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.