The blackest fish in the sea have 'really cool' camouflaging properties in their skin

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Karen Osborn's eureka moment started with some pretty mediocre photographs.

The marine scientist for the Smithsonian National Museum of Natural History's invertebrate zoology department was trying to capture images of deep-sea marine life. But the black fish she'd photographed were nearly impossible to decipher.

"You would crank up the lights and put as many flashes and strobes on it as you could … and still, I couldn't see anything of the fish. I just would get a silhouette," Osborn told As It Happens guest host Nil Köksal.

But those photographic misadventures spawned a new line of scientific inquiry, as Osborn and her collaborators set out to discover how those species of fish get their inky black camouflage. 

Their findings, published in the journal Current Biology this week, not only shed light on how these fish evolved, but have some important applications for technology. 

"Originally when I was trying to figure out why I couldn't get any decent photographs of these of these animals, it got me thinking about what happened, you know, why is all the light just disappearing into them?" Osborn said.

That led to a conversation with her friend, Sönke Johnsen, a biology professor at Duke University, about why these species of deep-sea fish are so black and hard to spot. They soon realized that no other team of scientists had set out to answer those questions.

"I started taking tissue samples to try to figure out what was going on with the skin to see if there was some kind of structure there," Osborn said. 

"Because typically ultra-black things have a layer of pigment, and then on the top of that layer of pigment, they have some kind of structure that works as a light trap."

But Osborn and her colleagues discovered that these species work differently. 

"They don't have any structure on top of their pigments. They just have a layer of pigment. And so what we found out about that layer of pigment was the really interesting thing here."

Light-absorbing camouflage

In these species, the pigment structure in their skin is unique in its ability to dissipate light. This layer on the fish's skin contains something called melanosomes — tiny cellular structures that store, transport and synthesize the common light-absorbing pigment melanin.

The researchers found that the size and shape of the melanosomes in these fish are optimal for reducing the amount of light that's reflected off them, which is what makes sea creatures visible to prey. 

"Being ultra-black for these deep-sea fish is a real advantage to them because they live out in the midwater, the open deep ocean where there is no structure; there's nothing to hide behind. There's no kelp, there's no rocks … so they have to find a way to camouflage in the middle of nothing," Osborn said.

"It's kind of like playing hide-and-seek in a football field, except at least with a football field … you're not worried about something finding you from below."

Of the 18 species of fish the researchers examined, 16 qualified as ultra-black — meaning that they reflect less than 0.5 per cent of the light that hits them.

"What was really cool about how broadly we found this phenomena in deep-sea fish is that of those 16 species, they come from six different orders of fish."

That means that this same ultra-blackness evolved over a long period of time in at least six different incidences. 

"And that's really cool, right? Because they're all solving a problem of how to camouflage in the deep, dark sea in the same way."

If these fish look a little scary, with their mouths full of sharp teeth, it's for good reason, Osborn said.

"They don't get to eat very often. They have to take advantage of what they can. So a lot of them have these huge teeth so big that they can't even close their mouths over them," she said. 

Applications for technology

The findings have interesting potential technological applications, the researchers say.

"Ultra-black materials are used for various technology, like lining the insides of really high-powered telescopes, lining the inside of cameras or camera lenses, so you don't have … any extra light bouncing around when you're really trying to measure really small amounts of light," Osborn said.

"Those materials currently are incredibly expensive and incredibly delicate to manufacturer. And so if we could take this even simpler system of creating an ultra-black material and replicate it, then potentially we could make much, much cheaper materials to do that with that are just as, or even more effective, than what's there.

"So that's that's kind of exciting." 

Written by Brandie Weikle. Interview produced by Menaka Raman-Wilms and Chloe Shantz-Hilkes.