The wings of the peppered moth are usually white with black speckles—a pattern that renders them invisible against the bark of a typical tree. But in the early 19th century, the trunks of English trees became jacketed in soot, which was belched forth from coal-fired factories. Against these trunks, the once-camouflaged moths shone out like beacons, making them easy prey for birds. But some lucky moths had mutations that gave them all-black wings. They successfully hid against the sooty trunks while their peppery peers were devoured, and as the decades ticked by, they became more and more common. By the turn of the 20th century, almost all the peppered moths in some parts of England were black.
The peppered moth’s turn to the dark side made it an icon of evolution. It’s a wonderfully evocative example of natural selection changing the body of an animal to meet an environmental challenge, by favoring certain genes over others—and we even know exactly which gene was responsible. It’s also the best-known case of industrial melanism—the phenomenon where animals become blacker in areas that are affected by industrial pollution.
Most cases of industrial melanism involve moths, butterflies, and other insects, but Claire Goiran, from the University of New Caledonia, has now discovered a surprising exception. In the waters around Australia, one species of sea snake, which usually has black and white stripes, has also become completely black. And just like the peppered moths, urban pollution is probably to blame.
Sea snakes include some of the most venomous serpents in the world, which use their super-toxins to rapidly immobilize fast-moving fish. But the turtle-headed sea snake is an exception. It dines on caviar, sucking fish eggs out of corals with its blunt (and oddly adorable) snout. Because of this unusual diet, it has tiny teeth, shrunken venom glands, and presumably watered-down venom. “They’re the perfect sea snakes for beginners,” says Goiran. “They’re easy to handle. They live in shallow water and can be studied by snorkeling from the beach. They swim slowly and are easy to catch.”
Goiran and her supervisor Rick Shine first started studying these snakes around 15 years ago, and quickly noticed that they varied in color. In most waters, they had black-and-white stripes or blotches. But in New Caledonia’s Noumea Lagoon, they were mainly black. “For many years we have been wondering why,” Goiran says. Black scales could protect snakes from the sun’s ultraviolet radiation, but that’s an unlikely explanation, because the Noumea snakes don’t get any more sun than those elsewhere. Darker snakes might better blend into certain backgrounds, but again, the Noumea population didn’t behave any differently to other turtle-headed sea snakes.
Unexpectedly, Parisian pigeons provided a clue. In 2014, French researchers showed that blacker pigeons have higher levels of zinc and other heavy metals in their feathers than their paler counterparts. That’s because melanin, the pigment that gives black feathers their color, is also coincidentally good at binding heavy metals. And in doing so, it detoxifies a pigeon’s body, keeping such metals away from internal organs and storing them where they do no harm. Perhaps, the researchers argued, that’s why dark pigeons are so common and successful in cities.
When Goiran read about the study, she immediately thought about the sea snakes. The rocks around New Caledonia naturally contain lots of nickel, and the levels of metal in the water have only increased because of mining and industrial runoff. Perhaps the sea snakes, like the pigeons, had adapted to these pollutants by upping the levels of melanin in their scales.
To test this idea, Goiran analyzed discarded skins that had been shed by turtle-headed sea snakes in different parts of Australia and New Caledonia. She found that snakes that lived near towns and industrial sites were more likely to be black than those from pristine waters. Their scales also contained higher levels of 13 heavy metals, like lead, zinc, cobalt, nickel, and manganese, at concentrations known to harm the health of other animals.
All of this supports the idea that urban sea snakes are ingesting lots of heavy metals, presumably through their food, and getting rid of these elements by storing them in dark scales. Indeed, Goiran found that the black snakes shed their skins more often than the banded ones.
“It’s a cool study,” says Ilik Saccheri from the University of Liverpool, who has studied the peppered moth. But he adds that the case isn’t solid yet, because the team hasn’t shown that the black snakes survive any better, or reproduce any faster, in their particular habitats. That’s the next step. It took many years to establish that the peppered moth is truly the evolutionary icon that it has long been portrayed to be. It will take a similar amount of work for Goiran and Shine to confirm if and why the blacker scales are beneficial, and when they started to slither into the dark side.