glow in the dark swell shark
A biofluorescent swell shark (Cephaloscyllium ventriosum)J. Sparks, D. Gruber, and V. Pieribone

Glow in the dark catsharks appear to use biofluorescence to communicate with one another in the ocean depths, scientists have discovered. A team of scientists developed a 'shark eye' camera, in order to find out exactly what these sharks see underwater – information that could shed light on the purpose of biofluorescence over their evolutionary history.

Unlike land animals, fish live in a world that is mostly blue. As the water gets deeper, it absorbs most of the visible light spectrum. Research over the last few years has shown many fish absorb the remaining light and re-emit it as biofluorescence in different shades, including reds, greens and oranges. However, this light is invisible to the human eye so observing the changes has been challenging.

glow in the dark shark
Fluorescent (A) and white light (B) image of a female swell shark.Gruber et al.

Scientists from the American Museum of Natural History and Scripps Institution of Oceanography used this information to design a light that replicates that found in the ocean, as well as cameras that capture the fluorescent light coming off the fish. This provided the team with a view into this hidden 'glow in the dark' world.

The team then studied the visual abilities two species of cat shark - chain catsharks and swellsharks. This allowed them to build a camera filter that simulates what a shark sees. Over a series of night-time dives, the researchers showed the contrast of their bright green pattern increases as the sharks swam at deeper depths. They also found differences between the biofluourescent patterns of male and female.

Researchers believe biofluorescence helps catsharks to recognise and communicate with one another, as well as possibly providing a reproductive strategy. Dimitri Deheyn, co-author of the paper, which was published in Scientific Reports, said: "This study provides the first evidence that sharks can see the fluorescence of their own species. It's not just beautiful but has an ecological purpose."

John Sparks, another co-author, added: "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. This is one of the first papers on biofluorescence to show this connection, and a big step toward a functional explanation for fluorescence in fishes."