The banded sea snake.

Credit: Rick Shine

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SYDNEY: Evolutionary biologists solve the "classical problem in evolutionary biology" for sea snakes – why are individuals different colours?

Until now, researchers have wondered why two colour forms existed in the sea snake Emydocephalus annulatus found in New Caledonia; the snakes are either totally black or they have a black-and-white, banded pattern.

The black sea snakes are more camouflaged, helping to avoid predation, but the rates of survival of the two forms were similar.

'Algal fouling'

Evolutionary biologists, led by Rick Shine form the University of Sydney, found that the black snake also experiences greater 'algal fouling' - algae accumulates on the darker surfaces.

"It is a classical problem in evolutionary biology, when two different forms of characters exist in the same population," Shine said.

The weedy covering creates drag when the snakes swim. "We showed that sea snakes covered in algae slowed down 20% in their swimming speed," said Shine, whose study was published in Proceedings of the Royal Society B in April.

More oxygen?

Lighter coloured sea snakes may have an advantage as they don't have to shed their skins to shed the algae. The covering might also help to camouflage the black snakes from predatory fish that swim overhead.

Another possible advantage is the algae may help with breathing. Sea snakes absorb 30% of the oxygen they need to survive through their skin while underwater.

A mass of oxygen-producing algae that surrounds a black snake could increase the length of time it can stay underwater, said Shine.

"Seaweed is so smart!"

To make sure that the colour was the factor that led algae to settle on the individual snakes, Shine and his team placed six plastic tubes filled with paper that was either banded or black.

These tubes were suspended in the ocean for a couple months and inspected later to confirm the researchers' hypothesis.

"The biggest shock to me, as a snake biologist, is to discover that seaweed is so smart!" said Shine. Algae in the ocean somehow detect darker places to live, which researchers believe is likely a survival tactic.