By Sabine Fuchs, the Branson School
The natural world is full of unsolved mysteries. Jacob Gorneu and Kate Montana, arachnid researchers at the California Academy of Sciences, disclosed one of the more perplexing enigmas of their studies when speaking at Marin Science Seminar: the fluorescence of scorpions. Fluorescence is when something glows or radiates visible light under black light, which is a type of light that emits ultraviolet (UV) radiation; in the case of these scaly creatures, they turn a bright glowing blue. Yet, there is no conclusive explanation for this phenomenon, merely a myriad of theories exploring the causation, chemical background, and possible uses for this fluorescence.
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| Image from Discover Magazine |
Attracting Prey
Many scientists have wondered why scorpions fluoresce instead of camouflaging themselves. An earlier hypothesis for scorpions' fluorescence was that it is used to attract prey. A study by Dumas Gálvez, Carolina Nieto, and Paola Samaniego in Neotropical Biodiversity, Vol. 6, explored this idea. They compared the activity of crickets near fluorescent scorpions and non-fluorescent scorpions that were painted black. However, no differences between the florescent and non-florescent scorpions were observed, and this theory was eventually discarded.
Warding Off Parasites
Further, in the American Chemical Society’s Journal of Natural Products, a group of researchers reported having found evidence that scorpions’ fluorescence might ward off parasites. β-carboline and 7-hydroxy-4-methylcoumarin were the only previously identified components of fluorescence in scorpions’ exoskeletons. Masahiro Miyashita and his colleagues decided to conduct further research, wondering what other chemical components contributed to the fluorescence of the hard outer shell. Using chemical conditions unlike to previous experiments, they took molted shells from the scorpion Liocheles Australasia and isolated compounds. After purifying one that showed strong fluorescence, they discovered its structure was a phthalate ester that has anti-parasitic properties. While this same molecule has been found in other species of scorpions, it likely contributes weakly to the exoskeleton’s fluorescence.
In a Marin Science Seminar interview conducted by Sabine Fuchs, Jacob Gorneau, a research assistant in the Entomology department at the Academy of Sciences in San Fransisco, was asked about his opinion on the myriad of theories for scorpion fluorescence.
“I think the fluorescence might more likely be a tool for simply detecting light,” says Gorneau. “There have been a few studies showing this, and this idea makes sense to me because it seems like a good way for a scorpion to detect the time of day by the amount of sunlight present, as well as being able to detect when it has found shelter, such as in a burrow.”
Detecting Light
Currently, this is one of the most compelling explanations for scorpions' fluorescence. Scorpions are nocturnal creatures, and the light predators use to find them is that of the moon and stars, a component of which is ultraviolet light. UV light is also the same light that illuminates their mysteriously fluorescent bodies. Douglas Gaffin, a Biologist at the University of Oklahoma, proposed the hypothesis that scorpions’ exoskeletons function as an “alarm system” that alerts them when they are exposed to predators. “You eventually wonder, ‘How do they find that one blade of grass and stay under it?’” said Gaffin in an interview with WIRED. Gaffin noticed while collecting scorpions that Paruroctonus utahensis, a grassland species of scorpions, was able to find shelter even in complete darkness. In a study published in Animal Behavior, titled “Scorpion Fluorescence and Reaction to Light,” Gaffin and his colleagues attempted to see if the exoskeleton alone could detect light. Previous studies have shown that scorpions' medial and lateral eyes are most sensitive to green light and less so to UV. Further, it is known that scorpions are negatively phototactic, meaning they move away from light. From this information, Gaffin and his colleagues predicted that the Paruroctonus utahensis would respond maximally to green light and minimally to UV light. However, in this study, they observed that the scorpions responded with abrupt bouts of locomotory activity to both green light and UV light. Next, the researchers covered the scorpion’s eyes with foil and again exposed them to UV and green light. The scorpions with blocked eyes still moved away from UV light but moved much less under green light than those without covered eyes. Gaffin and his team proposed that the exoskeleton functions as a photon collector that transforms UV light into cyan-green light before relaying it to the nervous system. Another study conducted by Carl T. Kloock, Abraham Kubli, and Ricco Reynolds, titled Ultraviolet light detection: a function of scorpion fluorescence, that was published in the Journal of Arachnology, explored this same idea. By exposing scorpions to prolonged UV light, researchers reduced their fluorescence. Next, they created an arena half in shade and half not. Three different light settings were tested: infrared (IR) light only, IR ultraviolet light, and IR white light. Then, the activity of fluorescent and non-fluorescent scorpions was compared. Under the IR ultraviolet light, fluorescent scorpions stayed more often in the shaded region, whereas the reduced-fluorescent scorpions had increased activity in both regions. However, in the IR only and IR white light, activity between both types of scorpions was the same. The researchers interpreted this as possible evidence that fluorescence aids in scorpions' detection of light.
Even with all of this research, the reason for scorpion fluorescence remains unclear. In this day and age, it is easy to think we have discovered all there is to know about the natural world. After all, we have bombs that can blow up cities, methods to reach other planets, and the capability to genetically engineer. Yet, the phenomenon of scorpion fluorescence makes it clear that there is still so much we have yet to know.
Works Cited
Author links open overlay panelDouglas D. Gaffin et al. “Scorpion Fluorescence and Reaction to Light.” Animal Behaviour, 19 Dec. 2011, www.sciencedirect.com/science/article/abs/pii/S0003347211005069?via%3Dihub.
Dailymail.com, Stacy Liberatore For. “Brown Scorpion and Its Babies Glow Stunning Shades of Blue and Purple under UV after Light.” Daily Mail Online, 21 July 2021, www.dailymail.co.uk/sciencetech/article-9811411/Brown-scorpion-babies-glow-stunning-shades-blue-purple-UV-light.html.
Full Article: Test of the Prey-Attraction Hypothesis for the Scorpion ..., www.tandfonline.com/doi/full/10.1080/23766808.2020.1844991. Accessed 1 Aug. 2023.
Kloock, Carl T., et al. “Ultraviolet Light Detection: A Function of Scorpion Fluorescence.” BioOne Complete, bioone.org/journals/the-journal-of-arachnology/volume-38/issue-3/B09-111.1/Ultraviolet-light-detection-a-function-of-scorpion-fluorescence/10.1636/B09-111.1.short. Accessed 31 July 2023.
Mosher, Dave. “Glowing Scorpion Exoskeletons May Be Giant Eyes.” Wired, 30 Dec. 2011, www.wired.com/2011/12/scorpion-fluorescence/.
Scorpion Fluorescence and Reaction to Light | Request PDF - Researchgate, www.researchgate.net/publication/256654998_Scorpion_fluorescence_and_reaction_to_light. Accessed 1 Aug. 2023.
“Scorpions Glow in the Dark to Detect Moonlight.” New Scientist, 8 Dec. 2010, www.newscientist.com/article/mg20827903-700-scorpions-glow-in-the-dark-to-detect-moonlight/.
“Scorpions Make a Fluorescent Compound That Could Help Protect Them from Parasites.” American Chemical Society, www.acs.org/pressroom/presspacs/2020/acs-presspac-march-4-2020/scorpions-make-a-fluorescent-compound-that-could-help-protect-them-from-parasites.html. Accessed 31 July 2023.
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