Did you know that some sharks can glow in the dark? Well, they can, and they do—you just can't see it. Biologists using a special shark’s-eye camera report that catsharks glow more brightly in deep water, which may help them find one another in the dark. They published their findings in the journal Scientific Reports.

As sharks go, catsharks are pretty small, maxing out at just over 5 feet long. The chain catshark (Scyliorhinus retifer) and swellshark (Cephaloscyllium ventriosum) are even smaller than that. Without bulk to intimidate, these sharks have resorted to some very strange tactics to deter predators, including wedging themselves in crevices and inflating their bodies with water so nobody can fish them out. In addition to those features, they also glow in the dark.

The ghostly glow of bioluminescence is the result of chemical reactions inside an organism. Biofluorescence, on the other hand, is a way of capturing existing blue light and re-emitting it as a red, green, or orange glow. Bioluminescence is a well-documented phenomenon among many sea creatures, including squid and jellies. But biofluorescence was more limited—or so we thought until 2014, when researchers revealed finding the glow in more than 180 marine species, including catsharks.

"Our next question was 'What does all this newfound biofluorescence we are finding in the ocean mean?" said author David Gruber, an associate professor of biology at Baruch College, in a press statement. "Can these animals see other animals that are biofluorescing in the deep blue sea? And are they using it in some way?"

To answer that question, they’d need some special equipment. "Some sharks' eyes are 100 times better than ours in low-light conditions," Gruber 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."

Needless to say, ours have not. So to see what the sharks see, Gruber and his colleagues built a special shark’s-eye camera.

First, they studied the configuration of rods and cones in the eyes of S. retifer and C. ventriosum. They found that the sharks’ eyes have long rods, which allow them to take in more light in the dark depths.

With this in mind, the researchers created a set of special filters that simulated the long rods. They attached these filters to waterproof cameras, then went for night dives in swellshark territory off the coast of California. To the divers’ eyes, the sharks looked like they always look: brown. But through the camera, they saw the sharks’ true radiant green display. 

Image Credit: © K. McBurnie

Back on land, they analyzed their pictures and found a trend: The deeper the shark, the brighter its glow. The researchers had spent their time at the very top of the sharks’ home range, which suggests that sharks even deeper down, in their comfort zone, would glow even brighter, broadcasting their presence to other sharks nearby.

A swellshark in white light, natural light, and depth-simulating blue light. Image Credit: © Gruber et al.

It's a discovery that could lead to future findings, according to John Sparks, a curator in the American Museum of Natural History's Department of Ichthyology and a co-author on the paper. He added, "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."