Marine scientists have found that biofluorescent catsharks, such as the swell shark from the eastern Pacific and the chain catshark from the western Atlantic, are not only able to see the bright green biofluorescence they produce, but that they increase contrast of their glowing pattern when deep underwater.

“We’ve already shown that catsharks are brightly fluorescent, and this work takes that research a step further, making the case that biofluorescence makes them easier to see by members of the same species,” said team member Dr. John Sparks from the American Museum of Natural History, who is a co-author on a study published this week in the journal Scientific Reports.

“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.”

Dr. Sparks and his colleagues recently released the first report of widespread biofluorescence in the tree of life of fishes, identifying more than 180 species that glow in a wide range of colors and patterns.

To further explore this phenomenon, they focused on the visual ability of two different catsharks: the chain catshark (Scyliorhinus retifer) and the swell shark (Cephaloscyllium ventriosum).

“Cephaloscyllium ventriosum occurs from relatively shallow habitats, down to over 1,180 feet (360 m), and S. retifer is found between 230-1,800 feet (70-550 m),” the scientists said.

“The deeper environments where these catsharks occur are dominated by the higher-energy, blue photons.”

The team used a technique called microspectrophotometry to determine how the sharks’ eyes absorb light, discovering that they have long rod pigments that help them see in low-light environments.

They also used this information to build a special camera filter that simulates how light hits a shark’s eyes.

They then went on a number of expeditions to Scripps Canyon in San Diego County, where they observed swell sharks in their native habitat, about 100 feet (30.5 m) underwater.

“Cephaloscyllium ventriosum is considered nocturnal and generally solitary, feeding at night by ambushing prey in a ‘lie and wait’ fashion,” the researchers explained.

“We observed C. ventriosum during the day and at night resting in groups of 2-5 individuals in crevices on the walls of Scripps Canyon and among rocks and kelp, as well as over sandy areas in the vicinity of Santa Barbara.”

During night dives, they stimulated biofluorescence in the sharks with high-intensity blue light arrays housed in watertight cases. The resulting underwater light show is invisible to the human eye.

To record this activity, the team used underwater cameras with green filters, which block out the blue light, as well as the newly developed ‘shark-eye’ camera to get a better idea of how the shark sees the underwater display.

“Some sharks’ eyes are 100 times better than ours in low-light conditions,” said lead author Dr. David Gruber of Baruch College and the American Museum of Natural History.

“They swim many meters below the surface, in areas that are incredibly difficult for a human to see anything. But that’s where they’ve been living for 400 million years, so their eyes have adapted well to that dim, pure-blue environment. Our work enhances the light to bring it to a human perspective.”

By mathematically modeling images from the shark-eye camera, the team found that the contrast of the patterns on the biofluorescent sharks increases with depth, suggesting that the animals can not only see the light, but are also likely using it to communicate with one another.

The scientists were only able to dive to the top depth range of where this shark lives, where blue and some green light exists.

Their model shows that at deeper depths, where the water is bluer, the contrast created by the fluorescence is even greater.

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David F. Gruber et al. 2016. Biofluorescence in Catsharks (Scyliorhinidae): Fundamental Description and Relevance for Elasmobranch Visual Ecology. Scientific Reports 6, article number: 24751; doi: 10.1038/srep24751