Some deep-sea fish species have developed a skin that is blacker than black to protect them from being eaten, or to help them sneak up on the fish they want to eat.
A study, published last week in the journal Current Biology, documents “ultra blackness” in 16 deep-sea fish species and suggests more could be found.
The discovery places deep-sea species among the few animals that evolved with ultra-black pigmentation, including Australasian birds of paradise and some butterflies and spiders.
But while other animals use ultra-blackness to bring out 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 for the predators they are hunting … some are preventing them from preying,” he said.
Ultra-black is defined as reflecting less than 0.5 percent of incoming light. In comparison, black paper actually reflects about 10 percent of incoming light, making it about 20 times lighter than ultra black.
The researchers have been working to create their own ultra-black materials. Last year, the Massachusetts Institute of Technology reported a material that reflects only 0.005 percent of light.
Ultra black fish are not that black, but they may be the blackest animals: the darkest are reflected between 0.044 percent and 0.051 percent from light, Davis said.
Davis and his colleagues spent weeks searching for ultra-black fish in the Gulf of Mexico and Monterey Bay in California. They were caught at depths of up to 1,000 feet at night, when the fish hunt higher in the water, and sometimes more than a mile deep during the day.
Some were very small: the blackest they found, a type of monkfish called a dreamer, was only two inches long.
It uses ultra blackness to hide, so it can take advantage of even smaller fish and shellfish attracted to the bioluminescent lure hanging 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 develop ultra-blackness.
But the deep sea is shining with the light of living creatures, Davis said.
“We have reflectors from predators, we have lures, we have marine snow that emits a glow, we have crustaceans that drop bioluminescent clouds when they freak out,” he said. “There are all kinds of light produced by animals down there.”
The blackness of the ultra black fish is not caused only by the pigment. Microscopes show that this is accomplished by shaped packages of black pigment that form an outer layer on the skin so that almost no light is reflected from the skin cells.
But there is a tradeoff: Ultra black skin skips a layer of collagen found on the skin of normal fish, making it fragile and gelatinous, Davis said.
Prosanta Chakrabarty, an ichthyologist and fish healer at the Louisiana State University Museum of Natural Sciences in Baton Rouge, said that even blacker fish could still 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 had seen ultra-black fish in his own studies. Ultrablack makes sense to survive in an environment where a predator can become prey the next moment, he said.
“It is a fish-eat-fish world,” he added.
