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Normally, people and sea snakes don’t swim in the same waters. The slithering creatures prefer hanging out in remote areas. They also tend to be venomous, so humans don’t normally seek them out. But turtle-headed sea snakes are a different story.

They don’t have long fangs or potent venom like their sea snake compatriots. Instead they have tiny fangs and typically chow down on fish eggs. They also live fairly close to human cities, like Noumea, the capital city of the French territory New Caledonia in the Pacific Ocean.

The fact that they are common and relatively easily accessible makes them a good target for research. Rick Shine, an evolutionary biologist at the University of Sydney, started studying these snakes almost 20 years ago.

“We’ve been able to conduct by far the most detailed ecological investigation of any “true” sea snake (one that never comes out on land) that has ever been attempted,” says Shine in an e-mail.

Over the course of his work with these watery reptiles, Shine noticed a difference between the turtle-headed snakes that lived near cities like Noumea, and the same species that lived elsewhere. The snakes near the cities were darker, with fewer distinctive stripes along their bodies. In a study authored by Shine and published today in Current Biology he explains that the most likely culprit is pollution.

“These sites are right beside the biggest city in New Caledonia, so they get all kinds of urban pollution—basically everything that is produced in a city and discarded into the drain.” Shine says.

Biologists have observed this color change in response to pollution for a long time. Arguably the most famous example of this is the Peppered Moth. During the Industrial Revolution in Europe, the Peppered Moth took on a darker color to blend in to the new sootier background. Other invertebrates made similar changes over time.

turtle headed sea snake
A turtle headed sea snake swimming through coral. Claire Goiran

But in this case, sea snakes aren’t having to hide in a dirtier environment. “We don’t think camouflage is important.” Shine says. “Camouflage mostly works if individuals stay still. In contrast, the snakes move about frequently, passing over live coral and coral rubble. There are no ‘black’ backgrounds,” Shine says.

Instead, Shine thinks that the change in color might be due to the fact that the darker melanin can bind to toxic trace elements within the snake’s body. When the snake sheds its skin, it also gets rid of pollutants building up in its body. The darker the skin, the more pollutants the snakes can excise from its system.

“It had never occurred to me that an animal could rid its body of contaminants in this way.” Shine says. But, he notes, there is at least one other vertebrate who uses the same adaptation to make life a little less toxic in an urban area.

“The only examples in vertebrates seem to be pigeons in cities and our sea snakes in polluted reefs,” Shine says. “in both of those cases the pattern may reflect the ability of melanin to bind trace-elements and thus help the animal expel those pollutants from its body when it sheds its feathers or sloughs its skin.”

But just because these sea snakes have found a way to live with our trash doesn’t mean that they are endlessly adaptable.

Shine says this study should serve as a reminder that although animals can show remarkable plasticity, that doesn’t mean we should treat these changes as benign. “Living systems are resilient, and the sea snakes are rapidly adapting to oceanic pollution, says Shine. “But there’s a limit to that resilience; if we keep treating the ocean as a toilet, we will lose many of the spectacular creatures that depend on it.”