In the darkness of the deep ocean a single photon of light can expose an animal to predators, so many species have evolved to survive. Researchers from the Smithsonian National Museum of Natural History and Duke University have discovered that at least 16 species of fish do it by being ultra-black. Their skin absorbs more than 99.5% of light.
They are blacker than the blackest paper, a new tyre and even electrical tape, making them near impossible to find in the shadows.
“Effectively what they’ve done is make a super-efficient, super-thin light trap,” says Smithsonian zoologist Karen Osborn. “Light doesn’t bounce back; light doesn’t go through. It just goes into this layer, and it’s gone.”
Osborn first became interested in the fish skin after trying, and failing, to photograph its detail.
“It didn’t matter how you set up the camera or lighting – they just sucked up all the light,” she says.
Back in the lab, she and colleagues carefully analysed the skin using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and light microscopy.
As they report in the journal Current Biology, the light absorption is dependent on melanin, the same pigment that colours and protects human skin from sunlight. Not only is it more abundant in the skin of the ultra-black fish, it is also distributed in a unique way.
“These pigment-containing structures are packed into the skin cells like a tiny gumball machine, where all of the gumballs are of just the right size and shape to trap light within the machine,” says co-author Alexander Davis from Duke University.
“This low reflectance puts deep-sea fishes on par with the blackest known animals,” they write, “surpassing the darkness of ultra-black butterflies, and equalling the blackest birds of paradise.”
And that might be of interest to more than just biologists. By mimicking this light-absorbing structure, they say, engineers could develop more durable, flexible and cheaper ultra-black materials for applications in telescopes, cameras and even camouflage equipment.
“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,” they write.
Amelia Nichele is a science journalist at The Royal Institution of Australia.
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