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Sharks are strange, often misunderstood creatures. They’ve been around for far longer than us humans, and as such, they’ve had time to develop some truly amazing characteristics. Since the deep sea is pretty dang dark, finding your friends is a little difficult, but certain species of sharks have figured out a way around it: they glow in the dark. It’s long been known that certain species of shark transforms the ocean’s blue light into a bright green color, but up until now, exactly how they do it has been unclear. A new study recently published in iScience has cracked the code.

“Studying biofluorescence in the ocean is like a constantly evolving mystery novel, with new clues being provided as we move the research forward,” says David Gruber, a professor at City University of New York and co-corresponding author of the study.  “After we first reported that swell sharks were biofluorescent, my collaborators and I decided to dive deeper into this topic. We wanted to learn more about what their biofluorescence might mean to them.”

Gruber, who worked with a professor at Yale University named Jason Crawford for the study, looked at two species of sharks in particular: the swell shark and the chain catshark. Both of the sharks have two skin tones, a light one and a darker one, so they took samples of the skin and analyzed them. They found that the light skin had a type of fluorescent molecule that wasn’t present in the dark skin.

Perhaps the most interesting part of the sharks’ biofluorescence is that, up until 2016, only other sharks could see it. Realizing there was something else going on that humans couldn’t see, researchers examined shark’s eyes and were able to build a special camera that showed us how they see. Now, three years later, they’re one step closer to unraveling yet another mystery about them.

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So how, exactly, do the sharks do it? Well, in that light-toned skin lies a previously unknown family of small-molecule metabolites. It’s a different mechanism of biofluorescence from most other marine creatures, and researchers believe it helps them identify each other in the dark depths of the sea. It might also be helpful in fighting against microbial infections.

“Sharks are wonderful animals that have been around for over 400 million years. Sharks continually fascinate humans, and they hold so many mysteries and superpowers,” Gruber added in Scimex. “This study highlights yet another mystery of sharks, and it is my hope that this inspires us to learn more about their secrets and work to better protect them.”

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