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Fish So Black, They Virtually Disappear

Common Fangtooth Fish
Common Fangtooth  Karen Osborn, Smithsonian








Scientists are still a bit far from recreating Harry Potter’s Cloak of Invisibility, but they are certainly trying. The best hope for a material that hides what’s behind it is to create something so black that no light bounces back to an observer. Now, scientists at the Smithsonian and Duke University have figured out just how certain deep-sea fish avoid detection by holding on to all the light their skin can hold.

Lead research zoologist Karen Osborn from the National Museum of Natural History was led to look into the super-dark fish after trying to photograph deep-sea creatures. Even though she was using some pretty powerful equipment, no matter how hard she tried, she simply could not get a good photo of some fish. It turns out that nothing was wrong with her camera—it was just that the fish absorbed more than 99.5 percent of the light that hit their bodies.

Melanin Mystery

After more research, Osborn and Duke University biologist Sönke Johnsen figured out that this amazing ability to go almost entirely invisible in the dark depths of the ocean is due to the arrangement of cells containing the pigment melanin, which is also found in human skin.

In the dark fish, microscopic compartments filled with melanin called melanosomes are packed into pigment cells. These cells are, in turn, butted up against each other very near the surface of the fish’s skin. When light hits the fish, the first cell absorbs what it can but, instead of bouncing the unabsorbed light back into the water, the unique cellular arrangement sends extra to light to other melanosomes that gobble the photons.

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

“These pigment-containing structures are packed into the skin cells like a tiny gumball machine, where all of the gumballs are of just the right size and shape to trap light within the machine,” said Davis.

Fangtooth and Dragon

In all, the researchers found 16 different species of related fish whose skin exhibits the unique light-trapping properties, including the common fangtooth (Anoplogaster cornuta) and the Pacific blackdragon (Idiacanthus antrostomus). It is believed that the light-grabbing adaptation came about as a way to protect the fish in extremely dark sea floors. 

Pacific Blackdragon Fish
Pacific Blackdragon Karen Osborn, Smithsonian

In fact, the Pacific blackdragon has other adaptations that help keep it hidden, including transparent, anti-reflective teeth and light-producing organs beneath its eyes. 

“If you want to blend in with the infinite blackness of your surroundings, sucking up every photon that hits you is a great way to go,” Osborn said. 

The researchers’ work appears in the journal Current Biology.