National Geographic reports the discovery that the hawksbill sea turtle (Eretmochelys imbricata), the most endangered of all marine turtles, is biofluorescent: it absorbs blue light from the ocean and, after that light is transformed into different light by photosensitive molecules, it’s reflected back as a panoply of different colors. This differs from bioluminescence, which is the emission of nonreflected endogenous light produced wholly by chemical reactions. Bioluminescence is found in many organisms, including fish, jellyfish, and marine microorganisms, while biofluorescence has been seen only in fish, corals, and now this turtle. Here’s what the fluorescent hawksbill looks like, filmed by the discoverer, marine biologist David Gruber. The colors are the reflection of the camera’s blue light, which matches wavelengths found in the ocean.
We have no idea why the turtle does this, or even whether it’s an adaptation. Perhaps it’s only a byproduct of some other aspect of the turtle’s metabolism or morphology. Gruber and Alexander Gaos (a researcher on turtles not involved in the discovery) speculate that the fluorescence helps camouflage the turtle at night, but of course we don’t know for sure:
“[Biofluorescence is] usually used for finding and attracting prey or defense or some kind of communication,” says Gaos. In this instance, it could be a kind of camouflage for the sea turtle. (See pictures of insects that are masters of camouflage.)
The hawksbill’s shell is very good at concealing the animal in a rocky reef habitat during the day, Gaos explains. “When we go out to catch them, sometimes they’re really hard to spot.”
The same could be true for a habitat rife with biofluorescing animals—like a coral reef.
In fact, Gruber pointed out that some of the red on the hawksbill he saw could have been because of algae on the shell that was fluorescing. The green is definitely from the turtle though, he says.
The problem I see with the “camouflage” explanation is twofold. First, as far as I know nothing preys on adult hawksbills except humans. Perhaps the camouflage is there to protect babies against predators, but that wasn’t suggested. Further, the prey of hawksbills isn’t likely to avoid them when they’re camouflaged, because their prey is largely sessile or nonvisual (the main diet of this turtle is sponges, supplemented with jellyfish). There’s not much need, then, to hide yourself from such prey. I could swell the suggestions by speculating that it’s a mate-recognition adaption, enabling males and females to find each other in the dark, but that too is pure speculation.