How the sea snake got its stripes

Professor Rick Shine and Dr Adele Pile from the School of Biological Sciences have discovered a sea snake's colouration can influence its susceptibility to algal fouling which can reduce swimming speed by up to 20 percent.

Their study, reported this month in Proceedings of the Royal Society B, sheds new light on how the transition from terrestrial to aquatic life has shaped the evolution of sea snakes.

Professor Shine said sea snakes evolved from venomous land snakes -- such as the highly toxic tiger snake -- who reinvaded the oceans around five million years ago.

"The fact that sea snakes have made the transition from terrestrial to aquatic life, makes them the perfect model to study evolution because we can compare traits between land snakes and sea snakes and hence identify selective forces unique to those habitats," he said.

"The shift from land to water brought with it a new set of challenges, and sea snakes evolved unique physical traits which enabled them to survive in the aquatic environment -- a paddle-shaped tail for swimming, valves to close their nostrils and large lungs to provide oxygen while under water.

"Another consistent attribute of sea snakes involves coloration: most are banded rather than unicoloured, blotched or striped. Fouling by algae has also been reported in several groups of sea snakes, and we wondered if maybe a snake's colour could influence its susceptibility to this."

To test this hypothesis, the scientists turned to a population of sea snakes in the tropical Pacific, in which members of the same species ranged from jet black to brightly black-and-white banded, and many patterns in between. Over a four-year period, the researchers examined free ranging individuals and found that black snakes supported significantly more algal cover than black-and-white snakes.

"Once we knew there was a relationship between a snake's colour and the amount of algal fouling, the next step was to determine if a snake's dark colour was the actual cause of the higher algal levels," Professor Shine said.

To do this, the researchers suspended plastic snake models -- in black, white and black-and-white -- in mid water and scored the amount of algal colonisation over the subsequent days. The results showed that colour directly affects the amount of algal growth, with black surfaces attracting the most algae, followed by black-and-white, and white the least.

"The spores of some marine algae settle out preferentially onto dark-coloured objects, which probably explains why the darker snakes hosted higher algal cover," he said.

The finding raises the crucial question: if snake colour influences rates of algal accumulation, what are the consequences of such accumulation?

"The most obvious such consequence is increased drag and things became really interesting when we tested to see if algal cover affected a snake's swimming speed. Our locomotor trials revealed a 20 percent reduction in swimming speeds in snakes covered with a heavy coating of algae."

Differences in colour involving black versus banded varieties of land snakes typically have been attributed to differences in heat transfer -- that is darker colours absorb more heat, even at the expense of looking more obvious to predators.

But Professor Shine said temperature based explanations can't be applied to the case in sea snakes.

"Unlike on land, colour does not affect the body temperatures of a snake under water. Our data suggests another potential fitness cost of colour in sea snakes, and potentially that of other aquatic animals: susceptibility to algal fouling," he said.

So why are some sea snakes black at all? Is there some hidden benefit to being black that outweighs the increased algal fouling?

"There is clearly a balance of costs and benefits of algal accumulation, which is why we see a variety of colours in the population. For example, a covering of seaweed may slow down the snake and reduce its ability to obtain oxygen from the water directly through its skin, because the algae form a barrier. But on the flip side, the algae might increase the snake's oxygen availability, because of algal photosynthesis, and hence benefit the snake."