A fascinating new study reveals how deep-dwelling sharks shine bright in deep waters, likely as a way to find a mate. Thanks to a new camera, scientists get a better picture of these glowing marine creatures that remain barely studied due to decreased light penetration where they are.

Researchers from the American Museum of Natural History studied biofluorescence in two catshark species, which like spending much of their time about 1,640 feet under the waters. While only blue light reaches their habitat, they give off a green biofluorescent glow through their skin, which differs in pattern depending on species sex and type.

For study author and Baruch College biology professor David Gruber, theirs is among the first biofluorescence studies to connect the small, deep-water sharks’ visual ability to fluorescence emission.

“We’ve already shown that catsharks are brightly fluorescent. [Now we show] that biofluorescence makes them easier to see by members of the same species,” he explained, adding that the species’ eyes have adapted well to dim and purely blue environments given 400 million years of living there.

The team arrived at these findings using a custom-built “shark-eye” camera that simulates sharks’ vision underwater. In recent years, they have found that many fishes absorb whatever blue light is there and re-emit it in shades of neon green, red, and orange.

In an amazing experiment, researchers designed special lighting that mimics ocean light. They then recorded underwater activity through cameras with blue light-blocking green filters, and paired these with their shark-eye camera to know how the sharks perceive the underwater display.

Images from the shark-eye camera were mathematically modeled, informing the team that the contrast of patterns on the biofluorescent animals increased along with depth. As the water got bluer and therefore deeper, the contrast made by fluorescence became even sharper.

This suggests that the sharks both see the light and are likely using it to their communication advantage. To another catshark, or camera that pretends to see like a shark, the fluorescence practically made it easier to see a glowing creature – much like “telling” others that they’re of the same family.

The findings were discussed in the journal Scientific Reports.