A glowing sea turtle: the world’s first biofluorescent reptile

October 1, 2015 • 12:00 pm

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.

h/t: Hempenstein

38 thoughts on “A glowing sea turtle: the world’s first biofluorescent reptile

    1. Maybe it absorbs it from micro-organisms it ingests that then are taken into some parts of the shell…? nah…
      Well someone should get a Phd out of it!

      1. Yes; similarly, I was thinking that if bioflourescence is already found in coral, and the turtle hides in coral, then maybe its got a thin layer of coral (or some related algae) on its shell.

  1. Neat!

    …but it’s worth noting that fluorescence is very common, all around us, in both animate and inanimate objects. Your own skin is weakly fluorescent, though I’m not sure how much of that is due to skin cells and how much to bacteria. Wood dimly fluoresces. Many minerals brightly fluoresce. Scorpions are well known for fluorescing. And, of course, all those fluorescent orange and green and pink and what-not papers and inks and plastics…they really do fluoresce, which is why they look so bright. All that UV light you can’t see is reemitted as visible light.

    There’re a few factors to consider.

    First, fluorescence mostly requires UV light. UV light is quickly attenuated under water, as I recall, so I doubt that undersea organisms gain any advantage to fluorescence. Those underwater scenes only look so striking because the photographers are pointing insanely bright artificial UV blacklights at them. Think of the last time you saw a disco or the like with blacklights and how everything looked different.

    This beautiful turtle’s fluorescent marking is highly patterened, which makes it striking. But the fluorescent patterns match the visible light patterns, which means that it’s the patterning that contains the fluorescence, not something else laid on top of the patterning that’s fluorescing.

    So…I’m guessing that the fluorescence is entirely irrelevant in the turtle’s case; it just so happens that the pigmentation fluoresces when subjected to UV light that the turtle never swims into.

    b&

    1. I remember this with black lights and Pink Floyd music while in college.
      No comment on what we were doing.

      1. Yes. Very handy for those looking to manually evict them.

        Fortunately, I’ve never lived anywhere with scorpions…but I know suburban places just several miles away that’re thick with them.

        …what I’ve got are black widow spiders…not sure they’re any better, but at least they haven’t shown any inclination to come indoors….

        b&

      1. Woah — had no clue about the evolutionary origins of teeth. The full text is available for free, and is short and reasonably accessible to the layperson. Thanks!

        b&

    2. Recently, we have been taking a lot of images of millipedes using UV light. Especially members of the order Polydesmida fluoresce. We get clear and detailed images of male millipede gonopods (their private parts) using UV light and fluorescence. Currently, I cannot think of an adaptive function for it. But it gets great images.
      Millipedes of the genus Mytoxia have bioluminsecence.

      1. A big problem, perhaps an insurmountable hurdle, for any proposal that fluorescence is adaptive…is that there are few, if any, natural environments where UV light predominates to the extent that fluorescence is responsible for more than a marginal amount of the visual appearance.

        You can go to your local office supply store and buy pens and paper that fluoresce pretty strongly. Yet none of these animals that fluoresce do so anywhere near as brightly nor as colorfully. At the same time, we’ve got lots of species, especially birds, that are all about the showy and the colorful…and they don’t use fluorescence to do so. But fluorescence would add an over-the-top dimension to those already-colorful organisms, something that would be even more strongly selected for than the existing bright coloration.

        So…I’m sure there’s more than one dissertation to be written on the subject, but the better ones are going to be more along the lines of explaining why we don’t see fluorescence rather than explaining the very weak fluorescence we do see.

        b&

  2. Wonderful, and I love sea turtles, but Tiger sharks eat sea turtles; their powerful jaws and adapted teeth can crack the hardest shell. Giant octopuses also sometimes eat sea turtles.

    1. In Alaska I once saw a giant octopus kill a seal; the Giant Pacific octopus is the largest in the world and can grow up to 100 lbs. The one we saw must have been full size. It was a grisly scene, but I couldn’t turn away. We figured the seal tried to eat the octopus before discovering it was HUGE and the octopus was having none of that. It basically suffocated the poor mammal.

  3. The adaptationist paradigm assumes that this must have an adaptive purpose. But perhaps it does not. The pigments they use, and the algae that cover them likely serve for camouflage, and that there is fluorescence could be for no adaptive reason whatsoever.

  4. I’m thinking about a 2003 paper: ‘The coevolution of blue-light photoreception and circadian rhythms.’ Not a perfect fit with biofluorescence, but there’s something here I can’t quite put my finger on about UV light, oceans…

    Abstract. Sunlight is a primary source of energy for life. However, its UV component causes DNA damage. We suggest that the strong UV component of sunlight contributed to the selective pressure for the evolution of the specialized photoreceptor cryptochrome from photolyases involved in DNA repair and propose that early metazoans avoided irradiation by descending in the oceans during the daytime. We suggest further that it is not coincidental that blue- light photoreception evolved in an aquatic environment, since only blue light can penetrate to substantial depths in water. These photoreceptors were then also critical for sensing the decreased luminescence that signals the coming of night and the time to re- turn to the surface. The oceans and the 24-h light– dark cycle therefore provided an optimal setting for an early evolutionary relationship between blue-light photoreception and circadian rhythmicity.

  5. Physics pedantry: fluorescence is indeed different from luminescence, but neither of them involves reflection. In reflection, a photon interacts with an atom by exchanging momentum with it, basically bouncing off it.

    In fluorescence, a photon is absorbed by an atom, which thereby enters an excited energy state. Some time later the atom falls back to a lower energy state, emitting one or more photons in the process.

    The green glow of a cat’s eyes at night is reflection, but that’s not what we’re seeing here.

    1. Indeed, there’re lots of ways that an inbound photon can result in an outbound photon. Even very, very indirect ways…when you burn a kerosene lamp, you can trace the resulting photons all the way back to the core of the Sun hundreds of millions of years ago. Well, not the same photons…but, at the same time, it’s rarely ever the same photon that comes out that went in, even with shiny mirrors.

      Optics is a very deep subject….

      b&

      1. Are any two photons ever the same photon? If you think of them as waves it would seem they don’t have much in the way of individual identity.

  6. “First, as far as I know nothing preys on adult hawksbills except humans.”

    Years ago I was guiding a snorkeling trip to an island in the Pacific Ocean off Costa Rica. In the ocean far from shore I saw a large sea turtle on the surface. We all got excited and I told the captain to stop the engine so we could take a look.

    But when I got in the water, I noticed the sea turtle’s head was strangely floppy. Then I noticed a big chunk had been taken out of its shell, as if someone had taken a bite of a giant cookie. There was blood in the water, and a big shark circling under the turtle.

Leave a Reply to Dominic Cancel reply