How some superblack fish disappear into the darkness of the deep sea

A layer of pigment-containing structures just below the skin’s surface traps nearly all light

side view of a Pacific blackdragon (Idiacanthus antrostomus)

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

K. Osborn/Smithsonian National Museum of Natural History

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

Some deep-sea fish have ultrablack skin capable of soaking up almost all light that hits it, making the fish nearly invisible. That camouflage is the result of a layer of densely packed pigment-containing structures just below the skin’s surface, researchers report online July 16 in Current Biology. The skin may hide the fish from predators, or prey, and might inspire new designs for ultrablack materials used in telescopes or fabric.

Although little light reaches the deep sea, bioluminescent organisms can brighten the inky darkness. For creatures trying to swim undetected, living in these depths is “like trying to play hide and seek on a football field,” says Karen Osborn, a marine biologist at the Smithsonian National Museum of Natural History in Washington, D.C. “There’s nowhere to hide.”

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

Enter superblack skin. Osborn and her colleagues captured 18 species of ultrablack fish from up to 2,000 meters deep in Monterey Bay off California and in the Gulf of Mexico. The team then measured how much light reflected off of the fish. The researchers also examined skin from nine species using electron microscopy and calculated how structures in the skin might absorb light.

The skin has a layer of closely packed, circular, melanin-containing structures called melanosomes that can absorb up to 99.95 percent of light with wavelengths similar to ambient sunlight in the ocean or light from bioluminescent animals. The melanosomes’ size, shape and arrangement may help direct light that isn’t absorbed by an individual melanosome to others in the layer, trapping even more light. Other dark-colored fish tend to have unpigmented gaps between melanosomes, which leads to more light being reflected and a more visible fish.

The newfound mechanism is simpler than how birds or butterflies achieve ultrablackness. Those animals’ feathers or scales have multiple layers of intricate micro- or nanostructures to absorb light (SN: 1/9/18). If engineers could mimic what the fish do, it may make producing ultrablack materials easier, Osborn says.

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