This deep-sea dragon fish has ultra-black skin capable of absorbing bioluminescent light that could blow its cover away. Credit: Photo by Karen Osborn, Smithsonian National Museum of Natural History.
Packaged pigment granules help them mix without blowing.
If there was a sea scene plotter, dressed in black to disappear into the dark behind the scenes, it could be the dragon fish. Or the common tusk.
These fish live in the ink depths of the ocean, where there is nowhere to protect themselves. Even beyond the reach of sunlight, they can get caught in the glow of bioluminescent organisms that light up the water to hunt. So they evade detection with a trick of their own: using stealth.
Scientists report that at least 16 species of deep-sea fish have developed ultra-black skin that absorbs more than 99.5% of the light that hits them, making it nearly impossible to distinguish them from shadows.
These fish owe their disappearance action to small pigment packets inside their skin cells called melanosomes. The melanosomes of ultra-black fish have different shapes and arrangements, at the microscopic level, compared to normal black fish, according to a study led by Duke University and the Smithsonian’s National Museum of Natural History.
The researchers say the work could lead to new light-trapping materials for use in applications ranging from solar panels to telescopes.
For the article, published on July 16, 2020, in the magazine Current biologyThe team used a trawl and a remotely operated vehicle to collect 39 black fish that swim up to a mile deep in the waters of Monterey Bay and the Gulf of Mexico, and take them to a boat to study.
Scientists report that some deep-sea fish have developed ultra-black skin that absorbs more than 99.5% of the light that hits them, making them almost impossible to distinguish from shadows. Credit: Karen Osborn, Smithsonian National Museum of Natural History.
Using a spectrometer to measure the amount of light reflected off the fish’s skin, the researchers identified 16 species that reflected less than 0.5% of the light, making them about 20 times darker and less reflective than everyday black objects.
“Ultra-black emerged more than once through the fish family tree,” said first author Alexander Davis, a Ph.D. of biology. student in Sonke Johnsen’s lab at Duke.
The darker species they found, a small monkfish not much longer than a golf tee, absorbs so much light that almost none (0.04%) bounces off the eye. Only one other group of black animals, the Papua New Guinea birds of paradise with their ultra-dark plumage, are known to match them.
Getting decent photos of these fish on board the boat was difficult; Her features were lost. “It didn’t matter how you set up the camera or the lighting, they just absorbed all the light,” said zoologist researcher Karen Osborn of the Smithsonian’s National Museum of Natural History.
The team found that when magnified thousands of times under electron microscopes, normal black skin and ultra-black skin look very different. Both have small structures within their cells that contain melanin, the same pigment that gives human skin its color. What distinguishes ultra-black fish, they say, is the shape and arrangement of these melanosomes.
Other cold-blooded animals with normal black fur have small pearl-shaped melanosomes, while ultrablacks are larger, more tick-tock. And ultra black skin has melanosomes that are tighter, forming a continuous sheet around the body, while normal black skin contains unpigmented gaps.
The researchers ran some computer models, simulating the skin of fish containing different sizes and shapes of melanosomes, and discovered that ultrablack melanosomes have the optimal geometry to swallow light.
Melanosomes are packed into skin cells “like a small chewing gum machine, where all the chewing gums are the right size and shape to trap light inside the machine,” Davis said.
Its ultra black camouflage could be the difference between eating and being eaten, Davis says. Being blacker than black, these fish manage to avoid detection even in ranges six times shorter.
Read how deep-sea ultramarine fish disappear to learn more about this research.
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Reference: “Ultra Black Camouflage in Deep Sea Fish” by Alexander L. Davis, Kate N. Thomas, Freya E. Goetz, Bruce H. Robison, Sönke Johnsen, and Karen J. Osborn, July 16, 2020, Current biology.
DOI: 10.1016 / j.cub.2020.06.044
Funding for this work was provided by a NOAA Ocean Exploration and Research Grant (NA17OAR0110208), a National Defense Science and Engineering Graduate Fellowship, the Smithsonian National Museum of Natural History, and the Duke Department of Biology.
