Ultrablack skin camouflages deep sea fish

In the darkest depths of the ocean, bioluminescence can make a creature stand out like a sore thumb—or tasty snack. Now, researchers have discovered that deep sea fishes have evolved a way to blend in using ultrablack skin. Their findings could inspire new synthetic black materials.

Karen Osborn, a marine biologist at the Smithsonian National Museum of Natural History, was inspired to investigate the skin of deep sea fishes after having difficulty photographing specimens. “It didn’t matter how I set up the camera,” she says. “All I got were these incredible silhouettes.”

After analyzing skin samples from 16 species of deep sea fishes, Osborn’s team found that their skin reflected less than 0.5% of light (Curr. Biol. 2020, DOI: 10.1016/j.cub.2020.06.044). This ultrablack coloration has never been found in aqueous organisms.

Other animals such as birds and insects achieve ultrablack coloration by embedding melanin in a matrix of keratin or chitin that helps the pigment trap and absorb light. But transmission electron microscopy images of fish skin reveal only densely packed layers of melanosomes—cellular structures packed full of melanin, suggesting that the pigment alone is responsible for the effect. The fish use melanosomes of “just the right size and shape so that any light they don’t absorb immediately bounces sideways and gets absorbed by its neighbor,” Osborn says.

Ali Dhinojwala, a materials scientist at the University of Akron who was not involved in the study, says that he was surprised to see such low reflectance from the fishes’ skin. The findings “give inspiration of how to design synthetic melanosomes . . . to create blacker material,” he says. But Dhinojwala adds that more refined models of the new system are needed to fully explain how the melanosomes are so effective at absorbing light.

This newly discovered melanosome structure is so effective that it can be found in completely unrelated fish species, suggesting that the mechanism evolved multiple times, Osborn says. That’s because camouflage is an important survival strategy in the abyssal depths of the sea. “There’s nowhere to hide,” Osborn explains. “It’s like playing hide-and-seek in a football field.”

The ultrablack skin not only protects fish like the Pacific blackdragon (Idiacanthus antrostomus) from the spotlights of predators but also from being sold out by its own bioluminescent lures. Further, the adaptation may help some fish block the glow of bioluminescent prey they’ve eaten, which might otherwise shine through their bellies.

Dakota McCoy, an evolutionary biologist at Harvard University who was not involved in the study, says this research not only shows a new structure for producing ultrablack in animals but also an ecologically novel use of ultrablack in predation. “You don’t think of the deep sea as a place where you’d have arms races of light and darkness,” McCoy says. “But there’s tons of bioluminescence, and that requires tons of camouflage.”