Scientists have discovered that a critically endangered sea turtle has an amazing ability never seen before in a reptile — it glows neon green and red when exposed to blue light.

The hawksbill sea turtle's glow is caused by biofluorescence, the ability of an organism to absorb light of one colour and use it to emit light of a different colour.

David Gruber, a marine biologist at the City University of New York, said many marine creatures biofluoresce because there are advantages to being able to produce other colours in the ocean, where mostly only the blue colour from sunlight can penetrate to any depth: "You're living in a one-colour world."

As I'm filming, this green and red spaceship of a turtle sails in front of my camera - David Gruber, marine biologist

Biofluorescent sea creatures include invertebrates such as jellyfish, corals and shrimp. More recently, Gruber has helped discover more than 200 new species of biofluorescent fish.

But turtles "really surprised us because it's a whole different group of animals," he said of the discovery in late July during an expedition funded by the TBA21 Academy.

Search for sharks

Gruber stumbled across the turtle while trying to film biofluorescent sharks at night off the Solomon Islands.

Because blue is the colour that travels best through water, most biofluorescent marine creatures absorb blue light and emit green, yellow or red. While it's sometimes possible to see the fluorescent colours in daylight, filming at night with a blue light makes the biofluorescent animals glow much more distinctly, so that's what Gruber did.

He was about 20 metres below the surface and 40 minutes into his dive when he ran into something unexpected.

"As I'm filming, this green and red spaceship of a turtle sails in front of my camera," he recalled.

The adult turtle's metre-wide shell had a glowing green pattern all over, with patches of glowing red.

Gruber followed the turtle over to an overhang, and then it disappeared down the side of a cliff.

"It was a beautiful moment where we were able to kind of witness this for the first time."

Hawksbill sea turtle

A hawksbill sea turtle as it cruises over a reef just off the shore of Curacao. The turtle's fluorescent colours can't be easily seen during the day. (Brian Witte/Associated Press)

The footage has been posted online by National Geographic, which supports Gruber under its Emerging Explorers program.

Gruber wanted to see if other hawksbill turtles also glowed, so he visited a nearby turtle farm that raised young hawksbills. Their shells also glowed green under blue light, but didn't show any red patches.

Gruber isn't sure whether the red patches might be caused by algae or are just something that develops as a turtle ages.

The discovery that hawksbill turtles can biofluoresce came as a big surprise to Alexander Gaos, director of the Eastern Pacific Hawksbill Initiative. He's been studying the endangered turtles for 12 years.

"Blew me away to hear about it," he said, in a phone interview from Hawaii, where he was preparing for a dive to look for hawksbill turtles, which live mainly in shallow, tropical waters near coastlines.

The turtle's biofluorescence isn't visible during the day, and Gaos had never thought to check what it looked like in blue light at night. But now he wants to see the phenomenon for himself.

"We have a blue light with us tonight that we're going to take it out here in Hawaii and see if we can confirm it here," he added.

More fluorescent turtles

Meanwhile, the hawksbill may have been the first fluorescent turtle ever discovered, but it's no longer the only one.

Gruber later visited an aquarium that kept loggerhead turtles and found that they also fluoresce green.

Still, a lot of mysteries remain. Researchers don't yet know how the turtles biofluoresce. Other marine animals produce special fluorescent proteins that are very different in some creatures, such as jellyfish, than others, such as fish.

Nor do they know why the turtles glow. As Gaos put it, "It just kind of opens up a lot of opportunities for research and discovery.