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.