Glow-in-the-dark predators: the curious case of scorpions and UV light

A Hottentota sp. on a nightly hunt on the shores of Aqua Outback, Tuticorin. The scorpions on these shores are typically out and actively hunting on new moon nights. All images by Samuel John.

It was 2014, and I was just getting into “wildlife” and “nature”. It started with birding, and I had just about started to reliably see the differences between a golden oriole and a mango. Around this time, I found myself travelling to Chengalpattu to go visit Chetana’s (and now, my) friend, Rom. Before we set out on our night walk during our first evening there, he handed us an ultraviolent (UV) torch. Chetana seemed to know what it was for. I did not. My only experiences with UV light until that point are neither relevant nor appropriate for this story.

My first scorpion under UV at Karadi Malai Camp in Chengalpattu.

We set out, UV torch in hand. And hardly 10 minutes later, there it was, a brilliant green animal in the middle of a dim purple spotlight. That was my first time seeing a scorpion in life, and it doubled as my first time seeing one fluoresce under UV light.

How on earth does a scorpion glow under UV?!

A Hottentota sp. from Aqua Outback, Tuticorin, seen in daylight.

A scorpion’s body is covered with a thin, near-transparent hyaline layer (derived from the Hyalinos, Greek for glasslike). The hyaline layer on scorpions contains molecules that are known to fluoresce. When these fluorescent molecules in the hyaline are struck by UV light, their electrons become excited with energy. In their jumpiness, they lose some of their newly gained energy as heat. To return to their original(pre-excited) state, the molecules emit the remaining energy back as light. What went into the hyaline layer was UV light, but after losing some of its energy, the light coming back from the molecules is a longer wavelength that shifted from the outside-of-visible-to-humans wavelengths of UV to a bright cyan green that is visible to our eyes.

A close-up of a scorpion's exoskeleton.

If the chemically inclined among us are curious about the names of the fluorescent compounds in the exoskeleton of a scorpion, the two we know of are β-carboline and 7-hydroxy-4-methylcoumarin.

I find the β-carboline aspect of a scorpion’s glow fascinating because it is a molecule that shows up in a biological process in humans. In their paper ‘The fluorescence of scorpions and cataractogenesis’, Stachel et al. reported that when certain proteins are chemically cross-linked by oxidation, the amino acid tryptophan can be converted into fluorescent β-carboline.

In scorpions, this happens as part of hardening the new exoskeleton after each moult, while in the human eye it happens with age as damage that clouds the lens into a cataract! A subtle reminder for me about the connectedness of life and the utter tangled mess of evolution that I struggle to comprehend in the slightest.

Why on earth does a scorpion glow under UV!?

This is the question I’ve been asked enough times to make me want to write this article. Let’s explore a few possibilities.

The hyaline layer of a scorpion's exoskeleton absorbs UV light and emits a cyan-green glow. The moon is a rich source of UV light and consequently has a crucial role to play in the life of a scorpion.

1.      Does the glow attract prey?

Studies have shown that the UV glow of a scorpion has no effect on attracting prey like crickets and may, in fact, act as a deterrent. In the study ‘Aerial Insects Avoid Fluorescing Scorpions’, Kloock does not mince words with the title, and finds that aerial insects avoid fluorescing scorpions on a full moon night (the moon is a rich natural source of UV light).

2.      Does the scorpion perceive itself glowing?

I know I’ve loaded that question with theory-of-mind-sounding phrasing. Sue me! But the leading hypothesis for the glow is that the scorpion’s hyaline layer that is fluorescing is also sending signals through the nervous system, so the entire body of the scorpion is detecting light. And work done by scientists like Dr Doug Gaffin shows that scorpions alter their behaviour based on their glow.

In a nutshell, if you’re a scorpion and you can sense your whole-body glowing, you’re likely exposed; and reduced activity or tucking yourself into a refuge may help you stay hidden from potential prey and predators.

In my own time watching scorpions at Aqua Outback in Tuticorin, I have seen them out and actively hunting on new moon nights. And on full moon nights, I’ve seen them sit inside burrows with only their pincers out (possibly a mode of ambush for passing prey).

3.      Is it an ancient trait that has just been conserved by chance?

A Heterometrus sp. from the Nilgiris under UV light. This large genus of scorpion conspicuously lights up the forest floor under a UV torch light.

Fluorescence appears to be a trait prevalent in almost all extant scorpions, with exceptions such as Chaerilus. The trait is also found in Horseshoe crabs leading to some hypothesising fluorescence to be a trait may have been present in some chelicerate ancestor of the scorpion and the horseshoe crab.

The next time you’re out on a nature walk, take a UV torch with you, look at lichen, look at termite hill geckos and look at scorpions. There’s plenty of mind-bending wonder to be seen under UV light and even more unanswered questions to be pondered.

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 About the author

John is a writer, photographer and researcher with a keen interest in spiders. ​ He regularly daydreams of using a calculator, a pencil and a cup of sambar to unravel the secrets of the universe.

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