Scientists have now uncovered the secrets behind the magical disappearing act of some of the fish lurking in some of the deepest parts of the ocean. These "ultra-black" fish are among the darkest creatures ever found, evolving to camouflage themselves to predators, even with no sunlight.
According to a study published Thursday in the journal Current Biology, certain exotic species of fish have adapted the shape, size and pigment of their skin to absorb 99.5% of the light that hits them — making them about 20 times darker than everyday black objects. These fish mark the first time ultra-black has been discovered in aquatic animals, researchers said.
Scientists at Duke University and the Smithsonian National Museum of Natural History studied 16 species of ultra-black fish, including the fangtooth, the Pacific blackdragon, the anglerfish and the black swallower, in the waters of Monterey Bay and the Gulf of Mexico. The fish spanned seven different orders, which are large groupings that each have a shared evolutionary history, to determine that the modifications occurred independently from each other.
Some of the fish inhabit parts of the ocean as deep as three miles, where very little sunlight can reach. At these depths, bioluminescence — light emitted by living organisms — is the only source of light.
With organisms illuminating the water themselves in order to hunt, ultra-black fish adapted to hide in plain sight. The camouflage is likely the difference between eating and getting eaten, scientists said in a press release.
"In the deep, open ocean, there is nowhere to hide and a lot of hungry predators," co-author and zoologist Karen Osborn of the Smithsonian Institution's National Museum of Natural History in Washington, told Reuters. "An animal's only option is to blend in with the background."
Scientists found that, compared with regular black fish, ultra-black fish have uniquely shaped melanosomes, the tiny packets of pigment with their skin cells. The skin of these fish is some of the blackest material ever discovered — they often appear as just silhouettes, even in bright light.
"The darkest species they found, a tiny anglerfish not much longer than a golf tee, soaks up so much light that almost none -- 0.04% -- bounces back to the eye," researchers said.
The findings rank the fish among the world's blackest-known animals: Ultra-black butterflies reflect between 0.06% to 0.5% of light and the blackest birds have 0.05% to 0.31% reflectance.
Photographing the fish proved extremely difficult for researchers.
"It didn't matter how you set up the camera or lighting -- they just sucked up all the light," said research zoologist Karen Osborn of the Smithsonian National Museum of Natural History.
Researchers say the discovery could lead to the development of light-trapping materials with practical applications on land — ranging from solar panels to telescopes — like Vantablack, the ultra-black coating designed for defense and space applications.