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This shark glows using a process previously unknown to science

For a shy shark that spends most its time resting on the sea floor (see video above), the chain catshark certainly wears a flashy outfit. Both it and the swell shark are the only sharks known to give off a green glow when exposed to light. Now, researchers have tracked down the source of this fluorescence—and it’s nothing like they’ve seen before.

Glowing jellyfish may be the most famous fluorescent sea creatures—radiating brilliant greens and blues—and scientists have used their proteins to track proteins in cells and even create glow-in-the-dark cats. On the trail of more such proteins, researchers surveying glowing fish—there are 180 known so far—focused on the chain catshark and swell shark, native to the waters off the coasts of the U.S. Northeast and Southern California, respectively.

When they took a piece of skin from living sharks and extracted all of the compounds, they were surprised: The glowing compounds weren’t proteins at all, but breakdown products of an unusual form of the amino acid tryptophan. In animals, most of the tryptophan goes toward creating proteins. But some is converted to a compound called kynurenine, which in turn is a building block for niacin, a vitamin, and is involved with diabetes, inflammation, depression, and even cancer. When an atom of the element bromine tags along, this kynurenine releases a green glow when exposed to the blue light below the ocean’s surface, the team reports today in iScience.

These sharks typically lie on the ocean bottom, waiting to ambush octopuses and other sea creatures. No one really knows why they glow, though it might help shark species tell each other apart or recognize potential mates, as males and females have different fluorescent patterns. But these compounds may also protect the sharks, as one of the glowing compounds kills bacteria. And given that these animals wallow in bacteria-laden sediments on the ocean bottom, such antibacterial properties likely come in very handy.

*Correction, 28 August, 11:25 a.m.: In this story, the locations of the swell shark and chain catshark have been changed to reflect their actual home ranges.