In the darkest depths of the ocean, where little to no light penetrates from the surface, unusual creatures thrive, many of whom create their own light via bioluminescence to hunt prey, among other things. But several species of fish have evolved the opposite survival strategy: They are ultra-black and absorb almost all the light that hits their skin, according to a new paper in Current Biology.
Karen Osborn of the Smithsonian’s Museum of Natural History became intrigued by the creatures when she found she couldn’t capture these ultra-black fish on camera while working in the field. She tried to photograph specimens caught in the team’s deep-sea trawl nets. “Two copies, the anoplogaster cornuta and the Idiacanthus antrostomus, were the only two fish over the course of six years of fieldwork that I was able to get decent pictures of,” Osborn told Ars.
To do this, she used a Canon Mark II DSLR body and a 65mm macro lens with four flashes, then tested different lighting setups by shooting loads of shots. Finally, she adjusted the contrast and applied a high-pass filter evenly across the images to bring out the details better. It was still not enough to catch most of the specimens caught in the trawl. “Over the years I’ve deleted thousands of failed photos of other fish because they were 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 exposure – they just sucked in all the light. I wish I had a few now to illustrate this.”
To find out why this was the case, Osborn teamed up with Duke University biologist Sönke Johnsen, among others, and lab measurements showed that these ultra-black fish did indeed absorb more than 99.5 percent of all the light that hit their skin. That’s a handy adaptation for survival in the dark depths of the sea, where even a few light photons — say from hungry nearby bioluminescent organisms — can betray a fish’s position to a predator.
The researchers also discovered the secret of this highly efficient light absorption: melanin, a pigment also found in human skin that protects us from damage from sunlight. The melanin is filled in granules known as melanosomes, which in turn are contained within cells known as melanophores. They form a continuous layer in the dermis (the deeper layers of skin), Osborn said. “This arrangement creates a continuous and uninterrupted layer of pigment-containing cells and ensures that this layer is the first light to encounter on the fish,” she said. “The pigment effectively absorbs most of the light that falls on the grain.”
The size and shape of those granules are also important because they scatter light that is not immediately absorbed sideways in the pigment layer so that it can be absorbed by adjacent pigment-containing cells. It is essentially a very thin, very efficient light trap. “The blackest fish was as black as VantaBlack,” Osborn said — that is, as black as one of the darkest substances known to date. “VantaBlack captures light in densely packed carbon microtubules, while these fish absorb the light with the pigment and do it extremely efficiently by optimizing the size, shape and packaging of the pigment granules themselves.”
“In virtually all ultra-black materials, you need both scattering and absorption,” said study co-author Alexander Davis, a graduate student at Duke. “In all the other animals we know of, the scattering in ultra-black coloring comes from a chitin or keratin matrix, such as a bird’s feather or butterfly shell, and the absorption comes from melanin embedded in those matrices. In these fish, the scattering and absorption both come from the melanosomes themselves. This makes the mechanism a bit simpler because no structural scaffolding is needed.”
Several ultra-black species seem to have evolved the exact same adaptation independently of each other; Osborn and her colleagues found these pigment patterns in 16 distantly related species. The ultimate goal of the research is to adopt an equally efficient design to make ultra-black materials, such as inner coatings for telescopes, cameras and other light-sensitive equipment, much cheaper and easier to produce.
DOI: Current Biology, 2020. 10.1016/j.cub.200.06.044 (About DOIs).