The ultra-black fish that disappear in the deep

Researchers have recently discovered 16 species of ultra-black fish, who have evolved specialised pigmentation in their skin which absorbs over 99.9% of the light that hits them, making them practically invisible in the deep ocean.

Pacific blackdragon
A Pacific blackdragon, one of 16 species of deep-sea fish to be classified as ultra-black in a new study. In reality they are much darker in colour, but these special photos help show us what they look like (Photo by Karen Osborne at Smithsonian NMNH)

A new study by researchers from Duke University and the Smithsonian National Museum of Natural History (NMNH), has revealed 16 species of ultra-black fish that are capable of absorbing at least 99.95% of the light that hits them, thanks to specially adapted pigments in their skin. Unlike other animals that qualify as ultra-black, such as birds of paradise, snakes, spiders and butterflies who all use their darkness to stand out, these deep-sea fish take the phenomenon to the next level and use their extreme blackness to disappear in the deep. Not only is this an extremely unique and interesting adaptation, but it could also hold the key to some useful human applications as well.

Ultra-black fish

The discovery of these ultra-black fish was first made by Dr Karen Osborne from the Smithsonian NMNH who came across one whilst trying to net deep-sea crabs on a research trip. What she instead pulled up was a fangtooth fish which, as hard as she tried, she was unable to take a proper photograph of. The reason behind this was that it was absorbing almost all of the light that hit it, which sparked a new study to see how many similar species could be found in the deep. Which basically involved randomly dropping nets into the deep ocean to see what they could pull up.

A fangtooth fish similar to the one first disvoered by Dr Osborne by accident (Photo by Karen Osborne at Smithsonian NMNH)

As a result the team found 16 species of deep-sea fish which reflected less than 0.5% of light, the minimum required to be classified as ultra-black in the animal kingdom. They were all described in a new paper released by the researchers, which was published in the journal Current Biology last month. However unlike other ultra-black animals found on land, some of these fish were capable of absorbing as much as 99.956%. For comparison most man-made black products absorb 6 times less light than these fish. Speaking to the New York Times lead author Alexander Davis, a PhD student at Duke University, described looking at the ultra-black fish as “like looking into a black hole”.

Disappearing act

You may think that living in the deep ocean disappearing would be the last thing you would have to worry about. Thousands of metres below the surface almost no sunlight penetrates to these depths that are naturally in a state of almost pitch blackness. However for the creatures that live there being seen is a bigger problem than you might expect. That is because most deep-sea predators are capable of creating their own light through what is known as bioluminescence, which can give them great sight over short distances. Some creatures try to get around this by becoming transparent, but often end up reflecting some amount of light back as a result. Therefore the best strategy is to absorb the light which shines on you, which is what makes the ultra-back fish so successful at avoiding predation.

Powerful pigmentation

The key to the absorption of light by ultra-black fish is the pigmentation in their skin. In particular melanin, which is also found in other species of fish and also in humans to protect us from UV rays. Using advanced microscopy the researchers found that the melanosomes of these fish, the cells that hold the pigmenting chemical, were different from other fish in three important ways – they were larger, more densely packed with pigment and capsule shaped rather than rounded. The result of this is that not only does each cell absorb more light, but what it can’t absorb is reflected into the adjoining cells rather than back towards the light source. This is what allows them to absorb almost all of the light that is shone upon them.

The specialised pigment containing cells within ultra-black fish is what allows them to absorb so much of the light that hits them (Photo by Karen Osborne by Smithsonian NMNH)

Potential applications

As previously mentioned these ultra-black fish are capable of absorbing much more light than any man made black object. Therefore scientists are now looking to see whether the pigmentation they use can be replicated in human technologies. One of the biggest areas of research is in military application, since being able to use ultra-black camouflage in covert night operations or on deep-sea submarines would obviously yield significant advantages. In addition to this being able to absorb so much light also gives any ultra-black material the ability to protect people from X-rays and other forms of radiation, which can be useful in a number of different ways. However, regardless of whether anything viable comes from this discovery it is yet another example of how further exploring the deep ocean and the creatures that live there can be extremely valuable and important.

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