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Snakes stay fresh
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New research may have solved the riddle of why sea snakes are distributed so patchily around the world.
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Researchers have found that, where it was previously thought that some 60 per cent of venomous sea snake species drink sea water, removing salt via filter glands, it seems that the creatures stick to fresh water - and so live in areas where they can get it.
The study involved three species of sea kraits caught off Taiwan. By placing samples in differently constituted water tanks, and weighing them regularly to establish their drinking and hydration levels, the researchers found that the creatures would drink only fresh water - even when they were thirsty.
In a report published in the journal Physiological and Biochemical Zoology, team leader Harvey Lillywhite, a zoologist at the University of Florida, said: 'Our experiments demonstrate that they actually dehydrate in sea water, and they'll only drink fresh water, or highly diluted brackish water with small concentrations of saltwater - 10 to 20 percent.'
Such behaviour would explain why sea snakes are found around freshwater springs or river outlets; and, for open-water survival, in regions that have plenty of rainfall. They can then drink from the small layer of fresh water that sits on the surface of the sea after it has rained, before the waters assimilate.
Large populations of sea snakes found in sea lagoons would be explained by the fact that the surrounding reefs make for protected, calmer waters, which allow the rainfall canopies to remain intact for longer.
One worry is that, as climate change leads to longer periods of drought in some tropical regions, sea snakes could be threatened by having to go for too long without a drink. 'There may be places where sea snakes are barely getting enough water now,' said Lillywhite. 'If the rainfall is reduced just a bit, they'll either die out or have to move.'
He observed that, in Australia, which has been suffering from prolonged drought, some types were in decline and threatened with extinction at Ashmore Reef, known for its broad array of sea snake species.
Lillywhite also raised the question of whether other marine reptiles, such as sea turtles, might also hydrate themselves by drinking fresh water rather than by processing sea water.
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