When catsharks glow, they become easier for others of their kind to see, the deeper they swim. And, what's more, all of that glowing might help the sharks communicate with each other.
Those were the findings of scientists from the American Museum of Natural History, who used a special "shark's eye" camera designed to reveal what biofluorescing sharks look like to one another.
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Biofluorescence –- absorbing light and then emitting, or "glowing" it back as different colors, such as neon greens -- is a relatively new finding among fish.
Earlier work by study co-author John Sparks and his fellow researchers had established that more than 180 fish biofluoresce. That finding begged the question "Why?" What was the meaning behind all of the glowing?
"Can these animals see other animals that are biofluorescing in the deep blue sea?" wondered co-author David Gruber, an associate professor of biology at Baruch College, in a statement. "And are they using it in some way?"
Those questions set the researchers on the path advanced in the new study.
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Sparks and his team observed catsharks -- small sharks that cruise on the floor of the ocean, where visibility is very poor -- in the Scripps Canyon underwater gorge in Southern California.
They stimulated biofluorescence in the sharks by training special light on them (not visible to the human eye) that mimics ocean light, all the while recording with the "shark's eye" camera.
When the team applied mathematical models to the recorded images -- pictures of the world as catsharks would see them - they noticed that the contrast of the catshark's pattern under biofluorescence got sharper the deeper underwater they went.
So, to another catshark - or camera pretending to see like a shark - the biofluorescence effectively made it easier to see the glowing shark. This told the researchers not only that the sharks could see each other but that the glowing also might help them communicate (perhaps by "telling" other catsharks that they are of the same species, the authors speculate).
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"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," said Sparks.
"We've already shown that catsharks are brightly fluorescent," he added. "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."
"Some sharks' eyes are 100 times better than ours in low-light conditions. They swim many meters below the surface, in areas that are incredibly difficult for a human to see anything," Gruber said. "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."
The team's findings have been published in the journal Scientific Reports.