Using an underwater camera equipped with "shark eye" vision, researchers reveal that catsharks can see each other’s bright green fluorescence. And the contrast between their glowing patterns and their dark surroundings increases the deeper they swim. The findings, published in Scientific Reports this week, suggest that catsharks use biofluorescence to communicate with one another. 

Like other marine fish, catsharks are surrounded by a mostly dim, deep blue since water absorbs most of the visible light from the Sun. But they have fluorescence distributed in patches and stripes all along their body. More than 180 species of fish are capable of biofluorescence: They absorb (mostly blue) light with short, high-energy wavelengths, transform it, and then emit it as a different color (green, red, and orange) with longer, lesser-energy wavelengths. Why they do this, however, remains unknown. 

To investigate, a team led by David Gruber of Baruch College studied the light-sensing photoreceptor cells of two catsharks: chain catsharks (Scyliorhinus retifer) of the western Atlantic and swell sharks (Cephaloscyllium ventriosum) of the eastern Pacific. They designed a camera that would allow them to simulate the light that sharks see, and then they imaged the catsharks in the lab. They also went diving in San Diego County’s Scripps Canyon to record swell sharks in their natural habitat. 

^{Scientific biofluorescent imaging camera and lighting system help researchers image sharks underwater in Scripps Canyon, San Diego. Kyle McBurnie}

After shining high-intensity blue light arrays in the water, the team combined their custom-made "shark eye" camera with another underwater camera that has green filters to block out blue light. The team discovered that bright green fluorescence makes the catsharks more visible to nearby catsharks in the low-light depths where they live. Additionally, female swell sharks also have a unique "face mask" with light spots in the center on each side and denser spotting on their abdomen that extends farther than in males.

At about 30 meters (98 feet), the researchers only observed the top of the shark's depth range. But using mathematical models based on their imagery, the team thinks that the contrast of catshark patterns will increase with depth. Furthermore, a study of their evolutionary relationships show that biofluorescence has evolved at least three times in cartilaginous fishes in distantly related families. This repeated evolution, combined with a visual adaptation to detect it, suggest that sharks communicate with each other using the light they produce.

"This is one of the first papers on biofluorescence to show a connection between visual capability and fluorescence emission, and a big step toward a functional explanation for fluorescence in fishes," study co-author John Sparks from the American Museum of Natural History said in a statement.

Dimitri Deheyn of Scripps added: "Sharks can see the fluorescence of their own species. It’s not just beautiful but has an ecological purpose."

^{Fluorescent (a) and white light (b) images of a 54-centimeter (21-inch) female swell shark (Cephaloscyllium ventriosum). D. Gruber et al., Scientific Reports 2016}