Ultrablack Fish from the Ocean Depths

Katherine Wu in the NY Times:

Alexander Davis admits that he can be a glutton for punishment. He staked part of his Ph.D. on finding some of the world’s best-camouflaged fishes in the ocean’s deepest depths. These animals are so keen on not being found that they’ve evolved the ability to absorb more than 99.9 percent of the light that hits their skin.

To locate and study these so-called ultra-black fishes, Mr. Davis, a biologist at Duke University, said he relied largely on the luck of the draw. “We basically just drop nets and see what we get,” he said. “You never know what you’re going to pull up.”

When he and his colleagues did cash in, they cashed in big. In a paper published Thursday in Current Biology, they report snaring the first documented ultra-black animals in the ocean, and some of the darkest creatures ever found: 16 types of deep-sea fish that are so black, they manifest as permanent silhouettes — light-devouring voids that almost seem to shred the fabric of space-time.

“It’s like looking at a black hole,” Mr. Davis said.

To qualify as ultra-black, a substance has to reflect less than 0.5 percent of the light that hits it. Some birds of paradise manage this, beaming back as little as 0.05 percent, as do certain types of butterflies (0.06 percent) and spiders (0.35 percent). A feat of engineering allowed humans to best them all with synthetic materials, some of which reflect only 0.045 percent of incoming light. (“Black” paper, on the other hand, returns a whopping 10 percent of the light it meets.)

But deep sea fish are good at this too:

One species profiled in the paper, a bioluminescent anglerfish in the genus Oneirodes, reflects as little as 0.044 to 0.051 percent of the deep-sea light it encounters. The other 99.95 percent, Mr. Davis and his colleagues found, gets lost in a labyrinth of light-swallowing pigments until it effectively disappears.

“I’m always arguing with bird people on the internet,” said Kory Evans, a fish biologist at Rice University who wasn’t involved in the study. “I say, ‘I bet these deep-sea fish are as dark as your birds of paradise.’ And then boom, they checked, and that was exactly the case.”

The point of combining extreme non-reflectivity with bioluminescence is so that none of the light from the fish's glowing lure reflects off its body, giving it away.

The photograph at the top, of a common fangtooth, looks pretty visible, but that's because of photographic tricks; if you were looking at it you would just see darkness.