Safety feature? Claire Goiran

Pollution from mining activities may be encouraging some sea snakes to evolve black skins – the first evidence of “industrial melanism” in a marine species.

Previous studies have observed industrial melanism in invertebrate species, most famously the peppered moth. During England’s Industrial Revolution, the frequency of dark-coloured moths skyrocketed. Schoolchildren are often taught that such insects blended in well with the soot-covered bark of trees in industrial areas, so their odds of surviving and breeding suddenly rose – although this might be an oversimplification.

Examples of industrial melanism in vertebrates are vanishingly rare, says Rick Shine at the University of Sydney – but the Indo-Pacific sea snakes he and his colleagues study may provide a good example.


The turtle-headed sea snake (Emydocephalus annulatus) is largely found in certain tropical waters near Australia. Usually, the snakes look like black-and-white banded candy canes. But Shine and his colleagues found that individuals living near polluted areas on the French island territory of New Caledonia, north-east of Brisbane – and in a nearby barrier reef atoll used as a bombing range – were entirely black instead.

The group already knew that pollutants such as arsenic or lead can bind to melanin, a dark pigment in the skin, and they wondered whether this might explain the black snakes. To find out, they collected and analysed the skins naturally shed by these snakes in industrial and non-industrial waters. The sea snakes typically shed – or slough – their skin a few times a year.

Saving their skins

Looking at 17 sloughs, Shine’s group found that the concentrations of 13 trace elements – particularly cobalt, manganese, lead, zinc and nickel – were higher in snakes near urban areas, and higher in darker skin. Shine says similar concentrations of those trace elements have been reported to cause severe health problems in many domesticated species, from cattle to poultry.

What’s more, Shine’s group found that the black sea snakes shed their skins twice as often as their lighter counterparts. This suggests that the black sea snakes are, indeed, adapting to deal with the pollution in the water they inhabit – both by developing skin with a better capacity to bind potentially harmful trace elements, and by shedding that skin more often to reduce the trace element load they must deal with.

Shine says the snakes primarily pick up the pollutants from the prey they eat: E. annulatus likes to nibble on the eggs of small fish like gobies and damselfish that breed close to the shore. These shallow waters, particularly near the New Caledonian city of Nouméa, are highly polluted by intensive mining activity and industrialisation. Exactly why the sea snakes are black around the atoll used as a bombing range is less clear. Shine and his colleagues say they need to check the trace element concentrations in the water there.

“Our urban, turtle-head sites are literally right beside the city of Nouméa, with water outfalls – taking water from the street – emptying directly into the study site,” says Shine. The city also has a large nickel factory.

The study does seem to suggest the sea snakes are evolving in response to industrial pollutants, says Susana Clusella-Trullas at Stellenbosch University in South Africa. But she wants to know whether black sea snakes have higher survival rates and more offspring than their paler peers in these polluted environments.

For Shine, the findings are a wake-up call about the dangers of polluting ocean environments. “Our research suggests that the snakes are rapidly evolving to help them deal with these new challenges, but there are limits to that resilience,” he says. “We need to stop treating the ocean as a toilet.”

Journal reference: Current Biology, DOI: 10.1016/j.cub.2017.06.073

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