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Deep into the ocean even the slightest glimmer give you away. Which is why some fish have evolved to be so dark that they absorb any light that hits them.

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Once you drop below about 200 meters deep into the ocean, there is almost no sunlight. But hiding in that darkness isn't as easy as you might think. That's because many creatures produce their own light through bioluminescence, which they use to lure in or expose prey.

And if you're trying to hide in the deep sea, reflecting even the tiniest bit of that light can be a dead giveaway. Which is why some species of fish evolved to absorb almost all of the light that hits them, making them nearly invisible in the deep sea. As unusual as this might sound to us land creatures, in the deep sea it actually isn't that rare.

A study published in the journal Current Biology in July 2020 found that 16 species of fish absorb more than 99% of the light that hits them, and that some of them are only distantly related. It turns out that in the deep sea being extremely dark is a pretty good way to avoid predators, so evolution has pushed a lot of species in that direction.

But these fish aren't just ordinary black like a t-shirt or fresh asphalt, these fish absorb so much light that it just completely disappears into them. Like one species absorbs 99.95% of the light that hits it. And that's not something we see in the everyday world.

To find out more about how they do this, the authors examined the skin of nine of the ultra-black fish from their study and they found that the fishes' strategy for making themselves so exceptionally dark is both unique and in some ways kinda simple. See there are a few other species with ultra black coloration like butterflies and spiders, and they get their color from complicated nanostructures that absorb light in a certain way. But these fish don't have anything like that. They just have a lot of melanin, which is the same pigment humans have in our hair and skin.

What makes them unique, though, is that these fish organize that pigment in a very special way. In their skin, structures called melanosomes, which contain melanin, are packed so tightly together that light can't reach other calls that don't have pigment. Not only that, but the size and shape of the melanosomes also prevents light from scattering outwards. Instead, any light that doesn't get immediately absorbed gets scattered into other melanosomes.

So their skin is essentially a thin, but endlessly absorbing, layer of darkness. And for the fish, that's pretty good for hiding from other deep sea creatures that basically have bioluminescent searchlights built into their bodies. Some of these ultra-dark fish even prey on those bioluminescent creatures, and they have evolved ultra-black pigmentation in their gut, so they can do so without their glowing bellies giving them away.

So in the end, this extreme pigmentation is incredibly helpful for surviving in the deep sea. But here on land, it might also be helpful to us. Engineers are interested in designing synthetic ultra-black materials for use in a wide range of applications, like cameras and optical instruments used in space, where a little bit of stray light can be a big problem. And while we have some materials like this already, they're complicated to manufacture. So scientists are looking into new ways to design them, and these incredible fish might just help us do that.

If you wanna learn more about engineers' attempts to create ultra-black materials, we have an episode about the darkest material ever made.

And as always, thank you for watching Sci Show.