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How Urban Sea Snakes Lost Their Stripes

Researchers studying turtle-headed sea snakes living on coral reefs in the Indo-Pacific noticed something unusual about the snakes’ colour patterns: sea snakes living in more pristine parts of the reef were decorated with black-and-white bands or blotches while those in places with more human activity were black.

A report in Current Biology has now found that these colour differences are due to the snakes’ exposure to pollution. The blacker skin of urban sea snakes allows the animals to more effectively bind and rid their bodies of contaminants, including arsenic and zinc, each time they shed their skins.

The findings add sea snakes to a growing list of species that show “industrial melanism” – a greater prevalence of dark-coloured varieties in industrial areas. “I think it’s remarkable to find industrial melanism in organisms as different as moths and sea snakes,” says Prof Rick Shine of The University of Sydney.

The study’s lead author, Dr Claire Goiran of Université de la Nouvelle-Calédonie, got the idea that blacker skin might be related to pollutant exposure after learning that the darker feathers of urban pigeons in Paris store more zinc than lighter feathers. Goiran and her colleagues wondered whether a similar thing might be happening in the sea snakes.

To find out, the researchers measured trace elements in the sloughed skins of sea snakes from urban-industrial versus other areas and in dark versus light skin. As predicted, concentrations of trace elements were higher in snakes from urban-industrial areas. Trace element concentrations were also higher in darker than in paler skin.

The researchers further found that darker snakes slough their skins more often. As a result, it appears that sea snakes whose skin is more heavily pigmented with melanin have an advantage over their lighter relatives in polluted areas.

Shine says that the findings are yet another example of rapid adaptive evolutionary change in action. He says it’s also a more sinister reminder that “even on an apparently pristine coral reef, human activities can pose very real problems for the animals that live there”.