David Gruber sees glowing life forms everywhere he looks. He’s found dozens of fluorescent corals in the Great Barrier Reef. In 2014, he reported on more than 180 fish species that fluoresce. Last year, he even stumbled across fluorescent sea turtles.
Now Gruber, a biologist at the City University of New York’s Baruch College, wants to know why all these species are glowing. He and his colleagues built a “shark-eye” camera to simulate how fluorescent sharks appear to each other, in part so that humans view these creatures a little more kindly.
Animals like fish and turtles don’t generate their own light, as a firefly does. Being biofluorescent means molecules in their skin absorb light of a certain wavelength, and bounce it back at a different wavelength. In the ocean, that usually means they absorb blue light and transform it into green, red, or orange. It’s hard to notice with human eyes in the dim ocean, though a person might detect a greenish cast to a shark’s skin, for example.
Finding biofluorescence in so many sea animals led Gruber to wonder what advantage it conferred upon a species. He and his co-authors have begun to answer that question for two biofluorescent sharks, the Atlantic-dwelling chain catshark and the Pacific-living swell shark. They have done so by looking deep into their eyes—not in the romantic sense, but in the dissection sense. They found that although these species seem to have excellent low-light vision, they’re monochromats. That means unlike humans, who build color vision using three types of pigment molecules in our eyes, these sharks have just one pigment. It detects blue-green light.
That makes sense, Gruber says. “The ocean is this huge blue filter, and it becomes more perfectly blue as you go deeper.” If there aren’t any other colors of light to see, why bother?
Next the team asked what parts of a shark’s body fluoresce. Both species have mottled patterns, which in an aquarium’s light would appear light beige-and-dark gray, or light-beige-and-black. (The chain catshark’s pattern looks almost like a giraffe’s.) The researchers studied sharkskin in the lab, and used a special camera setup to vividly capture the sharks’ fluorescence in the wild. They dove at night, shining blue light onto the animals. Then they used a camera with blue light filtered out of it to capture only the green fluorescence that shone back at them.
The fluorescence mostly came from the sharks’ beige patches. But the swell shark also revealed “these twinkling, very bright spots all over,” Gruber says. Additionally, the researchers saw fluorescence shining eerily from the sharks’ eyes.
Finally, the team used what they’d learned about vision in the two species to create the shark-eye camera. It’s “a very high-resolution movie camera,” Gruber says, with filters added to simulate what the monochromatic animals would see.
The result doesn’t look too splashy. But the real question is what difference it makes to a shark. Does the extra green light make a shark’s patterns easier to see against its ocean backdrop? In a model, the researchers found that as sharks swim deeper in the ocean, their fluorescent patterns should stand out more strongly to other sharks’ eyes. They published their results in Scientific Reports.
Not everyone is convinced by the team’s model. Nathan Hart, a biologist at Macquarie University in New South Wales, Australia, who studies shark vision, wonders whether blue light in the deep ocean is really bright enough to make the sharks’ fluorescence stand out. Christine Bedore, of Georgia Southern University, adds that she’s “pretty doubtful that the fluorescence has any ecological relevance.”
Gruber stresses that the study is only a first pass at figuring out how sharks see their own glow. And biofluorescence seems to have evolved many times in fish—a clue that it has a purpose. “It makes perfect sense if you think about life in the blue ocean,” Gruber says. “Why wouldn’t they come up with a way to make their world richer in texture?”
If fluorescence does help sharks see other members of their species, it could help them find each other for mating or socialization. But biofluorescing might also make the sharks more obvious to predators. Gruber says it’s not clear what animals eat these species—maybe other, bigger sharks—or what their own visual abilities might be. Very few shark species have been “brought to the eye doctor,” he says.
“This study really opened my eyes up,” Gruber adds (no pun intended), “to how little we know about shark vision.”
One of his next steps will be to create cameras representing other animal eyes, thanks to a new technology called a hyperspectral camera. This kind of camera could let researchers record footage underwater, then write algorithms back in the lab to transform the footage into the viewpoint of various species.
Ultimately, Gruber hopes seeing the world through other animals’ eyes will have practical benefits. It’s hard to convince people about the importance of protecting the ocean, he says, when they can’t relate to the animals that live there. People may think of marine creatures as mysterious, or scary, or simply food. But if we put ourselves into their perspective, Gruber believes, “It could draw us closer to these species.”