Originally published 2021-08-29.
Some deep-sea fish are so dark that they reflect less than 0.5 percent of the light that hits them, which means they absorb more than 99.5 percent. Scientists call them ultra-black. A study published in Current Biology on 16 July 2020 identified 16 such species and showed how they do it: their skin contains a continuous sheet of tightly packed, unusually shaped melanin granules that work together as a light trap.
The darkest fish in the study, a tiny anglerfish not much longer than a golf tee, reflected only about 0.04 percent of light. For comparison, the researchers described ultra-black fish as roughly 20 times darker than everyday black objects such as black paper or electrical tape.
A widely shared headline said these fish "absorb 99.9% of sunlight". That is not quite right on either count. The threshold for ultra-black was absorbing over 99.5 percent, only the very darkest fish went beyond 99.9 percent, and the light in question is not sunlight at all, as the next sections explain.
Who did the research and what they found
The work was led by Duke University and the Smithsonian's National Museum of Natural History. The authors were Alexander Davis, then a Duke PhD student, Kate Thomas, Freya Goetz, Bruce Robison, Sönke Johnsen and Karen Osborn, a Smithsonian research zoologist.
The team collected 39 black fish using trawl nets and a remotely operated vehicle in Monterey Bay off California and in the Gulf of Mexico. Some were living as deep as about a mile below the surface. On board, they measured how much light each fish's skin reflected using a spectrometer. Sixteen species, from distantly related families, crossed the ultra-black line. They included the common fangtooth (Anoplogaster), a dragonfish (Idiacanthus) and the small anglerfish that took the record.
The discovery started with a practical problem. Osborn had struggled for years to get usable photographs of these animals. As she put it, "It didn't matter how you set up the camera or lighting—they just sucked up all the light."
Why darkness matters where there is no sunlight
Sunlight fades quickly with depth and does not reach beyond a couple of hundred metres. Below that, the only light comes from animals themselves. Most deep-sea creatures are bioluminescent: they make light through chemical reactions, to attract prey, find mates, confuse predators or illuminate the water around them like a searchlight.
In that world, ordinary black is not good enough. A fish with normal dark skin still bounces back a few percent of any light that falls on it, enough for a predator's glowing lure or searchlight to pick it out. Ultra-black skin cuts that reflection so far that the researchers calculated these fish can stay unseen at about one-sixth of the distance at which a normally black fish would be spotted. That can be the difference between eating and being eaten, and it works both ways: ultra-black predators can approach prey without being noticed, and ultra-black prey can slip past hunters.
So the common idea that the fish are dark "because there is no light down there" gets it backwards. They are dark precisely because there is some light down there, produced by their neighbours.
How the light trap works
All black skin contains melanin, the same pigment that colours human skin and hair. It is packed into tiny structures called melanosomes inside pigment cells. In most cold-blooded animals with black skin, the melanosomes are small and round, like pearls, and the pigment cells leave unpigmented gaps between them.
Ultra-black fish are different in three ways:
- Size: their melanosomes are larger than usual.
- Shape: they are elongated, capsule-shaped, often compared to a Tic Tac.
- Arrangement: they are crammed into a continuous layer just under the skin surface, with no gaps.
Light that is not absorbed straight away scatters sideways into the layer, where neighbouring granules soak it up. Davis compared the packing to a gumball machine. Computer models of skin with different granule sizes and shapes showed that the geometry found in ultra-black fish is close to optimal for absorbing light. Osborn summed it up: "Effectively what they've done is make a super-efficient, super-thin light trap."
Evolved more than once
The 16 ultra-black species are scattered across the fish family tree. That means the trait did not come from a single shared ancestor; it evolved independently several times, a strong sign that it gives a real survival advantage. Biologists call this convergent evolution.
Fish are not the only ultra-black animals. Some birds-of-paradise in New Guinea have ultra-dark plumage, and some butterflies have very black wing scales. Those animals, however, combine melanin with microscopic structures in their feathers or scales that trap light. According to the Smithsonian, the deep-sea fish are the only known system that uses the arrangement of the pigment itself to capture light that would otherwise escape.
What engineers might borrow
Human-made ultra-black materials already exist and are used in telescopes, cameras and other optical instruments to cut stray light. Many of them are fragile, difficult to produce and expensive. The fish suggest a simpler route: a thin layer of the right-sized, right-shaped pigment particles, packed correctly, might achieve similar darkness more cheaply and durably. The researchers mentioned possible uses in optics, camouflage and light-absorbing surfaces such as solar panels.
Common misconceptions
- "They absorb 99.9% of sunlight." They absorb over 99.5 percent of light in general, and the light that matters to them is bioluminescence.
- "All 16 species are equally dark." Reflectance varied between species; the 0.04 percent figure belongs to the darkest anglerfish.
- "They were all caught in one net at 200 metres." Fish came from two regions and a range of depths, some down to about a mile.
- "The colour helps them avoid their prey." It helps them avoid being seen, both by predators and by the prey they hunt.
Sources: Smithsonian press release, 16 July 2020; Duke Today, July 2020; Davis A. L. et al., "Ultra-black camouflage in deep-sea fishes", Current Biology, 2020. Originally reported in English by Big Think.
COMMENTS