The fangtooth fish is among 16 species of ultra-black fish that can avoid detection by absorbing the light around them (Credit: Karen Osborn / Smithsonian NMNH)

Over the years, researchers have made numerous attempts to make Harry Potter's invisibility cloak a reality. While each version has been slightly better, none have come close to the one donned by the young wizard. Now, scientists may be able to get some tips from an unlikely source — ultra-black deep-sea fish that have evolved the capability to hide in plain sight to avoid detection.

Karen Osborn, a marine biologist at the Smithsonian's National Museum of Natural History, and her colleagues stumbled upon the ultra-black fangtooth fish while netting crabs just off California's Monterey Bay. Before releasing the specimen back in the ocean, Osborn decided to take a photo. However, despite shining two bright strobes of light directly at the fish, she was unable to obtain a clear image of the creature.

"I was trying to take pictures of it, and I was just getting these silhouettes," Osborn says. "They were terrible." The expert soon realized that the blurry images were not a result of operator error. The tiny fish, with a mouth full of pointy teeth and a teardrop-shaped body, was somehow absorbing all the light and remaining "invisible."

The terrifying ultra-black Pacific dragonfish combines its camouflage skills with a bioluminescent lure, to attract prey (Credit: Karen Osborn / Smithsonian NMNH)

"We had no idea there were any fishes at all that were ultra-black," says Alexander Davis, a a biology Ph.D. student at Duke University and the lead author of the study. "As far as we knew, the only vertebrates that were ultra-black were these birds-of-paradise and a couple of other bird species. It was the first case we have of something this black really being used as camouflage as well."

Curious to see if there were other "ultra-black" fishes lurking in the ocean, the scientists collected specimens of 18 species of deep-sea fish and used a black-reflectance probe to observe how much of the light being directed towards the skin was reflected. To be classified as "ultra-black" the fish had to absorb over 99 percent of the light. The team was surprised to find that 16 species, including the accidentally-discovered fangtooth fish, met the criterion.

"Effectively what they've done is make a super-efficient, super-thin light trap," said Osborn. "Light doesn't bounce back; light doesn't go through. It just goes into this layer, and it's gone."

The ultra-black Pacific dragonfish, which lives in the deep waters of the eastern Pacific Ocean, can absorb about 99.5 percent of light (Credit: Karen Osborn / Smithsonian NMNH)

A closer examination of the ultra-black fish's tissue revealed that their melanosomes — the cells that hold the pigmenting chemical melanin — were structured differently from those of fish that do not possess the camouflaging capabilities. The cylinder-shaped cells were larger, more densely packed, and close to the skin surface, producing a continuous layer around the body. Conversely, the skin of regular black fish sported circular melanosome cells that were separated by gaps that did not contain any pigment.

The scientists believe that the ultra-black fish's tissue's unique shape and arrangement causes the photons of light that strike the skin's surface to be absorbed by both the cell they hit and the adjacent cells. "So basically, by changing the shape and the size of those granules," says Osborn, "instead of letting light that's not immediately absorbed escape and signal their presence, they control it so that the light goes into the layer and side-scatters into the granules next to it."

Ultra-black fish, like the ultra-black ridgehead above, absorb light by using the size and shape of pigment-filled cells (Credit: Karen Osborn / Smithsonian NMNH)

Interestingly, the structure of the melanosomes was not uniform across the different species, leading the scientists to suspect that the ultra-black fish did not have a common ancestor but rather evolved their deep black skin independently. "Some of them have just a big jumble," says Osborn. "Some of them have three layers, some of them have two layers. Some of them have really thick layers, some of them have thinner layers."

Given that the fish species dwell in the darkest part of the ocean, which gets little to no, sunlight their need to develop such a survival mechanism may appear a little excessive. However, the researchers say the oceans are lit up by numerous bioluminescent animals, many of whom are out seeking a meal. "There is a ton of animals down there, but their density is relatively low, which means you probably very rarely meet your lunch. So, when you do meet your lunch, you want to make sure that you catch it," says Ron Douglas, a marine biologist at the City University of London who was not involved in the study.

Davis and his team, who published their findings in the journal Current Biology on July 16, 2020, believe their discovery could help with the development of coatings for submarines and other vessels, or perhaps even be used to create stealth armor for night operations!