Even in bright light, the fish appears to be just a silhouette.

Researcher Karen Osborn, affiliated with Smithsonian’s National Museum of Natural History , was with colleagues hunting deep-sea fish some time ago. And successfully. Using trawls, they retrieved several dark deep-sea fishes, which Osborn then wanted to photograph in her enthusiasm. But no matter how hard she tried, she couldn’t capture details in the fish skin. “Hat didn’t matter how you set the camera or exposure – they just suck up all the light,” says Osborn.

Ultra-black
It intrigued her and she and colleagues decided to take a closer look at the deep-sea fish. Laboratory measurements now show why it was not possible to properly photograph the fish. Many deep-sea fish have been found to absorb more than 99.5 percent of the light hitting their surface. And thus they even look like silhouettes in bright light; details cannot be seen.

The fish are ultra-black . Blacker than a new car tire. Blacker than a black sheet of paper.

Camouflage
The ultra-black fish skin is very useful for fish deep in the ocean. Since sunlight reaches only a few hundred meters below sea level, it is pitch dark in the habitat of the deep sea fish. To see something – or to be seen yourself – many organisms themselves provide light by means of bioluminescence. With that self-produced light, they attract potential partners, trick predators or lure prey. But that light can also fall on animals in the environment, which are therefore clearly visible against your will. Unless those animals have a good camouflage technique. And that’s what these ultra-black deep-sea fish have. “If you want to blend into the infinite black of your surroundings, soaking up every photon (light particle, ed.) That hits you is a great way to do that,

An ultra-black deep-sea fish, belonging to the species Anoplogaster cornuta . You can see the same fish at the top of this article. Image: Karen Osborn, Smithsonian.

Light cannot escape
Laboratory research also provides more clarity on how the fish acquire this ultra-black color. It is thanks to melanin, the same pigment that colors our skin and protects it from sunlight. The deep sea fish not only appear to have a lot of this pigment. The pigment also appears to be spread in a unique way in the skin. Melanosomes – parts of skin cells filled with melanin – are packed tightly together and together form a layer close to the surface of the skin that light cannot actually escape. Because the size and shape of the melanosomes and the way they are arranged ensures that all light that they do not absorb directly is sent to nearby melanosomes, which can then absorb it. “What the deep sea fish actually have is a super-efficient one, super thin trap for light, ”says Osborn. “Light does not bounce back, light does not pass through, it just disappears in this layer and it is gone.”

Sixteen species
That this approach works is evident from the fact that Osborn and colleagues have found no fewer than sixteen different types of deep-sea fish that are ultra-black. The most successful example was a small fin-arm that reflected only 0.04% (!) Of the light. Incidentally, the sixteen species are only very distantly related, indicating that the ultra-black surfaces have evolved more than once in different branches of the fish family tree.

Here you can see an ultra-black deep-sea fish, belonging to the species Idiacanthus antrostomus. It is the second blackest fish the researchers examined in their study. In order to capture the fish – and also details on the fish skin – Osborn had to go far, she tells Scientias.nl when asked. “These two fish (the fish in the photo above and the fish in the photo at the top of this article, ed.) Are the only two fish where I had enough time and patience to try enough different exposures and capture the details. It’s really a matter of using a lot of light, trying a lot of different angles and settings to get to those details. ” And then she also tried to edit the photos to bring out as many details as possible. “And even that rarely worked.

Birds
Deep sea fish are not the only ones to have ultra-black surfaces. Earlier, ultra-black feathers have already been found in some birds. They often lie next to colored feathers that appear even more vibrant in color due to the ultra-black. But where these animals acquire the ultra-black color through a combination of melanin and structures – a kind of small tubes, for example – that can still catch any escaping light, the deep-sea fish tackle it slightly differently in their unique environment. “This is the only system we know of that uses only the pigment itself to control light that is not initially absorbed,” says Osborn.

The research not only provides more insight into deep-sea life and the unique adaptations that some species have undergone to make it a success here. We may also be able to learn from these fish and mimic the way they create ultra-black surfaces to create ultra-black objects that can be useful, for example, in sensitive optical equipment such as telescopes or cameras. Nowadays, the strategy often used by birds with their ultra-black feathers is still used when producing ultra-black materials. But that could possibly be better. “Instead of building a kind of structure that catches the light, you can achieve the same absorption by making absorbent pigments of the right size and shape, but a lot cheaper.”

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