Deep in the ocean, where there is no light, a group of fish with light-absorbing skin literally disappear into the darkness. A team of scientists found that the skin of some deep-sea fish absorbs more than 99.95 percent of the light that hits them, making them appear ultra-black.
The team at Duke University and the Smithsonian National Museum of Natural History have now discovered at least 16 species of ultra-black fish. Each of these fish is equipped with a special skin that allows them to evade detection while hunting or hiding in the dark.
Karen Osborn, co-author of the new study, first became interested in the skin of these deep-sea fish when she tried to photograph some specimens that had been brought back to the surface. “It didn’t matter how you set up the camera or the lights – they just sucked up all the light.”
She set up a Canon DSLR camera with four flashes and then tested various lighting configurations by taking many, many photos. Finally, she adjusted the contrast and evenly applied a high pass filter to the images to better bring out the details.
The ultra-black Pacific dragon (Idiacanthus antrostomus), the second blackest fish examined by the research team. Photo credit: Karen Osborn, Smithsonian Institution
“Over the years, I’ve deleted thousands of failed shots of other fish as useless because I couldn’t bring out the details in the photos,” she added. “It didn’t matter how you set up the camera or the lights – they just sucked up all the light. I wish I had some of these now to illustrate this. “
Because sunlight is no more than a few hundred meters below the surface of the sea, most deep-sea creatures create their own light, known as bioluminescence. Bioluminescent lights are used to attract mates, distract predators, and lure prey.
They can also expose nearby animals – by thwarting a predator’s stealth approach or setting a beacon at potential prey – unless those animals are properly camouflaged. “If you want to blend in with the infinite blackness of your surroundings, it’s a great way to soak up every photon that hits you,” said Osborn.
Nearly complete light absorption by ultra-black fish depends on melanin, the same pigment that colors human skin and protects it from sunlight. Osborn and her colleagues discovered that not only is this pigment abundant in the skin of ultra-black fish, but it is also distributed in a unique way.
Cell compartments filled with pigment, called melanosomes, are tightly packed in pigment cells, and these pigment cells are arranged in a continuous layer very close to the surface of the skin of an ultra-black fish. The size, shape, and arrangement of the melanosomes cause any light they don’t absorb immediately to be directed towards neighboring melanosomes in the cell, which will then soak up the remaining light.
“In fact, they made a super-efficient, super-thin light trap,” said Osborn. “Light doesn’t bounce back; Light does not come through. It just goes into that shift and it’s gone. ” [EurekaAlert]