Scientists have discovered a secretive group of sharks that survive deep under water and use a form of glowing to make themselves more visible to one another.

Researchers report the group resides in a dark blue realm of low light aided by glowing known as biofluorescence. Presumably, the raised level of visibility makes it easier for the animals to come together for the purposes of mating.

Biofluorescence is not believed to be a new phenomenon and researchers report a history of it in fishes. In all, it is thought to use by more than 200 species, including sharks, bony fish and marine turtles.

Biofluorescence Also Detected in Catsharks

A recent Scientific Reports study also highlights the presence of biofluorescence in catsharks and swell sharks, described as small, nocturnal sharks up to three feet long that survive on the bottom in crevices.

"The cool thing about this research is it literally shines a light on animals that are often overlooked," said David Gruber, the study's lead author and a National Geographic Emerging Explorer.

Catsharks typically reside so deep under water that they are bathed only in blue light. Researchers further note the animals have a special, as-yet-unidentified pigment in their skin that absorbs blue light and re-emits it as the color green.

To get a better handle on the phenomenon of biofluorescence, Gruber and his team focused on studying the eyes of the animals, learning that many of them carry really long rods which help them to navigate in lower light.

In addition, they detected only one visual pigment for color detection, which allows the animals to make their way in the blue and green spectrum.

Humans Have Three Times the Visual Color Pigment

By comparison, humans have three color pigments-red, green, and blue-allowing us to see a wider range of colors.

Armed with such knowledge, scientists created a "shark-eye" camera to approximate vision. They carried out their plan by adding filters in front of a lens of a Red Epic camera to restrict the wavelengths of light passing through, a process that mimicked the eyes of the sharks.

To further enhance the effect of fluorescence, they occasionally shined blue lights. The team conducted their search for catsharks along the San Diego coast of Scripps Canyon.

The sharks were hardly visible under normal conditions, but through the use of the shark-eye camera researchers were able to detect how they glowed a bright green hue.

"Imagine being at a disco party with only blue lighting, so everything looks blue," says Gruber. Some things will look lighter blue and others will look darker blue. "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."