“These two species are often caught by fishermen as bycatch and we studied them not far from San Diego’s best surfing beaches, but no one had looked at them,” says Gruber.
The catsharks generally live deep enough that they are bathed only in blue light, since the rest of the wavelengths of light are blocked by the water above. The sharks have a special, as-yet-unidentified pigment in their skin that absorbs blue light and re-emits it as the color green, in a process called biofluorescence. This is different from bioluminescence, where animals either produce their own light through a series of chemical reactions, or host other organisms that give off light.
To better understand how the biofluorescence works, Gruber and team examined the sharks’ eyes. They found really long rods, which help the animals see in low light. They also found one visual pigment for color detection, which lets them see in the blue and green spectrum. Humans, in contrast, have three color pigments—red, green, and blue—allowing us to see a wider range of colors. At the high end, mantis shrimp have 12 pigments and can see an even wider array of colors.
Once the scientists worked out how the sharks likely see, they created a “shark-eye” camera to approximate the vision. They did this by adding filters in front of a lens of a Red Epic camera to restrict the wavelengths of light passing through, mimicking the shark's eye. To enhance the effect of fluorescence, they also sometimes shined blue lights. The team donned scuba gear and swam into Scripps Canyon, off San Diego, to find catsharks.
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