In the darkest depths of the ocean, where little to no light from the surface penetrates, an unusual array of creatures thrives, many of which create their own light via bioluminescence to hunt for prey, among other uses. But there are also several species of fish that have evolved the opposite survival strategy: they are ultra-black, absorbing nearly all light that strikes 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 upon finding she was unable to capture these ultrablack fish on camera while working in the field, trying to photograph specimens caught in the team's deep-sea trawl nets. "Two specimens, the Anoplogaster cornuta and the Idiacanthus antrostomus, were the only two fish over the course of six years of field work that I was able to get decent photographs of," Osborn told Ars.
To do so, she used a Canon Mark II DSL R body and 65 mm macro lens with four strobes, then tested various lighting setups by taking lots and lots of photographs. Finally, she adjusted contrast and applied a high pass filter uniformly across the images, the better to bring out the details. It still wasn't sufficient to capture most of the specimens caught in the trawl net. "Over the years I deleted thousands of failed shots of other fish as useless because I couldnt bring out the details in the photos," she added. "It didn't matter how you set up the camera or lighting—they just sucked up all the light. I wish I had a few of them now to illustrate this."
To discover why this was the case, Osborn teamed up with Duke University biologist Sönke Johnsen, among others, and laboratory measurements showed that, indeed, these ultra-black fish absorbed more than 99.5 percent of any light that hit their skin. That is a handy adaptation for survival in the dark depths of the sea, where even a few photons of light—say, from hungry nearby bioluminescent organisms—can give away a fish's position to a predator.
Another angle on the ultra-black fish species Anoplogaster cornuta. Karen Osborn/Smithsonian This Anoplogaster cornuta fish was so lively after being sampled and documented that the research team released it back to the deep via submarine the day after being caught in a trawl net. Karen Osborn/Smithsonian The ultra-black Pacific blackdragon (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 not for their ultra-black skin and transparent, anti-reflective teeth, the reflection of their lure would scare prey away. Karen Osborn/Smithsonian The Pacific blackdragon also has light-producing organs below their eyes that scientists expect might be used as a searchlight to spot prey. Karen Osborn/Smithsonian The ultra-black ridgehead (Poromitra crassiceps). These fish are also commonly known as bigscales because of the few giant scales they possess. Their ultra-black skin covers their scales, but the skin and scales detach easily when a predator tries to grab them. Karen Osborn/Smithsonian Read More – Source [contf] [contfnew] 
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