drink seawater, your kidneys try to get rid of the excess salt by diluting it in urine, and you expel more water than you take in. The same applies to other land animals, and those that return to the sea have special adaptations for coping with salt. Many sea animals avoid swallowing seawater entirely, and get fresh water from the food they eat. Turtles, sea birds and marine iguanas have special glands for getting rid of salt.
Sea snakes have similar glands under their tongues. When Harvey Lillywhite from the University of Florida started studying these serpents a few decades ago, all the textbooks said that they used these glands to get rid of salt.
But Lillywhite had a terrible time trying to get sea snakes - in particular, the yellow-bellied sea snake, Pelamis platura, a species found in tropical waters throughout the world - to survive in aquariums filled with seawater. The snakes require fresh water to survive, and in experiments actually refuse to drink salt water. Imagine becoming dehydrated even while surrounded by water.
So how do the snakes survive? The same way most terrestrial critters do. Rainwater.
When it falls over the ocean, it doesn't mix with the seawater straight away. Instead, it forms a layer that is either fresh or only mildly salty. If the conditions are right, these "freshwater lenses" can be both deep and persistent. And the yellow-bellied sea snake, it seems, drinks from them.
Since snakes, like all reptiles, require air to breathe, it's simple enough to drink at the surface. In addition, sea snakes have the unique ability to use their bodies as reservoirs, storing up lots of water to get them through the less rainy parts of the year. Making them, essentially, sea camels.
Don't miss out on reading the entirety of Ed Yong's beautifully-written post, over at Not Exactly Rocket Science.