Most turtle-headed sea snakes throughout the Indian and Pacific Oceans have black-and-white banded bodies—unless they swim in city waters. Instead of the distinctive white rings around their bodies, the snakes that live closest to industrial areas have much darker bodies. In fact, turtle-headed sea snakes in the polluted waters around New Caledonia, in the Pacific, are often entirely black. These city sea snakes aren’t making a fashion statement. Having darker skin, scientists have learned, gives them an edge over their banded brethren when living near large human populations.
The evolutionary adaptation of darker pigmentation is called melanism, named for the chemical melanin that makes skin, feathers, scales and other external body parts darker. Scientists have long seen evolutionary melanism in other animals. Peppered moths living in polluted cities, for example, are much darker than those in the country. Their darker color helps them blend in better in a sooty city. But it’s not improved camouflage that helps sea snakes. Their advantage is similar to that of pigeons in Paris whose darker feathers store more zinc than lighter ones. Darker scales help the sea snakes rid their bodies of extra chemicals from pollution.
Melanin efficiently absorbs metals and similar trace elements, so instead of remaining in the snake’s body, contaminants like arsenic and zinc end up in the snake’s skin. When the snake sheds its skin, it sheds excess pollutants with it. Even better, the darker sea snakes shed more often, preventing potentially toxic chemicals from building up in their bodies. For these ocean city dwellers, black isn’t a wardrobe choice. It’s a method of survival.