Biologists Identify 16 Deep-Sea Fish Species with Ultra-Black Camouflage

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In a paper published today in the journal Current Biology, a team of marine biologists from the United States and the United Kingdom investigated the distribution and production of ultra-black camouflage in deep-sea fish. They found that at least 16 deep-sea fish species across seven distantly related orders have a continuous layer of pigment-filled cellular compartments called melanosomes in their skin that are optimized in size and shape to allow them to reflect less than 0.5% of light.

This deep-sea dragonfish Idiacanthus sp. has ultra-black skin capable of absorbing the bioluminescent light that might blow its cover. Image credit: Karen Osborn, Smithsonian’s National Museum of Natural History.

This deep-sea dragonfish Idiacanthus sp. has ultra-black skin capable of absorbing the bioluminescent light that might blow its cover. Image credit: Karen Osborn, Smithsonian’s National Museum of Natural History.

For the study, Duke University Ph.D. student Alexander Davis and his colleagues from Duke University, the Natural History Museum, London, the Smithsonian’s National Museum of Natural History, and the Monterey Bay Aquarium Research Institute used a trawl net and a remotely operated vehicle to scoop up 39 black fish swimming up to a mile deep in the waters of Monterey Bay and the Gulf of Mexico.

Using a spectrometer to measure the amount of light reflected off the fishes’ skin, they identified 16 species that reflected less than 0.5% of light, making them some 20 times darker and less reflective than everyday black objects.

“Ultra-black arose more than once across the fish family tree,” Davis said.

The darkest species they found, a tiny anglerfish not much longer than a golf tee, soaks up so much light that almost none — 0.04% — bounces back to the eye.

Only one other group of black animals, the birds-of-paradise of Papua New Guinea with their ultra-dark plumage, is known to match them.

The researchers found that, when magnified thousands of times under electron microscopes, normal black skin and ultra-black skin look very different.

Both have melanosomes that contain melanin — the same pigment that lends human skin its color.

What sets ultra-black fish apart is the shape and arrangement of these melanosomes.

Other cold-blooded animals with normal black skin have tiny pearl-shaped melanosomes, while ultra-black ones are larger, more tic-tac-shaped.

And ultra-black skin has melanosomes that are more tightly packed together, forming a continuous sheet around the body, whereas normal black skin contains unpigmented gaps.

Histology and TEM of the skin of Oneirodes sp. (A) and Idiacanthus antrostomas (B-F) reveal a continuous layer of melanosomes (m) immediately beneath the epidermal basement membrane (ebm). Melanosomes are membrane-bound organelles (black arrowheads). Melanophores (black arrows) are seen beneath the stratum compactum (white arrows). Scale bars - 50 mm in (A and E), 5 mm in (B, C, and F), 0.5 mm in (D). Image credit: Davis et al, doi: 10.1016/j.cub.2020.06.044.

Histology and TEM of the skin of Oneirodes sp. (A) and Idiacanthus antrostomas (B-F) reveal a continuous layer of melanosomes (m) immediately beneath the epidermal basement membrane (ebm). Melanosomes are membrane-bound organelles (black arrowheads). Melanophores (black arrows) are seen beneath the stratum compactum (white arrows). Scale bars – 50 mm in (A and E), 5 mm in (B, C, and F), 0.5 mm in (D). Image credit: Davis et al, doi: 10.1016/j.cub.2020.06.044.

The scientist also ran some computer models, simulating fish skin containing different sizes and shapes of melanosomes, and found that ultra-black melanosomes have the optimal geometry for swallowing light.

“Effectively what they’ve done is make a super-efficient, super-thin light trap,” said senior author Dr. Karen Osborn, a research zoologist at the Smithsonian’s National Museum of Natural History.

“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,” Davis added.

“Their ultra-black camouflage could be the difference between eating and getting eaten.”

“By being blacker than black, these fish manage to avoid detection even at six-fold shorter ranges.”

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Alexander L. Davis et al. 2020. Ultra-Black Camouflage in Deep-Sea Fishes. Current Biology 30: 1-7; doi: 10.1016/j.cub.2020.06.044

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