In the light of day, catsharks appear to be murky brown in coloration, nothing spectacular. But deep underneath the ocean, when there's nothing but pure blue light shining through, these sharks make an amazing transformation – their biofluorescent skin absorbs that blue light and transforms it into a greenish hue. The best way to see this colorful change is to observe it through the eyes of the shark itself; and that's exactly what a team of researchers led by scientists from the American Museum of Natural History (AMNH) have attempted to do.

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Creating a custom-built "shark-eye" camera, the researchers were able to dive deep into the way sharks perceive each other and their research has been 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," John Sparks, a curator in the AMNH Department of Ichthyology and a co-author on the paper, said in a press release. "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."

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The researchers developed a special camera filter after analyzing the eyes of chain catsharks and swell sharks and then observed the sharks in the wild in Scripps Canyon in San Diego County as far as 150 feet below the surface. They shone a blue LED light to record biofluorescence. 

"Some sharks’ eyes are 100 times better than ours in low-light conditions," David Gruber, an associate professor of biology at Baruch College and research associate at AMNH, said. "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."

The scientists' models show that the deeper into the sea a shark swims, the brighter its fluorescence contrasts with the rest of the sharks' bodies. They even found differences in patterns on male and female sharks in both swell sharks and chain catsharks. Researchers think the animals may be using the biofluorescence to communicate with each other in these depths. 

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"Imagine being at a disco party with only blue lighting, so everything looks blue," Gruber said in an interview with National Geographic. "Suddenly, someone jumps onto the dance floor with an outfit covered in patterned fluorescent paint that converts blue light into green. They would stand out like a sore thumb. That's what these sharks are doing."

The question remains: do prey or predators of these sharks also perceive the biofluorescence in the same way? This study opens up a new door to study more biofluorescent species in the ocean's depths. 

Gruber and his fellow researchers hope their work will help humans see the world through shark's eyes figuratively as well as literally. 

"Hopefully it will also inspire us to protect them better," he told National Geographic. 

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