A team of researchers led by scientists from the Museum has found that catsharks are not only able to see the bright green biofluorescence they produce, but that they increase contrast of their glowing pattern when deep underwater.
The study, conducted with a custom-built “shark-eye” camera that simulates how the sharks see underwater, shows that that fluorescence helps catsharks see each other and may even offer them a way to communicate. The work was recently published in the journal Scientific Reports.
“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 John Sparks, a curator in the Museum's Department of Ichthyology and a co-author on the paper. “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.”
Since water absorbs the majority of the visible light spectrum, fishes live in a world that is predominantly blue. In recent years, Sparks and his colleagues have discovered that many fishes absorb the remaining blue light and re-emit it in neon greens, reds, and oranges. The team recently released the first report of widespread biofluorescence—a phenomenon by which organisms absorb light, transform it, and emit it as a different color—in fishes, identifying more than 180 species that glow in a wide range of colors and patterns.
“Our next question was ‘What does all this newfound biofluorescence we are finding in the ocean mean?” said paper author David Gruber, an associate professor of biology at Baruch College and a research associate at the Museum. “Can these animals see other animals that are biofluorescing in the deep blue sea? And are they using it in some way?”
To further explore this phenomenon, the researchers focused on the visual ability of chain catsharks (Scyliorhinus retifer) and swellsharks (Cephaloscyllium ventriosum). With the help of Cornell University veterinary expert Ellis Loew, the researchers used a technique called microspectrophotometry to determine how the sharks’ eyes absorb light and built a special camera filter that simulates the phenomenon.
Diving at night, the team stimulated biofluorescence in the sharks with high-intensity blue lights, creating an underwater light show that is invisible to the human eye. The researchers then used the newly developed “shark-eye” camera, as well as custom-built underwater cameras which block out blue light, to get a better idea of how sharks see this display. Analysis of images from the shark-eye camera found that the contrast of the patterns on the glowing 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.