Elusive sharks that hang out in rock canyons just off the Atlantic and Pacific coasts have a secret: They communicate with each other by releasing glowing light from their skin, in patterns that are unique to each species and even sex. This signaling ability isn’t visible to the naked human eye and was only discovered a few years ago, after scientists shined the right light on the animals. Now a team of researchers has discovered how these animals create the glowing effects.
The research, published this week in the journal iScience, shows that the glowing green spots and stripes, a form of biofluorescence, is created by a pathway different from any other known form of biofluorescence. The process involves molecules that were previously unknown to science.
Why do sharks glow? After discovering more than 200 biofluorescent marine creatures, scientists are studying the role of this phenomenon.
What’s even more exciting, says study co-author David Gruber, is that these newly discovered fluorescent compounds also appear to act as antimicrobial agents in the sharks’ bodies, likely protecting them from disease and algae. The findings “open up new mysteries and give yet another amazing property to sharkskin,” says Gruber, a National Geographic Explorer and a researcher at City University of New York’s Baruch College and the American Museum of Natural History.
One of the shark species studied also directs the glowing light through tiny, rigid denticles on its skin to enhance the fluorescent effect. Gruber calls these structures "optical light guides" or "lightsabers." They’re similar to structures used by some self-glowing, or bioluminescent, sharks but had never been seen before on a biofluorescent shark.
How sharks glow
Scientists know of more than 200 species of sharks and bony fish, as well as marine turtles, that glow, though they think the numbers could be much higher in the ocean. Many invertebrates are also known to glow, from corals to jellies to crustaceans. Biofluorescence works when blue light—the dominant colour present in ocean water—is absorbed by an animal’s skin pigments and then re-emitted as the colour green. That’s a distinct process from bioluminescence, in which animals either produce their own light through a series of chemical reactions or host other organisms that give off light.
Gruber and colleagues discovered in 2014 that a pair of related species of catsharks living off the U.S. glow by releasing green light. This includes the chain catshark (Scyliorhinus retifer) and the swell shark (Cephaloscyllium ventriosum). These fish grow to about 90 centimetres long and spend much of their time on the bottom, hiding in crevices.
Over the past two years, Gruber enlisted an interdisciplinary team of scientists to study these glowing sharks. He obtained samples of skin from catsharks hauled up as bycatch by fishermen and from an aquarium. Gruber sent the samples to chemists Hyun Bong Park and Jason Crawford, and others, at Yale.
Park worked to isolate the colour pigments from the skin. He quickly realised that this was something different than had been seen before. Most animals that use biofluorescence rely on molecules known as green fluorescent proteins (GFPs), which have since been adopted for use in the biomedical industry as handy ways to track genes in experiments. But the catsharks weren’t using GFPs.
Park, Crawford, and colleagues put the skin pigments through a series of advanced tests, eventually discovering that the pigments are much smaller molecules than GFPs. They are a form of metabolite, or small molecule that forms as an intermediate step in metabolism inside a living thing. Specifically, they are brominated (or bromine-containing) tryptophan-kynurenine metabolic products.
Those compounds had never been seen before. But what’s even more intriguing, they say, is that the molecules are closely related to compounds that play an important role in the human body. Minus the bromine, similar molecules impact depression and other mental processes, and they are related to the compounds in medications that are currently under study.
There’s thus much more that needs to be learned about these processes, says Crawford. “The overwhelming majority of metabolites in any organism remain largely unknown,” he says. “And the chemistry of biofluorescence in the ocean is hardly studied at all. It really remains a black box—a colourful black box—of unknown chemistry.”
Park and Crawford were surprised to discover that the metabolites in the sharkskin, in addition to causing glowing, also killed potentially harmful bacteria—specifically, groups of bacteria known to cause MRSA and cholera. The sharkskin is “chock full of these compounds,” says Crawford.
Crawford says he doubts the metabolites would prove useful as a human antibiotic, because the concentration needed would likely be impractical. But, he says, “the fact that sharks accumulate it to such a high concentration suggests that it is a natural way of protecting the skin from potential microbial infection.”
Park adds that the pigments may also help protect the sharks from getting sunburned by UV rays underwater.
“The fact that they discovered a different pathway to fluorescence is pretty exciting,” says biologist Ann Money, who was not involved in the research. Money, who studies biofluorescence in corals as the director of education and research at the Oklahoma Aquarium, says it’s likely additional pathways will be discovered in other animals.
She finds the antimicrobial aspects the most exciting part of the new research. “That’s been a hot topic: What is it about sharkskin that helps protect them from infection?” she asks.
One of the studied species, the chain catshark, was found to channel the glowing light along tiny denticles protruding from the rough sharkskin. The denticles look like tiny teeth—in fact, one theory holds that teeth actually evolved from these skin structures. The scientists don’t yet know much about how the lightsabers work, or why some species have them while others don’t. The lightsaber structures may even prove useful to other applications—as an analog, scientists are already designing swimsuits and aircraft inspired by sharkskin to reduce drag.
The next step is to figure out how sharks make these compounds, Park says. Crawford says he and his colleagues hope to keep “trying to decode the chemistry of this colourful world.”
Gruber adds that better understanding of how and why sharks glow can provide new insights into how they live and reproduce. Catsharks, in particular, are poorly documented and remain largely mysterious. That makes it harder for conservationists to figure out how best to protect them in a world where humans kill an estimated 100 million sharks a year and many species face steep declines.
“We’ve just barely scratched the surface of biofluorescence,” Money says. “We know so little about it because no one could see it. But we’re starting to figure it out.”
Such research “calls to attention how well adapted, beautiful, and impressive these animals are,” says Gruber.