Washington (AFP)

This is not necessarily the best way to go unnoticed, but some sharks, carpets at the bottom of the oceans, emit a fluorescent green light. We now know how.

Researchers have identified the molecules behind this fluorescence, according to their study published Thursday in the journal iScience.

"This is very different from all other forms of marine fluorescence", including jellyfish or coral, says to AFP one of the co-authors, David Gruber, a professor at City University in New York.

"It's a small molecule rather than a protein, which shows that animals are independently evolving this ability to absorb the blue light of the ocean and transform it into other colors," says the marine biologist.

The study examines two species of sharks, the flying fox (Scyliorhinus retifer) and the swollen shark (Cephaloscyllium ventriosum), which the researcher was able to observe while diving in the Scripps Canyon, off San Diego, California.

"They are about a meter long, stay at the bottom, are pretty shy and do not swim very well," says the researcher, who noted with his colleague Jason Crawford, of Yale University, that their skin had two tones: a light and a dark.

- Absorb the blue light -

The two men extracted a fluorescent molecule found only in the lightest areas of the skin and allowing the sharks to absorb the blue light - the only one to reach them in the depths (30 meters or more) where they live. - to emit green.

David Gruber noted during his dives that sharks evolved in groups, from two to ten individuals. Their fluorescence, he argues, could help them to identify or differentiate between the sexes.

According to the researchers, a better knowledge of the fluorescence of marine animals could eventually lead to advances in medical imaging.

Their recent discovery shows in any case how much sharks, animals yet ancestral, still contain many secrets.

"They are great creatures with fascinating super powers, such as their incredible sense of smell or their Lorenzini bulbs that allow them to detect electromagnetic fields," said David Gruber.

The sharks studied "are close to the San Diego Pier and yet we only see this mystery now," he adds.

© 2019 AFP