How Sharks Glow to Each Other Deep in the Ocean

Let us dive into the sea, beyond the colorful world of the sun. About 1,000 to 2,000 feet down, we’ll arrive at a place where only blue beams in sunlight can penetrate. This is the home of the swell shark and chain catshark. Look at them with your human, land-ready eyes, and all you’ll see are a couple of unimpressive fish, spotted in shades of brown, beige and gray.

But peer through a blue filter, more like the way these sharks see each other, and behold beaming beauties robed in fluorescent green spots.

Recently scientists discovered that these sharks see the world totally differently than we do. They’re mostly colorblind, with eyes that can detect only the blue-green spectrum. This means when the sharks appear to change color in the blue water, they’re almost projecting a secret code to other sharks: One pattern male, the other female — come and get it. But just how they take blue light from their dull environment and transform it into a neon sign has been a mystery.

ImageHow Sharks Glow to Each Other Deep in the Ocean
David Gruber with his camera.CreditKyle McBurnie

In a study published Thursday in iScience, researchers reveal the secret behind this magical transformation: Molecules inside their scales transform how shark skin interacts with light, bringing in blue photons, and sending out green. This improved understanding of these sharks’ luminous illusions may lead to improvements in scientific imaging, as the study of biofluorescence in other marine life already has.

A swell shark in Scripps Canyon. Video by David Gruber

“I think this is just yet another amazing feature of shark skin that we didn’t already know about — just adding to their list of superpowers,” said David Gruber, a marine biologist at City University of New York and an author on the paper.

These aren’t the charismatic predators “that make it onto shark week,” he adds. “But if we look at them deeply, we’ll see that there are hidden secrets and beauty.”

Shark skin is weird, plated in toothlike scales called denticles. To find their hidden beauty, Dr. Gruber worked with Jason Crawford, a biochemist at Yale University, and other colleagues. Dr. Crawford helped determine what metabolic pathways, or sequences of chemical reactions inside cells, could explain the neon shark skin of the swell and catshark.

Under the microscope, they noticed several types of dendrites that, depending on their size, emitted green light of varying intensities in response to blue light. Some dendrites focused the light, channeling it outward like a kitschy fiber optic light display.

Denticles, fluorescing, right and not fluorescing, on a swell shark.CreditJames Weaver
Chain catshark denticles, fluorescing, right, and not.CreditJames Weaver

Examining the skin more closely, the team found a set of molecules that were derived from tryptophan, an amino acid important for sleep and mood in humans. These molecules also contained bromine, which changes how the molecules, and essentially the skin, interacts with light. Those in the light skin, for example, could turn blue light into green light, creating that neon beacon for the other sharks to see.

Many fluorescent marine animals seem to have evolved their own techniques for transforming light. The most well-studied among them is the use of green fluorescent protein, which can make neon rainbows out of jellyfish, corals and other animals in shallower depths. But the sharks’ process was unlike any of these.

These small-molecule metabolites in the shark skin may have stuck around because it helps sharks recognize mates in dark blue places. But some also have known antibiotic properties, which may explain why the shark skin stays so clean around so much bacteria on the seafloor.

Still, many questions remain: Why do these sharks fluoresce while other species that aren’t so different from these two don’t?

“It’s all like a big mystery novel,” said Dr. Gruber. “I’m sure there are many more species of fluorescent sharks out there in the ocean that we have yet to encounter.”

A biofluorescent chain catshark. Video by David Gruber

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An earlier version of this article misstated the name of the tooth-like projections on shark skin. They are denticles, not dendrites.

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