Some species of deep-sea fish have evolved blacker-than-black skin to protect them from being eaten – or to help them sneak up on fish they want to eat.
A study, published last week in the journal Current Biology, documents “ultra-blackness” in 16 species of deep-sea fish and suggests more could be found.
The discovery places the deep-sea species among the few animals to evolve ultra-black pigmentation, including Australasia’s birds of paradise and some butterflies and spiders.
But while other animals use ultra-blackness to highlight their brightest colors, deep-sea fish use it so they can’t be seen, said lead author Alexander Davis, a biologist at Duke University in Durham, North Carolina.
“Some of these fish are using it as camouflage from predators that are hunting … some are preventing giving themselves away to prey,” he said.
Ultra-black is defined as reflecting less than 0.5 percent of incoming light. By comparison, black paper actually reflects about 10 percent of incoming light, so it’s about 20 times lighter than ultra-black.
Researchers have been working to create their own ultra-black materials. The Massachusetts Institute of Technology last year reported a material that reflects only 0.005 percent of light.
The ultra-black fish aren’t quite that black, but they may well be the blackest animals – the darkest reflected between 0.044 percent and 0.051 percent of light, Davis said.
Davis and his colleagues spent weeks trawling for ultra-black fish in the Gulf of Mexico and the Monterey Bay in California. They captured them at depths of up to 1,000 feet during the night, when the fish hunt higher in the water, and sometimes more than a mile down during the day.
Some were very small – the blackest they found, a type of anglerfish called a dreamer, was only two inches long.
It uses ultra-blackness to hide so it can prey on even smaller fish and crustaceans attracted to the bioluminescent lure that dangles from its forehead, Davis said.
If there was no light at all in the depths of the ocean, then everything would be completely black and there would be no need to evolve ultra-blackness.
But the deep sea is glittering with light from living creatures, Davis said.
“We’ve got searchlights from predators, we have lures, we have marine snow that's giving off a glow, we've got crustaceans spewing out bioluminescent clouds when they're startled,” he said. “There's all sorts of light produced by animals down there.”
The blackness of ultra-black fish isn’t caused by pigment alone. Microscopes show it’s achieved by shaped packets of black pigment that form an outer layer on the skin, so that almost no light is reflected from the skin cells.
But there’s a trade-off: ultra-black skin omits a layer of collagen found in the skin of normal fish, making it fragile and gelatinous, Davis said.
Prosanta Chakrabarty, an ichthyologist and the curator of fishes at Louisiana State University’s Museum of Natural Science in Baton Rouge, said such even-blacker fish might yet be found in the largely unexplored depths of the oceans.
“We send more people into outer space," he said.
Chakrabarty was not involved in the new research but said he’d seen ultra-black fish in his own studies. Ultra-blackness makes sense for survival in an environment in which a predator can become a prey in the next moment, he said.
“It’s a fish-eat-fish world out there," he added.