Superblack fish can disappear in the deep sea’s darkness

Pigment-containing structures just below their skin’s surface trap nearly all light

Some fish living in the deep sea have ultrablack skin, including this Pacific blackdragon (Idiacanthus antrostomus). It absorbs almost all light that hits it. Such skin may help hide the animals from predators or prey.

K. Osborn/Smithsonian National Museum of Natural History

In the ocean’s depths, it might take more than a little light to illuminate some of the planet’s darkest fish.

Their ultrablack skin can soak up almost all light that hits it. That makes these deep sea fish nearly invisible. This camouflage comes from a layer of densely packed, pigmented structures just below the skin’s surface. Researchers described this online July 16 in Current Biology. The skin may hide the fish from predators or prey. It might also inspire new designs for ultrablack materials used in telescopes or fabric.

Little of the sun’s light reaches the deep sea. But bioluminescent organisms can brighten this inky darkness. They create their own light. For creatures at these depths, trying to swim here unseen is “like trying to play hide and seek on a football field,” says Karen Osborn. Explains this marine biologist, “There’s nowhere to hide.” Osborn works at the Smithsonian National Museum of Natural History in Washington, D.C.

Enter superblack skin.

Osborn was part of a team that captured 18 species of ultrablack fish. These fish had been living at depths of up to 2,000 meters. Some came from Monterey Bay off of California. Ohers came from the Gulf of Mexico. The team measured how much light reflected off the fish. They also studied the skin from nine of the species using electron microscopy. Later, they calculated how structures in that skin might absorb light.

The skin has a layer of closely packed, circular structures. They’re called melanosomes (Meh-LAN-uh-soams). Their name comes melanin, a dark pigment, inside of them. These structures can absorb up to 99.95 percent of the light having wavelengths similar to ambient sunlight in the ocean or to light from bioluminescent animals.

microscope image of melanosomes
Unlike normal black fish, ultrablack deep-sea fish have a layer of tightly packed, pigment-containing structures in their skin. Called melanosomes (red arrows), these clustered circular structures (shown in electron micrographs of the Pacific blackdragon) absorb nearly all light that hits them. That includes the bioluminescence from other fish. Melanosomes occur beneath a membrane (blue arrows) just under the skin’s surface.K. Osborn/Smithsonian, A.L. Davis et al/Current Biology 2020

The melanosomes’ size, shape and arrangement may help direct light that isn’t absorbed by an individual melanosome to others in the layer. That would trap even more light. Other dark-colored fish tend to have unpigmented gaps between melanosomes. This lets more light be reflected — and makes a fish more visible.

The newfound mechanism is simpler than how birds or butterflies achieve ultrablackness. Their feathers or scales have multiple layers of intricate micro- or nanostructures to absorb light. If engineers could mimic what the fish do, it may make producing ultrablack materials easier, Osborn says.

Erin I. Garcia de Jesus is a staff writer at Science News. She holds a Ph.D. in microbiology from the University of Washington and a master’s in science communication from the University of California, Santa Cruz.

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