At the deepest depths of the ocean where light barely reaches, deep-sea creatures have evolved all sorts of adaptations to help them see, communicate, and hunt. Some, however, would quite like to sneak around undetected, thank you, and now scientists have discovered just how some of the stealthiest manage to disappear into the darkness.
Reporting in Current Biology, researchers from the Smithsonian National Museum of Natural History and Duke University have revealed at least 16 species of deep-sea fish that have evolved an incredible stealth tool for avoiding even the passing glow of bioluminescent creatures: ultra-black skin.
Like stagehands disappearing into the background, these fish can move around undetected because their skin absorbs more than 99.5 percent of light, on a par with the fabulous feathers of the superb bird of paradise (yes, that’s its whole name), which absorb 99.95 percent of light, and the human-made vantablack, which absorbs 99.96 percent.
Interestingly, the researchers discovered the fish skins used quite a different mechanism to achieve blacker than black, which could have multiple applications, creating durable, flexible, ultra-black materials to be used in everything from solar panels to telescopes, cameras to camouflage.
Smithsonian zoologist Karen Osoborn first became intrigued by the light-absorbing skin when she tried to photograph some black fish she and her colleagues had caught in trawler nets while collecting deep-sea samples. Despite their sophisticated camera equipment, she struggled to take photos of the fish that captured the details of their features. "It didn't matter how you set up the camera or lighting – they just sucked up all the light," she explained in a statement.
Laboratory testing revealed why the slippery customers were so tricky to image. The fishes’ skin absorbed nearly all light directed at it. Osborn and colleagues discovered that the pigment melanin – which protects human skin from sunlight as it can absorb 99.9 percent of UV radiation – was not just abundant in the skin, but distributed in an interesting way. Melanosomes, pigment-filled cellular compartments, are packed into pigment cells, which in turn form a closely-packed continuous layer very close to the surface of the skin. The melanosomes absorb most of the light directed at them, but the size, shape, and arrangement of these cells also cause them to direct any remaining light at other melanosomes within the cell to suck up the rest.
"If you want to blend in with the infinite blackness of your surroundings, sucking up every photon that hits you is a great way to go," Osborn said. "Effectively what they've done is make a super-efficient, super-thin light trap. Light doesn't bounce back; light doesn't go through. It just goes into this layer, and it's gone."
Further analysis and distance modeling suggest that this ability to reflect such an infinitesimal amount of light could reduce the distance a predator might be able to spot the fish by six times. At least three of the species studied are known ambush predators with bioluminescent lures. The researchers suspect the ultra-black skin serves to hide them from their own light. They even found ultra-black skin around the gut of one species, Cyclothone acclinidens, which they hypothesize may be to conceal light emitted from any recently consumed bioluminescent meal.
Understanding this new mechanism for producing ultra-black skin could improve the manufacturing of materials that currently use the structure found in birds and butterflies. The researchers think adopting this technique could result in more robust, efficient materials. "Instead of building some kind of structure that traps the light, if you were to make the absorbing pigment the right size and shape, you could achieve the same absorption potentially a lot cheaper and [make the material] a lot less fragile," Osborn said.