Marine biologists have stumbled upon a number of fish species that make themselves invisible in the sea, by absorbing 99.956% of the light that hits them.

Karen Osborn, marine scientist at the Invertebrate Zoology Department of Smithsonian’s National Museum of Natural History, and her colleagues were documenting deep-sea fish on board a research vessel not far from Monterey Bay, California, when they discovered something quite out of the ordinary.

When Osborn was trying to take pictures of a fangtooth – a deep-sea fish with large, sharp teeth – all she could capture were the outlines of the fish. Despite her experience photographing such animals, and the custom-built camera set-up, she couldn’t capture any detail in the photos, almost as if the fangtooth had sucked up all the light. But then Osborn remembered that it wasn’t the first time she had issues taking pictures of deep-sea fish.

“I had tried to take pictures of deep-sea fish before and got nothing but these really horrible pictures, where you can’t see any detail. How is it that I can shine two strobe lights at them and all that light just disappears?” Osborn told Wired.

After a thorough examination, the team of researchers have found that cameras couldn’t capture the animal in detail because of a pigment in the skin of the fish, called melanin – the same pigment that colors human skin and hair. In the fish’s case, however, it enables the animal to absorb more than 99.5% of the light that hits it.

So, because of the melanin that is distributed in a layer of the skin and makes the fish ultra-black, these animals can pretty much become invisible in the sea. “Effectively what they’ve done is make a super-efficient, super-thin light trap,” said Osborn. “Light doesn’t bounce back; light doesn’t go through. It just goes into this layer, and it’s gone.”

But why do these fish need to absorb light, when sunlight never reaches the depths where they live anyway?

Well, even though sunlight doesn’t penetrate that deep, there are other sources of light down there. In fact, light is very common in the dark deep, in the form of bioluminescence. This biological light is produced by deep-sea creatures. Some of them may use this bioluminescent glow to lure prey, while others just want to attract mates.

However, these living light sources could attract attention for the wrong reasons as well. It can blow the cover of a stealthy hunter, or could expose potential prey – unless they have the right features to stay hidden. Therefore, having ‘ultra-black’, light-absorbing skin can help these creatures blend in to the darkness of the deep, increasing their chances of survival.

All in all, Karen Osborn and her team have identified 16 species of fish that camouflage themselves by absorbing more than 99% of the light that hits their skin. Surprisingly, there wasn’t even 1 common ancestor among these 16 fish that could have passed on this disguise, and rather, the different species have developed it independently.

That being said, there are probably more than just 16 species of ‘ultra-black’ fish in the world; they are just yet to be found. After all, most of what is in the depths of our oceans is still unknown to us.