How Deep-Sea Fish Are So Exceptionally Black

In the darkness of the oceans' depth, fish have evolved a strategy to become nearly invisible to prey.

By Elizabeth Anne Brown
Published 20 Apr 2018, 13:47 BST
The threadfin dragonfish lives more than two miles deep in the eastern Atlantic.
The threadfin dragonfish lives more than two miles deep in the eastern Atlantic.
Photograph by Sonke Johnsen

In the vast, featureless darkness of the oceans, fish take camouflage to a new art form. How do you blend in with nothing?

Viperfish and creatures like it have evolved ever blacker—we're talking blacker than black—so they can hide in plain sight.

"When you look at them, especially in the water, it’s just like a hole in the universe," says Sönke Johnsen, a marine biologist at Duke University who studies the denizens of the deep.

Scientists are learning how these "super black fish," a catch-all term for dark deep-sea creatures, make their bodies effectively disappear. 

In new research, Johnsen and colleague Karen Osborn discovered how complex nanostructures in the fishes' skin trap incoming photons, absorbing almost all the light that touches them. In January, the team presented their research at the Society for Integrative and Comparative Biology annual meeting.

"We were just expecting copious amounts of pigment," says Osborn, an invertebrate zoologist at Smithsonian's National Museum of Natural History. "What we found was really organised—tuned to make them as black as possible."

But why is this optical trickery even necessary in perpetual darkness?

Deep-Sea Pinball

Since food is scarce in the deep sea, everyone's on the menu. Animals such as anglerfish have evolved an arsenal of tools to search out prey, like whiskery spines that detect movement.

Many also scan their surroundings with photophores, bioluminescent organs that produce light.

"Imagine a world where if you shine your flashlight, nothing comes back," Johnsen says. "Except for every now and then, something will hit it, almost like a radar hit." 

The deep-sea fish's only defence is to mimic endless water. "You basically have to suck up every ounce of that searchlight that hits you," he says.

Eustomias pacificus is a dragonfish that lives off the Hawaiian Islands.
Photograph by Sonke Johnsen

To absorb all that light, it's not enough to have tons of black pigment, Johnsen says. The key is the skin's surface.

If a fish's skin is simple and smooth, light photos bounce right off into the waiting eyes of a hungry predator. But the more complex the surface structure, the more likely photons will get trapped, ricocheting around like they're in a pinball machine.

In recent experiments, Osborn examined the surface structure of seven species of super black fish, acquired from the wild. She found tiny packets of melanin, the same black pigment found in humans, arranged in dizzyingly complex microscopic structures—a very difficult game of pinball. Light doesn’t stand a chance.

According to Johnsen and Osborn's study, some super black fish achieve up to 99.9 percent light absorption. In other words, only one out of every thousand photons escapes.

The Blackest Black

That puts super black fish in some very dark company—they're within shooting range of the blackest creatures known to humankind.

Australasia's birds of paradise currently hold the record for the "blackest black," their plumage clocking in with a maximum absorption rate of 99.95 percent.

Related: See 13 Amazing Pictures of Birds

Scientists believe the complex microstructures on the males' feathers evolved because a blacker black helps accentuate the bird’s brightly coloured spots—with the ultimate goal, naturally, of impressing the ladies.

Richard Prum, an ornithologist at Yale University, was impressed by the "novel mechanism" that these deep-sea fish use to absorb light.

"In bird feathers and in butterfly scales, you basically have microscale cavities that trap the light," Prum explains, but super black fish use their own internal optical mechanism for absorbing light via pigment granules.

This unique structure is of interest to scientists working to create a synthetic super black for products such as cameras, telescopes, and solar panels, especially as current production techniques are costly.

Maybe the threadfin dragonfish, with its winning smile and luminous purple photophore, will unlock the next big technological innovation.


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