Sharks have a "superpower" that until recently, has been completely invisible to humans - they can glow in the dark.
Certain species of the ocean predator can turn themselves bright green, and only other sharks can see it.
And their bioluminescense is unlike any other creature's, new research has found.
"They have a completely different view of the world that they're in because of these biofluorescent properties that their skin exhibits and that their eyes can detect," said Yale University chemical biology professor Jason Crawford.
"Imagine if I were bright green, but only you could see me as being bright green, but others could not."
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Two species - the swell shark and the chain catshark - use brominated tryptophan-kynurenine small-molecule metabolites to glow, says study co-author David Gruber of City University of New York. That's completely different to how other marine creatures such as jellyfish do it, he says - they use fluorescent proteins, which are much bigger.
"It's a completely different system for them to see each other that other animals cannot necessarily tap into," said Dr Gruber.
The molecules do more than make the sharks glow too - they also have anti-bacterial properties.
"Sharks are wonderful animals that have been around for over 400 million years," said Dr Gruber.
"Sharks continually fascinate humans, and they hold so many mysteries and superpowers. 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."
Since 2014 at least four species of sharks have been discovered to have bioluminescent properties, but this is the first time scientists have figured out how.
The research was published Friday in journal iScience.