Scientists have unlocked the secret of how these ultra-black fish absorb light

Anoplogaster cornuta. A unique arrangement of pigmented granules allows some fish to absorb almost all of the light that hits their skin, so barely 0.05% of that light is reflected. “/>
Enlarge / A specimen of the ultranoir fish species Anoplogaster cornuta. A unique arrangement of pigmented granules allows some fish to absorb almost all of the light that hits their skin, so barely 0.05% of that light is reflected.

In the darkest depths of the ocean, where little or no light from the surface penetrates, unusual creatures thrive, many of which create their own light via bioluminescence hunting for prey, among other uses. But several species of fish have developed the opposite survival strategy: they are ultranoir, absorbing almost all the light that hits their skin, according to a new paper in current biology.

Karen Osborn of the Smithsonian Museum of Natural History became intrigued by the creatures when she discovered she was unable to capture these ultra-black fish on camera while working in the field. She was trying to photograph specimens caught in the team’s offshore trawls. “Two specimens, the Anoplogaster cornuta and the Idiacanthus antrostomus, were the only two fish in six years of fieldwork that I was able to get decent photographs of, ”Osborn told Ars.

To do this, she used a Canon Mark II DSLR body and a 65mm macro lens with four strobes, then tested various lighting setups, taking lots and lots of pictures. Finally, she adjusted the contrast and applied a high-pass filter evenly over the images, to better bring out the details. This was still not enough to capture most of the specimens caught in the trawl. “Over the years, I deleted thousands of failed shots of other fish as unnecessary because I couldn’t bring out the details in the photos,” she added. “He didn’t matter how you set up the camera or the lighting – they just sucked in all the light. I would like to have a few now to illustrate that. “

To find out why this was the case, Osborn teamed up with University biologist Duke Sönke Johnsen, among others, and laboratory measurements showed that, indeed, these ultra-black fish absorbed more than 99.5% of any light that touched their skin. This is a practical adaptation for surviving in the dark depths of the sea, where even a few photons of light, say, from hungry bioluminescent organisms nearby, can reveal a fish’s location to a predator.

Researchers have also discovered the secret of this highly efficient light absorption: melanin, a pigment also present in human skin that protects us from damage caused by the sun. Melanin is contained in granules called melanosomes, which in turn are contained in cells called melanophores. They form a continuous layer in the dermis (the deepest layers of the skin), according to Osborn. “This arrangement provides a continuous, uninterrupted layer of cells containing pigment and ensures that this layer is the first thing encountered by light hitting the fish,” she said. “The pigment effectively absorbs most of the light that hits the granule.”

The size and shape of these granules is also important, as they scatter any light that is not immediately absorbed laterally into the pigment layer so that it can be absorbed by neighboring cells containing pigment. It is essentially a very fine and efficient light trap. “The blackest fish was as black as VantaBlack“Osborn said, being as black as one of the darkest substances ever known.” VantaBlack traps light in tightly packed carbon microtubules while these fish absorb light along with the pigment and do this extremely efficiently by optimizing the size, shape and packaging of the pigment granules themselves. “

“In just about any ultra-black material, you need both diffusion and absorption,” said co-author Alexander Davis, graduate student at Duke. “In all other animals that we know of, the diffusion of the ultra-black coloration comes from a matrix of chitin or keratin, like a bird feather or a butterfly scale, and the absorption comes from the encrusted melanin. in these matrices. In these fish both diffusion and uptake are from the melanosomes themselves. This makes the mechanism a bit simpler as there is no structural scaffolding required. “

Several ultranory species appear to have independently developed exactly the same adaptation; Osborn and his colleagues found these pigment patterns in 16 distantly related species. The end goal of the research is to adopt an equally efficient design to produce ultra-clear materials, such as linings for telescopes, cameras and other light-sensitive equipment, much more easily and at lower cost.

DOI: current biology, 2020. 10.1016 / j.cub.2020.06.044 (About DOIs).

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