The Mystery of the Glow-In-The-Dark Scorpion

Here is a biological riddle that’s been fascinating me lately. While scorpions are not a “rare” animal, per se, they possess a trait that has yet to be fully explained by evolutionary science: they glow under ultraviolet light.

This isn’t bioluminescence. They are not generating their own light. Only under a source of ultraviolet, like a blacklight, do they show their true colors. But scorpions are nocturnal and stay out of the sun… and raves in the desert, while apparently not uncommon, aren’t exactly natural. So why would scorpions evolve the ability to glow when they don’t seem to use it? After the jump, some hypotheses, and why they are probably wrong.

Hypothesis: While dark to our UV-insensitive eyes, scorpions seem to glow to each other.
Contradiction: A tempting theory, but there are two problems with it. The first is simply that while scorpions do see into the UV spectrum, they don’t seem to see fluorescence. So no, they don’t see themselves glow. The second is that the fluorescence only happens when UV light excites the fluorescent pigment, causing the photons to be partially absorbed and partially dissipated as glowing radiation. No UV light = no fluorescence. And the moon doesn’t reflect enough UV to trigger the effect. If it did, we’d probably be able to see the scorpions glowing ourselves. Also, vampires would get moonburns.

Hypothesis: While dark to our UV-insensitive eyes, scorpions glow as a means of warning their nocturnal predators.
Contradiction: See above. Also, if the scorpion’s predators could see their fluorescence, the scorpion’s prey would evolve to see the fluorescence. Imagine how hard it would be to sneak up on prey when you’re lit up like a Vegas wedding cake.

Hypothesis: Oh yeah? What if they glow to attract prey?
Contradiction: Many animals use bioluminescence to attract prey, but only a few jellies use fluorescent lures. Besides, we don’t know enough about how the scorpion’s insect prey sees visible and UV light, even if the fluorescence was activated.

Hypothesis: The fluorescence serves no practical purpose. It’s the chemical byproduct of another, unknown process – basically glowing waste material.
Contradiction: Many animals excrete fluorescent, UV-sensitive waste products. But usually it’s in their urine. Why, then, would the scorpion put waste products in the cuticle of their exoskeleton?

Hypothesis: The ability to fluoresce is a vestigial trait, left over from when scorpions were diurnal creatures, and acts as a sort of sunblock.
Contradiction: This is my favorite hypothesis so far. The ability of fluorescent pigments to absorb some UV and spit out the rest as light makes them an excellent sunscreen, and there are plants that do just that. (If you ever walk under the maples in Autumn and they seem like they’re glowing, it’s because they are.) My question is: since scorpions, the first land predators, went nocturnal during the Carboniferous, around 350 million years ago, why haven’t they shaken the useless glow yet? Evolution is slow, but not that slow.

Hypothesis: The fluorescence is a fashion thing. Scorpions are stuck in the Eighties.
Contradiction: None.

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About quantumbiologist

Christian Drake, AKA The Quantum Biologist, is a naturalist and poet formerly of Albuquerque, NM and currently living deep in the backwoods of the Connecticut Berkshires. He has worked in aquariums and planetariums, national parks and urban forests. When not birding or turning over rocks to find weird bugs, he enjoys rockabilly music, gourmet cooking, playing harmonica and writing dirty haiku. View all posts by quantumbiologist

7 responses to “The Mystery of the Glow-In-The-Dark Scorpion

  • Chris Reeve

    If you assume, like most other theorists, that the Earth has always revolved around the Sun, then findings like these can be difficult to resolve.

    However, one only has to go back around 5,000 years to find entire cultures that worshiped the planet Saturn as a God … Which begs the question of WHY?

    Uniformitarianism is actually an assumption even though most theorists treat it like an unerring philosophy.

    The notion that theorists will find some bridge to truth by examining the implications of just one cosmology — the Big Bang — is short-sighted. Cosmologies do not always bridge into one another. And it’s very easy to end up on a theoretically-stranded island.

    But to theorists who have only been taught one cosmology, all observations which are not supported by their favored school-taught cosmology will fall into one of two categories: either it proves the cosmology they were taught or it should be shelved until later until it can be made to prove it.

    Meanwhile, there exists an alternative cosmology which can be used to better explain ALL of those shelved enigmatic observations. My guess is that this particular discussion of the scorpion is yet another.

    We live in a plasma universe. We’ve known this since the 50′s. But the ramifications of that one concept are still working their way through the scientific disciplines to this day.

  • quantumbiologist

    Wh… what? Who was talking about cosmology? Is this spam?

  • Bromandudeguy

    Kinda late, but he/she is basically saying you’re looking into things to much. It just glows for whatever random ass reason, accept it.

    Personally, it being an ancient vestigial trait could still work, considering that coelacanths havent evolved for some millions of years. Think of it this way, the trait never got in the way of the scorpion during evolution, but didnt help it in any way either. As such the trait stuck because evolution doesnt get rid of traits that arent necessary, just traits that DO get in the way of survival. And it stuck around.


  • quantumbiologist

    Whoa, Bromandudeguy. I thought I was supposed to be the romantic, and the real scientists were supposed to shoot me down.

    I never argued that scorpion fluorescence was an an extraneous adaptation. I only wondered why it evolved in the first place, which is the point of the post. (And shame on me if that isn’t clear.) If the fluorescence is, in fact, the vestigial trait left over from a diurnal lifestyle, then you and I are both of the same mind, since that’s my going theory. No need to bring coelocanths into this!

  • Kristoffer

    My Hypothesis: A ways to warn day-active predators.
    Contradiction: Many animals especially birds can se UV-light. Experiments with birds have shown that some birds choose their mate by their uv-colour. The ability to use strong colours is often used by insects as a means to warn predators that they are poisonous. I believe that though we humans cant see anything on the scorpions in daylight, I believe birds can. This way birds are warned of the scorpions deadly poison. This also makes sence in regard to evolution ;)

    • quantumbiologist

      Indeed! Not a bad theory! I’d say the only problem with the argument is that scorpions are strictly nocturnal, and are usually hiding under a rock when the hawks are up and about.

      The only other problem is that scorpions date back to the Silurian Period, about 425 mya, while the first full bird we know of, archaeopteryx, dates back only 150 mya. This would make the scorpion’s fluorescence a very recent adaptation — presuming dinosaurs couldn’t see UV, based on the fact that modern reptiles can’t — and a fairly extreme adaptation for something the nocturnal scorpions aren’t going to show off much. That said, it’s not impossible. Maybe there are enough scorpions out in the daytime that their fluorescence confers a strong evolutionary advantage as aposematic warning coloration.

  • Yolande van der Merwe

    Kristoffer, on your theory about the glow as a warning to the birds – I have witnessed starlings, bee eaters, kingfishers, rollers and drongos in South Africa and Namibia feeding on scorpions. They merely wacked the scorpion against a branch breaking off the tail, or in the case of the starlings only a few days ago, they rubbed the scorpion on the ground until the tail broke of and then completely consumed the scorpion.

    Is it definate that they can’t see each others flourescense?

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