It was like a dazzling spacecraft, the scientist recalls.
Beautifully striped, like an incandescent UFO gliding through the ocean floor, the hawksbill sea turtle was glowing — the first known instance of a reptile exhibiting biofluorescence, an ability to absorb the blue light of the water and emit it as a different color. In other biofluorescent animals, the gleaming result is often neon green, red, and orange.
Marine biologist David Gruber had been in the Solomon Islands in late July to film biofluorescence in the South Pacific, reports National Geographic. While searching for glowing coral reefs and small sharks, Gruber and his team instead encountered a luminous and very friendly turtle.
"Out of the blue, 40 minutes into the dive – it almost looks like a bright red and green spaceship – it came underneath the camera," Dr. Gruber says in a video. "It just bumped into us.... Came in front of my lens and then hung out with us for five minutes."
Ecstatic about his find, the marine biology professor at City University of New York explains that the only animal known to glow two colors is coral. Other sea creatures that glow through biofluorescence include sharks, sea horses, a number of fish, and tiny crustaceans called copepods.
Biofluorescence differs from bioluminescence, which is a chemical reaction within the body that creates radiant light. Bioluminescent animals include jellyfish and deep sea fish, who use their glowing to navigate, communicate, and lure prey.
Gruber and his team recorded the turtle before it dove deep into the dark waters using a video camera that emitted an artificial blue illumination identical to the blue light of the ocean. Later, when the marine biologist found a local community that kept several captive young hawksbills, he found that they all glowed red.
"I've been [studying turtles] for a long time and I don't think anyone's ever seen this," Alexander Gaos, director of the Eastern Pacific Hawksbill Initiative, told National Geographic. "This is really quite amazing."
Neither Mr. Gaos nor Gruber is sure why the hawksbill sea turtles biofluoresce. Gaos, who was not involved in the discovery, speculated that It could be a kind of camouflage in this instance.
Even without biofluorescence, hawksbill sea turtles have famously colorful shells, which led to massive population declines as the animals were hunted to make tortoiseshell products.
Named for their birdlike beak and narrow head, these turtles feed mostly on sponges and linger in rocky areas and coral reefs. They have a mutually beneficial relationship with corals, as the sponges the turtles consume can suffocate reefs. The largest hawksbill populations are in the Caribbean Sea, the Seychelles, Indonesia, Mexico, and Australia.
Globally, hawksbill sea turtles are critically endangered. Just in the last century, they’ve diminished by more than 80 percent. Before international tortoiseshell trade became illegal, they were hunted almost to extinction. Today, their populations are jeopardized by poaching, entanglement in fishing nets, destruction of habitat, coastal development, and climate change. Nearly all sea turtle species are endangered.
Even though his encounter with the turtle was brief, Gruber says it was enough to show that turtles are biofluorescent and to open up a new discussion on why that might be.
"I wanted to let him go after a little bit," he says. "I feel like he came and divulged a secret."