16 species of fish have been discovered whose skin absorbs 99.9% of light

Last year, a team of marine biologists studying the deep waters of California discovered 16 species of ultra-black fish that absorb more than 99% of the light falling on their skin, making them virtually invisible to other creatures living in the deep sea.

The fish were caught at a depth of over 200 metres near California’s Monterey Bay. At this depth, sunlight disappears completely and this is one of the reasons why many deep-sea species have developed the ability to illuminate dark waters using bioluminescence. The researchers published their findings in the journal Current Biology.

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As it turns out, however, there are species of deep-sea fish that do not want to be noticed. To counteract bioluminescence, some of them have evolved to create an ultra-black skin that absorbs light extremely well. Only a few other species are known to have this strange trait, including birds of paradise and some spiders and butterflies, among others.

When scientists first saw the new deep-sea species, it was not immediately obvious that their skin was ultra-black and had such unusual properties. However, marine biologist Karen Osborn, co-author of the paper, noticed something strange in the photos of the fish she took

I tried to take pictures of these fish, but the results were terrible and no detail was visible in the photographs taken. I couldn’t believe how it was possible that after pointing two flashes at the subject, all that light just disappeared,” said Osborn

Only after examining skin samples under a microscope did scientists discover that the skin contains a layer of organelles called melanosomes, which contain melanin, the pigment that also gives colour to human skin and hair. It is this layer of melanosomes that absorbs most of the light that falls on them.

Interestingly, what is not absorbed scatters sideways and is absorbed by neighbouring pigments that are right next to it. So what we have here is an extremely efficient system that uses only pigment particles to create a kind of light trap, Osborn added

It is also interesting to note that this unusual ability, was not passed on to these species by a common ancestor. According to the researchers, each of them developed it independently and, as a result, different species use their ultra-blackness for different purposes. Echiostoma barbatum, for example, only has black skin during adolescence, when it is defenceless.

Other species e.g. Oneirodes, which use bioluminescent lures to attract prey, probably developed ultra-black skin in the process of evolution to avoid reflecting the light produced by their own bodies. Meanwhile, species such as Cyclothone acclinidens have ultra-black skin only around their intestines, presumably to hide the light of the bioluminescent fish they have eaten.

Given that the newly described species are all ones the team found off the coast of California, there are probably many more of them swimming in the deep ocean and perhaps much darker, ultra-black fish.

Photo: Karen Osborn/Smithsonian NMNH