Anoplogaster cornuta. A unique arrangement of pigment-filled granules allows some fish to absorb almost all of the light that reaches their skin, so only 0.05 percent of that light is reflected. “Karen Osborn / Smithsonian
In the darker depths of the ocean, where little or no surface light penetrates, an unusual variety of creatures thrive, many of which create their own light through bioluminescence to hunt prey, among other uses. But there are also several species of fish that have developed the opposite survival strategy: They are ultra black and absorb almost all of the light that hits their skin, according to a new article in Current Biology.
Karen Osborn of the Smithsonian’s Natural History Museum was intrigued by the creatures when she discovered that she was unable to capture these ultra-black fish on camera while working in the field, trying to photograph specimens caught in the deep-sea trawls of the equipment. “Two specimens, the Anoplogaster cornuta and the Idiacanthus antrostomus, they were the only two fish in the course of six years of fieldwork that I could get decent pictures of, “Osborn told Ars.
To do so, he used a Canon Mark II DSL R body and a 65mm macro lens with four strobes, then tested various lighting settings by taking lots of pictures. Finally, he adjusted the contrast and applied a high-pass filter evenly across the images, to better highlight the details. It was still not enough to capture most of the specimens caught in the trawl. “Over the years, I deleted thousands of failed photos of other fish as useless because I couldn’t bring out the details in the photos,” he added. “It didn’t matter how you set up the camera or the lighting, they just absorbed all the light. I wish I had some of them now to illustrate this.”
To find out why this was the case, Osborn teamed up with Duke University biologist Sönke Johnsen, among others, and laboratory measurements showed that these ultra-black fish actually absorbed more than 99.5 percent of any light. to hit your skin. That’s a handy adaptation for surviving in the dark depths of the sea, where even a few photons of light, for example from hungry, nearby bioluminescent organisms, can give away a fish’s position to a predator.
-
Another angle on ultra black fish species Anoplogaster cornuta.
Karen Osborn / Smithsonian
-
Is Anoplogaster cornuta The fish was so lively after being sampled and documented that the research team returned it to the depths by submarine the day after it was caught in a trawl.
Karen Osborn / Smithsonian
-
The Ultra Black Pacific Black Dragon (Idiacanthus antrostomus), the second blackest fish studied by the research team.
Karen Osborn / Smithsonian
-
The Pacific Black Dragon has a bioluminescent lure that they use to attract prey, and if it weren’t for its ultra-black skin and transparent, anti-reflective teeth, the reflection of its lure would scare off prey.
Karen Osborn / Smithsonian
-
The Pacific black dragon also has light-producing organs under the eyes that scientists hope can be used as a reflector to detect prey.
Karen Osborn / Smithsonian
-
The ultra black crest (Poromitra crassiceps) These fish are also commonly known as large scales because of the few giant scales they possess. Its ultra black skin covers its scales, but the skin and scales come off easily when a predator tries to grab them.
Karen Osborn / Smithsonian
The researchers also discovered the secret to this highly efficient light absorption: melanin, a pigment also found in human skin that protects us from damage from sunlight. Melanin is filled into granules known as melanosomes, which in turn are contained within cells known as melanophores. They form a continuous layer in the dermis (the deepest layers of the skin), according to Osborn. “This arrangement provides a continuous and uninterrupted layer of pigment-containing cells and ensures that this layer is the first thing you find when light hits the fish,” he said. “The pigment effectively absorbs most of the light that hits the granule.”
The size and shape of those granules are also important as they scatter any light that is not immediately absorbed sideways to the pigment layer so that it can be absorbed by neighboring pigment-containing cells. It is essentially a very thin and highly efficient light trap. “The blackest fish was as black as VantaBlack,” said Osborn, meaning as black as one of the darkest substances known so far. “VantaBlack traps light in tight carbon microtubules, while these fish absorb light with the pigment and do so extremely efficiently by optimizing the size, shape and packaging of the pigment granules.”
“Almost all ultra-black materials require dispersion and absorption,” said co-author Alexander Davis, a Duke graduate student. “In all the other animals we know of, the dispersion in the ultra-black coloration comes from a chitin or keratin matrix, like a feather or butterfly scale, and absorption comes from the melanin embedded in those matrices. In these fish , the dispersion and absorption come from the melanosomes themselves. This makes the mechanism a little bit simpler because there is no structural scaffold needed. “
Several ultra black species appear to have independently developed the same adaptation; Osborn and 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 liners for telescopes, cameras, and other light-sensitive equipment, much more economically and easily.
DOI: Current Biology, 2020. 10.1016 / j.cub.2020.06.044 (About DOI).
