Exploring the Mystery of Scorpion Fluorescence

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.

 

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. 

"Arachnophilia! Using Museums to Understand and Conserve Arachnids" - An Interview with Jacob Gorneau and Kate Montana

 By Sabine Fuchs, Katherine Branson School

Spiders and scorpions and arachnids, oh my! The study of these “creepy crawlies,” their evolution, habitat, and, for some, their mysterious bioluminescence, is explored by Jacob Gorneau and Kate Montana at the California Academy of Sciences. Gorneau, who received his bachelor’s in entomology from Cornell University and his master’s in Biology at San Fransisco State University, is a research assistant in the Entomology department at the Academy of Sciences. Montana, who received her undergraduate degree in biology and anthropology, is currently a graduate student researcher in the arachnology lab at the California Academy of Sciences. Gorneau and Montana have worked together using morphological and molecular data to revise the evolutionary history of the marronoid clade of spiders. Jacob and Kate presented Arachnophilia! Using Museums to Understand and Conserve Arachnids at Marin Science Seminar on February 8, 2023. Kate and Jacob were kind enough to answer some questions about their fascinating work surrounding arachnids.

Kate Montana in the field.

1. Jacob: You discussed how scorpions are found in a wide variety of climates. What traits do scorpions possess that make them so adaptable? Are there large differences between scorpions that live in a forest compared to those that live in the desert? 

Physically, scorpions are quite similar even though they are found in a wide range of habitats and climates — fossils of scorpions from nearly 400 million years ago look exactly as scorpions look today! The main exception is some cave-dwelling scorpions no longer have eyes, which is a common phenomenon in animals that exclusively live in caves. Scorpions do, however, have specific habitat preferences that we call microhabitats, and these are generally pretty consistent regardless of the climate and often involve darker spaces like burrows that are, in the case of deserts, generally slightly more humid than the surrounding landscape. This also helps them escape the extreme weather conditions that they can experience in deserts. Scorpions also have a strong exoskeleton that prevents them from losing too much water, and they are so efficient at this that they usually get enough water from their food and never need to drink water.

2. Kate: Since your work centers around fieldwork, what practices do you implement to ensure that the environment is not negatively affected by your studies? 

We take care to be as minimally invasive as possible. We do often collect spiders and scorpions that will be sacrificed for DNA extraction, but we only sample populations that are not in danger of being depleted. We try our best to only take the focal species that we need, though some bycatch does occur. When we do catch something that we did not intend to catch, we reach out to colleagues to find out if it might be useful for their research so as not to waste the specimen. We spread out our sampling geographically, which helps get a wide range of geographic representation as well as keep our sampling to a minimum at any particular site. 

3. Jacob: There is a lot of uncertainty and debate over why scorpions fluoresce under ultraviolet black light. As scientists and researchers, what explanation(s) are you most drawn to? 

Thoughts about why scorpions fluoresce range from allowing scorpions to navigate using the sky, to recognize members of the same species, or to detect light. While I personally find the idea that scorpions can use fluorescence under ultraviolet light to somehow navigate using the stars or moon really exciting, I think the fluorescence might more likely be a tool for simply detecting light. 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.

Jacob Gorneau in the field.

4. Kate: What does the process of revision of the evolutionary history of a species look like, and how does this revision impact the scientific world?

We start with a thorough search of the scientific literature to find out all we can about what is already understood about a particular species, genus, family, or other designation of a group of organisms. We use tools like Google Scholar or the library to find papers that contain this information. Then we can decide which samples we need in order to answer our question about how one particular species or genus is related to others in the spider tree of life. In addition to the samples that we are directly interested in, we need outgroups—taxa whose relationship we already know to our focal species in order to root the tree. Then we can collect both molecular data through DNA sequencing and morphological data through careful observation of the physical characteristics of the organisms. We take these data and run various computational analyses in order to produce a phylogeny that will visualize the evolutionary relationships between species. The scientific community can then use this tree of life to inform questions about behaviors, range of species, species interactions, and morphological characteristics. Since evolution is the process by which all organisms come to be the way they are, we need a solid understanding of the intricate evolutionary relationships between organisms in order to fully understand how they came to be.

5. Jacob: You explained that cattle grazing has impacted scorpion habitats since it compacts the earth, which prohibits scorpions from burrowing. In what other ways are humans negatively impacting scorpions or their habitats? 

In California, there are two other main threats to scorpions and their habitats: wildfires and land development. Firstly, while wildfires are an important and natural part of California’s ecosystem, the way that humans manage land today has resulted in wildfires larger in scale and intensity than they have ever been before. We don’t know if scorpions can survive these wildfires. Since scorpions are slow dispersers, meaning they move through the environment quite slowly and can’t run away from a fire, populations that exist where major wildfires have occurred might go extinct. Similar to how wildfires might block out groups of scorpions, developing land, either because of agriculture or building construction, can also remove important scorpion habitats. One way to combat this issue is contributing to wildlife corridors or making sure that as land is developed, there is enough connected natural land for organisms to move freely. People often think of wildlife corridors in terms of larger animals, but this connectivity of habitats is also useful for scorpions.