<img src = "https://cdn.arstechnica.net/wp-content/uploads/2020/07/blackfish1-800×533.jpg" alt = "A specimen of the ultra black fish species Anoplogaster cornuta. A unique array of pigment-packed granules allows some fish to absorb almost all of the light that hits their skin, so that only 0.05 percent of that light is reflected back. "/>
Enlarge /. A specimen of the ultra-black fish Anoplogaster cornuta. A unique arrangement of pigment-filled granules allows some fish to absorb almost all of the light that hits their skin, so that only 0.05 percent of this light is reflected back.
In the darkest depths of the ocean, where little to no light penetrates from the surface, unusual creatures thrive, many of which generate their own light, including through bioluminescence, to look for prey. However, some fish species have developed the opposite survival strategy: they are ultra-fast and, according to a new publication in Current Biology, absorb almost all of the light that hits their skin.
Karen Osborn of the Smithsonian Museum of Natural History was intrigued by the creatures when she found she couldn't hold these ultra-black fish in front of the camera while working in the field. She tried to photograph specimens caught in the team's deep-sea trawls. "Two specimens, the anoplogaster cornuta and the Idiacanthus antrostomus, were the only two fish in six years of field work that I could take decent photos of," said Osborn to Ars.
To do this, she used a Canon Mark II DSLR housing and a 65mm four-flash macro lens, and then tested various lighting configurations by taking many, many photos. Finally, she adjusted the contrast and evenly applied a high pass filter to the images to better highlight the details. It was still not enough to catch most of the specimens caught in the trawl. "Over the years, I've deleted thousands of failed shots of other fish as useless because I couldn't get the details out of the photos," she added. "It didn't matter how you set up the camera or the lights – they just soaked up all the light. I wish I had a couple of them now to illustrate this."
To find out why this was the case, Osborn worked with, among others, Duke University biologist Sönke Johnsen. Laboratory measurements showed that these ultra-black fish actually absorbed more than 99.5 percent of the light that struck their skin. This is a practical adaptation for survival in the dark depths of the sea, where even some light photons – for example from nearby hungry bioluminescent organisms – can give the position of a fish to a predator.
Another perspective on the ultra-black fish Anoplogaster cornuta.
This anoplogaster cornuta fish was so lively after sampling and documentation that the research team brought it back in a submarine trawl the day after it was caught.
The ultra-black Pacific Black Dragon (Idiacanthus antrostomus), the second blackest fish studied by the research team.
The Pacific black dragon has a bioluminescent bait to attract prey, and without its ultra-baked skin and transparent, non-reflective teeth, the reflection of its bait would scare the prey away.
The Pacific black dragon also has light-producing organs under the eyes that scientists expect to be used as searchlights to spot prey.
The ultra-black ridge head (Poromitra crassiceps). These fish are also known as large scales because of the few huge scales they have. Her ultra-black skin covers her scales, but the skin and scales easily release when a predator tries to grab them.
The researchers also discovered the secret of this highly efficient light absorption: melanin, a pigment that also occurs in human skin and protects us from damage from sunlight. The melanin is filled into granules known as melanosomes, which in turn are contained in cells known as melanophores. According to Osborn, they form a continuous layer in the dermis (the deeper skin layers). "This arrangement provides a continuous and uninterrupted layer of pigment-containing cells and ensures that this layer is the first to be struck by light that strikes the fish," she said. "The pigment effectively absorbs most of the light that falls on the granules."
The size and shape of these granules also play a role, since they scatter any light that is not immediately absorbed laterally into the pigment layer so that it can be absorbed by neighboring pigment-containing cells. It is essentially a very thin, highly efficient light trap. "The blackest fish was as black as VantaBlack," said Osborn – that is, as black as one of the darkest substances known to date. "VantaBlack traps light in tightly 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."
"You have to scatter and absorb in pretty much all ultra-black materials," said co-author Alexander Davis, a PhD student at Duke. "In all the other animals we know, the scatter in the ultra-black color comes from either a chitin or keratin matrix, such as a bird feather or butterfly scale, and the absorption comes from melanin, which is embedded in these matrices. Scattering and absorption come from both from the melanosomes themselves. This makes the mechanism a little easier because it doesn't require a structural framework. "
Several ultra-black species appear to have developed exactly the same adjustment independently; Osborn and her colleagues found these pigment patterns in 16 distantly related species. The ultimate goal of the research is to develop a similarly efficient design to make ultra-black materials – such as interior coatings for telescopes, cameras, and other light-sensitive devices – much cheaper and easier.
DOI: Current Biology, 2020.10.1016 / j.cub.2020.06.044 (About DOIs).