Black sea snakes are prone to being weighed down by algal spores, making them slower swimmers than their black-and-white-banded cousins, say researchers.
Dr Rick Shine of University of Sydney and colleagues report their findings today in the Proceedings of the Royal Society B.
"As anyone that owns a boat knows if you get an algal build up on the bottom of your boat, it slows it down," says Shine.
"It creates all sorts of friction against the water column as you're moving through it, and sea snakes seem to have the same problem."
Shine says simple evolutionary theory suggests a particular environment will favour a particular colour and species of that colour will dominate there.
So it's a puzzle, how different coloured animals of the same species come to thrive equally in the same environment.
In the case of land snakes scientists have suggested the answer to this puzzle lies with a trade-off between advantages and disadvantages of particular colours.
For example, being black gives land snakes the advantage of heating up quicker, but it also makes them more obvious to predators, says Shine.
But, the story isn't so clear in the case of sea snakes of the species Emydocephalus annulatus, from New Caledonia, which come in two colours - black and black-and-white-banded.
The effect of seawater means there is no heat advantage to being black. In fact, Shine and colleagues found there was a disadvantage.
In experimental tests, using model snakes made out of plastic tubes, they found that black models accumulated more algae than the black-and-white-banded ones.
Shine and colleagues then tested the impact of this algae on the snakes' swimming speeds and found it slowed the black ones down.
"We ran a bunch of snake Olympics where we took the swimming speeds of these guys and showed, sure enough, there was about a 20% decrease in speed," says Shine.
So, what is the trade-off that allows black sea snakes to thrive alongside their black-and-white cousins?
Shine says it could be that the algae produces oxygen, which is absorbed through the black sea snake's skin.
"Sea snakes actually get about 30% of their oxygen through their skin from the water," says Shine.
"They may be able to stay under water for longer by having their pet algae producing oxygen for them."
Shine says he and colleagues don't yet know if this is the case and further research is needed.
"There's very likely to be some corresponding advantage to that black colour and the challenge is now for us to go out and work out what that is," he says.