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.