Scorpion Fluorescence – August 2013

By Kevin Scott

Man’s highly developed color sense is a biological luxury- inestimably precious to him as an intellectual and spiritual being…. -Aldous Huxley inThe Doors of Perception

Aldous Huxley concisely described man’s fascination with things that shimmer and glow. The earliest admiration of the luminous was probably lightning at night, wildfires, the sun or a full moon.

To be sure, these things were revered and their causes unknown to the point that gods and goddesses were invoked, whose attributes described these phenomena.

Fast forward several millennia to the advent of artificial ultraviolet light and another curious phenomenon is observed: the fluorescence of scorpions under a black light. After the initial curiosity one comes to wonder what the purpose of this could be and how works.

Wild california scorpion while illumated with a blacklight

Light in the ultraviolet wavelengths is certainly not abundant at night (although the proportion of ultraviolet to white light is higher at night), so why the glow? The wavelengths in question abound during the day, but scorpions are decisively nocturnal animals that have well developed sensory mechanisms, allowing them to efficiently hunt and mate in low levels of light. The chart below describes some of these adaptations (Blass and Gaffin 2008).

Although all of these adaptations allow scorpions to capture prey, navigate and reproduce in low-light conditions, they do have several photosensory organs as well.

Emperor Scorpion, showing both fluorescing color and normal color

Scorpions have lateral and median eyes that are capable of detecting light magnitude changes and image formations, respectively (Gaffin, et al. 2011).   The median eyes are most sensitive to wavelengths around 500 nm (green) and secondarily sensitive to light in the 350-400 nm (ultraviolet) range (Machan 1968, Fleissner & Fleissner 2011).

In addition, scorpions have a metasomal element that is sensitive to light in the green area of the visible spectrum (Zwicky 1968, 1970; Rao & Rao 1973).

These technical details may at first seem superfluous until one thinks about the fact that when one views a scorpion under a black light, the black light and the green glow are in the same areas of the light spectrum as those to which the scorpion’s eyes are most sensitive; this is probably not a coincidence.

But this still does not explain the function of the fluorescence. Although the exact purpose is not known, there are a few hypotheses. There is the obvious possibility that no function is served at all.

Gary Polis suggested that fluorescence could act as a lure to draw prey in, but subsequent tests of this hypothesis show that insects will actually avoid scorpions that are fluorescing (Polis 1979; Kloock 2005).

One other theory suggests that fluorescence may play a role in courtship behavior, allowing one scorpion to tell whether a near by scorpion is of the same species and/or of the opposite sex (Kloock 2008). This would allow, from a distance, a scorpion to decide to approach (or be approached by) another that is either of the same sex or of a different species – either would be futile in a mating attempt. A negative photoresponse is observed in scorpions suggesting that the cuticle may serve as a photodetection device, however, it is not clear that fluorescence plays any role in this. One study showed that prolonged exposure to ultraviolet light caused a decrease in fluorescence (Kloock 2009).

Although the exact function has yet to be elucidated, mechanism by which fluorescence occurs in scorpions is relatively well understood. The scorpion’s exoskeleton is made from chitin, like other invertebrates. The compounds that fluoresce are found near the surface of the cuticle and relatively recently two molecules (4-methyl-7-hydroxycoumarin and beta carboline) have been isolated, both of which fluoresce in the presence of ultraviolet light (Stachel et al. 1999; Frost et al. 2001). Interestingly, Polis points out in his book The Biology of Scorpions that scorpions that have newly undergone ecdysis do not exhibit total fluorescence until 48 hours thereafter.

Fluorescence is observed in many life forms. Some of their functions are understood and some remain a mystery. Significant progress has been made with respect to that of scorpions, but more work is needed to fully understand the function thereof. For anyone who is interested in a deeper understanding of any of the topics discussed here, please explore some of the books and papers referenced below.

Blass, G. R. C & Gaffin, D. D. 2008. Light wavelength biases of scorpions. Animal Behaviour, 76, 365-73.

Gaffin, D. D., Bumm, L. A., Taylor, M. S., Popokina, N. V., & Mann, S. 2011. Scorpion fluorescence and reaction to light. Animal Behaviour, 83, 429-36.

Fleissner, G. & Fleissner, G. 2001. Night vision in desert scorpions. In: Scorpions 2001; In Memoriam Gary A Polis (Ed. by V. Fet & P. A. Selden), pp. 317-324. Burnham Beeches, Bucks: British Arachnological Society.

Frost, L. M., Butler, D. R., O’Dell, B. & Fet, V. 2001. A coumarin as a fluorescent compound in scorpion cuticle. In: Scorpions 2001; In Memoriam Gary A Polis (Ed. by V. Fet & P. A. Selden), pp. 363-368. Burnham Beeches, Bucks: British

Arachnological Society.

Kloock, C. T. 2005. Aerial insects avoid fluorescing scorpions. Euscorpius, 21, 1-7.

Kloock, C. T. 2009. Reducing scorpion fluorescence via prolonged exposure to ultraviolet light. Journal of Arachnology, 37, 368-370.

Machan, L. 1968. Spectral sensitivity of scorpion eyes and the possible role of shielding pigment effect. Journal of Experimental Biology, 49, 95-105.

Polis, G. A. 1979. Prey and feeding phenology of the desert sand scorpion Paruroctonus mesaensis (Scorpionida: Vaejovidae). Journal of Zoology, 188, 333-346.

Rao, G. & Rao, K. P. 1973. A metasomatic neuronal photoreceptor in the scorpion. Journal of Experimental Biology, 58, 189-196.

Stachel, S. J., Stockwell, S. A. & Van Vranken, D. L. 1999. The fluorescence of scorpions and cataractogenesis. Chemical Biology, 6, 531-539

Zwicky, K. T. 1968. A light response in the tail of Urodacus, a scorpion. Life Sciences, 7, 257-262.

Notes From The Field – July 2013

By Kevin Scott

I remember that day, and recall with delight, as I walked through the desert and took in the sight of the spring desert flowers that followed the rain, giving life and bold color to the spring desert plain. See, spring is my favorite season for herping, the locusts were buzzing and song birds were chirping, the beetles were digging their holes in the sand while the rock lizards lie in the sun as they tanned. Coyotes left evidence that they had been here, but of course they’re always the first to disappear. In search of a creature – no particular kind – just taking my chances on what I might find, I cautiously tip-toed my way through the cactus, an activity in which I have had lots of practice. After the cactus I trudged on ahead, and wandered along an old, dried riverbed.

I stopped in my tracks at the moment I saw (my jaw hanging low with exuberance and awe) the curious creature that caused much confusion, for which kind it was I could draw no conclusion. At first glance I thought it could be some amphibious creature, but it was simply much too hideous. On closer inspection I saw it had scales, some sort of carapace and two lengthy tails. Some of the scutes were partially keeled, and flipping it over is when I revealed that this creature was somewhat obscurely chelonian, but appeared somehow older… silurian, or devonian? Its digits were webbed and its neck rather long, but something was missing, and something seemed wrong. You see, it had gills on the side of its head, right behind frills that were easily spread. But lacking in water, this distant location, could hardly have produced this strange adaptation. No eyes could be seen on the primitive face, not even a remnant or residual trace of an organ deemed worthy of visual perception, for locating food and for predator detection. Small holes were presumably there for olfaction but the quantity thereof evoked an exaction of closer inspection for what they might be, for there weren’t just two, but indeed there were three! Three nostrils, a notion entirely absurd, surely not even biologists have heard of something occurring in such repetition, alas, evolution has brought it to fruition.

The anomalous creature had but seven teeth, with four on the top and then three more beneath, but their shape made it hard to discern what it ate; they were kind of roundish, though more or less straight. They couldn’t be used for herbivorous chewing but the creature was clearly not made for pursuing anything other than immobile prey, perhaps it just grabbed things that came past its way. It seemed quite content to concede submission, as though for defense it had lost its volition, so I sought to expose its purpose ulterior, and subsequently moved to inspect its posterior. The subcaudal scales were smooth and divided, however no insight was thusly provided, though it possessed two sets of quaint hemipenes, the first advantageous trait I had seen.

I’ve read of strange creatures in Carrollian fiction, but nothing like this in scientific depiction. Of all of my lectures and myriad books, I’d seen nothing bearing ridiculous looks like this beast that left but a pale question mark, it was strange as a Jabberwocky, Jub Jub or Snark. It seemed hardly a question of natural history but more of a serious supernatural mystery. For I bet even Darwin’d be slightly confused, and good old Linnaeus’d be likewise bemused in attempts at this queer critter’s classification, a fruitless endeavor with no explanation. Even Lamarck couldn’t have found a prerequisite force to give rise to something so exquisite, quirky, peculiar unusual and odd… but perhaps a creator, a maker… a god?

But I digress, let’s get back to the narrative, because what comes next is especially imperative for everyone reading this to understand, why I left this thing there in the hot desert sand. I thought “if I bring this thing back to society, it would get much attention and then cause great anxiety amongst the paleo-scientific community who’d want to dissect it with carefree impunity.”

However, it just didn’t seem well adjusted to this undisclosed desert but somehow I trusted that if it had come to make it this far, despite that its looks were extremely bizarre, it would keep on surviving, prolong its existence, with a little bit of luck and extravagant persistence.

So, live and let live’s the philosophy I followed, the lump in my throat I painfully swallowed, as I shed a tear and prepared to depart with the creature that won a small place in my heart.