Despite their name, shy, slow-moving catsharks aren’t all that catlike—or even sharklike, really—compared to their chompier cousins of Jaws fame.
Running just 2 to 3 feet in length, these bashful bottom feeders spend much of their time holed up in seafloor crevices, where their intricately spotted skin camouflages them against the rocks.
David Gruber, a marine biologist at Baruch College, puts it bluntly: “These are not the Michael Phelps of sharks,” he says.
But something about catsharks like the swell shark (Cephaloscyllium ventriosum) and chain catshark (Scyliorhinus retifer) has captivated Gruber for years—a secret power that can make even the most demure of these underwater wallflowers light up a room.
Swell sharks and chain catsharks can transform the ocean’s blue light into a luminous green hue. This phenomenon, called biofluorescence, has been studied in other marine creatures that are known to convert light through the action of a well-known protein.
But as Gruber and his colleagues report today in the journal iScience, catsharks have found their own way to glow, relying instead on an unusual class of small molecules in their skin to produce their signature shimmer. In doing so, these fluorescent fish initiate a discreet dialogue with their peers—one that might play a role in mating displays and more.
“This is a reminder that fish may look different to each other than they do to us,” says Natalya Gallo, a marine ecologist at the Scripps Institution of Oceanography who was not involved in the study. “When we’re thinking about marine ecology, we need to consider this landscape of light.”
Unlike air, water isn’t terribly good at transmitting light. By the time the sun’s rays have trickled down to the seafloor, most wavelengths have been filtered out, save for those that our eyes see as blue.
One of the most effective ways to stand out in this monochromatic world is to undergo a colorful costume change—a strategy that’s been taken up by a menagerie of marine creatures, from jellyfish to corals. In most cases, this trick involves a protein, aptly named green fluorescent protein, or GFP, that absorbs the ocean’s abundant blue light and emits it as a far rarer shade of green. (This is totally different from bioluminescence, in which animals create their own light from scratch, or host bacteria that do it for them.)
Several years ago, Gruber led a team of researchers in describing how the skin of swell sharks and chain catsharks accomplishes a similar feat. They also found that the sharks had evolved specially adapted eyes to match, allowing them to glimpse each other while flying under the radar of less enlightened fish (and, perhaps, nosy scientists).
For sharks, at least, fluorescence can create “high contrast in a dim environment,” says Edie Widder, CEO and senior scientist at the Ocean Research and Conservation Association (ORCA), who was not involved in the study.
What’s less clear, though, is how the sharks produce their green glow in the first place.
Under normal white light, swell sharks are covered in splotches of tan and brown, with tiny beige spots sprinkled throughout. But when viewed through a special camera that mimics shark vision, the tan skin glows green. Brightest of all are the beige freckles, which, when illuminated, almost make the fish look like finned Christmas trees.
To home in on the source of the twinkling, Gruber recruited Baruch chemist Jean Gaffney to identify chemicals more abundant in the lighter parts of swell shark skin. After a battery of procedures and tests, Gaffney was pretty sure she’d managed to extract the glowing catshark compound. But the find came with an unexpected twist: Unlike the ocean’s most famous fluorescent agent, GFP, this new molecule didn’t appear to be a protein.
This revelation put Gruber, a protein chemist, a little out of his depth. So he contacted Jason Crawford, a biochemist at Yale University whose team quickly confirmed that the culprit was a small molecule—a series of them, actually. Together, the molecules comprised a metabolic pathway that processed a modified form of tryptophan, a protein building block that’s found throughout the bodies of humans and the foods we eat. A follow-up experiment revealed that the same compounds were present in the skin of chain catsharks, which are mottled with a more snakeskin-like pattern than their cousins.
Similar metabolic pathways exist in humans and many other animals, where they’re thought to play roles in immunity, brain function, and more. This versatility sets these molecules apart from proteins like GFP, Crawford says, which tend to serve only one explicit function: fluorescence. So the researchers weren’t all that shocked when they discovered that some of the catsharks’ fluorescent molecules also halted the growth of two potentially pathogenic bacterial strains.
It’s not known what came first—the molecules’ microbe-fighting properties, or their eerie green glow. Either way, Crawford says, this likely represents a case of evolutionary co-opting, in which one set of biological tools was repurposed for a second, perhaps equally important, task.
“This study is really well done,” says Rene Martin, a marine biologist studying deep-sea fish at the University of Kansas who was not involved in the study. “Biofluorescence is a field that hasn’t been as widely explored [compared to bioluminescence]...and it’s great to have a more molecular approach.”
But chemistry isn’t the whole picture, Widder points out. Researchers still aren’t entirely sure why these catsharks evolved into toothsome glowsticks.
Gruber thinks a hint might lie in the intricate patterns of light the fish give off. Shark skin is almost entirely blanketed by rough, pointed scales, and in the chain catshark, a subset of these spiny projections are studded with transparent stripes, illuminating a path like the lit aisle of an airplane. Species-specific patterns like these, Gruber says, could help sharks tell each other apart—or avoid cases of mistaken identity.
What’s more, males and females of both catshark species glow in noticeably different ways. Though it hasn’t been confirmed, Gruber says it’s possible green light functions as a sort of sexual beacon, allowing catsharks to identify or even attract potential mates. In other words, fluorescent might be kind of flirty.
Whatever catsharks are using their fluorescence for, they’re probably not alone. Biofluorescence evolved several times in different families of fish, and is present in at least 180 marine species. And there are probably many more that have yet to show their true colors—which means catsharks will almost certainly have to share their subtle spotlight. “We don’t know how deep this goes yet,” Gruber says. “We’re just at the tip of the iceberg.” (For what it’s worth, Gruber is pretty sure that the whale shark, a catshark cousin that can weigh up to 20 tons, is not among the luminous. He’s checked.)
“Even in this blue environment, [these sharks] have found a way to make their world more rich,” Gruber says. “I hope this calls attention to these animals, and the mysteries that are still out there.”