Scorpions possess a remarkable ability that has puzzled scientists and captivated nature enthusiasts for years: they glow in the dark.
But why do scorpions light up when night falls?
In this article, you will know about the anatomy of scorpions that enables them to glow, as well as numerous theories about its origin. Do not worry, I will keepthings easy to understand.
Scorpions and Biofluorescence
Scorpions are no ordinary creatures. They possess a unique ability known as biofluorescence, which is the key to their nighttime illumination.
| Do not confuse biofluorescence with bioluminescence. They may sound similar but they are not the same.
The main difference is that bioluminescence is an active process and requires a lot of resources. Bioluminescence is the ability of certain organisms to produce their light through a chemical reaction. For example, Fireflies produce light by mixing luciferin and luciferase enzymes in the presence of oxygen. |
However, in scorpions, their body simply reacts to external sources of ultraviolet (UV) light, like moonlight or blacklight. This process does not require anything from the scorpion. Unlike bioluminescence, there are no chemical processes involved.
When exposed to these UV rays, they just absorb the energy and re-emit it as visible light (blue).
The substance known as luciferin naturally absorbs photons and then emits them back, creating a self-sustaining process.This is what creates the eerie glow that’s captured the curiosity of many.
| Biofluorescence
Fluorescence is a form of luminescence (non-thermal light emission).Itoccurs in response to external irradiation and ceases with it. Therefore, when exposed to UV light (with a wavelength in the range of 350–400 nm), the scorpions emit blue-green light (with a wavelength of 450–500 nm). |
The Glow Mechanism
The glow mechanism of scorpions was first described in 1954 by the Italian zoologist M. Pavan, and at that time, it was established that the source of the effect was the cuticle.
So, let’s examine the scorpion’s exoskeleton in detail to comprehend how this functions.
Scorpions have a specialized layer in their cuticle (the outer shell) that contains specific molecules capable of fluorescing.
Now, the cuticle has also 3 layers:
- Epicuticle. This is the thinnest waxy layer that protects against drying out.
- Exocuticle. This is the middle layer. It provides rigidity and protection. It also acts as a structural framework and contains pigments that give coloration to the outer shell.
- Endocuticle. The innermost and thickest layer. It functions as a reserve of minerals (primarily calcium). This layer is used during molting or for the arthropod’s overall health.
The glow comes from the middle layer. In scorpions, this layer also contains specific molecules, known as fluorophores, that react to UV light by absorbing and re-emitting it as visible light, creating the characteristic glow.
The middle layer is where the glow originates. This layer of the scorpion also includes certain molecules called fluorophores, which react to UV light by absorbing it and then reemitting it as visible light, giving the creature its distinctive glow.
| Strictly speaking, the exocuticle is the primary layer during molting. It is responsible for providing maximum rigidity.
During molting, the epicuticle and exocuticle are shed. That is why a scorpion that has recently molted does not fluoresce for a few days. Additionally, newly born scorpions (first instar) with an unhardened cuticle are also incapable of fluorescing but the emission becomes strong in the 3rd and later instars. |
Therefore, when UV light hits this layer, the molecules just absorb energy.
Interesting fact: According to the study, the glow effect gradually fades if scorpions are maintained continuously in UV light and, after about 1 week, disappears entirely.
Exocuticle Layer in Detail
What’s fascinating is that the exocuticle in scorpions is unique. According to the study, it is made of 2 layers as well:
- the hyaline exocuticle.
- the inner exocuticle.
This hyaline layer is made mainly of sugars and proteins. This is a very thin layer (<4 µm). Inside this layer, scientists found these magic ingredients for their glow, which are called fluorophores.
Note: The inner exocuticle provides strength and rigidity to the exoskeleton.
In scorpions, the 2 main fluorophores are beta-carboline, which are connected to the amino acid tryptophan, and 4-methyl-7-hydroxycoumarin. These compounds in the hyaline exocuticle are what create the scorpions’ glow.
So, when we shine UV light on them, these compounds soak up the short-wavelength UV light and then re-emit it as longer-wavelength visible light. That’s what makes them glow green or blue when you see them under a blacklight.
But Why Do Scorpion Glow?
So, science has uncovered the mechanism of luminescence in scorpions. But why do they do it? What reasons have led scorpions to develop such a fascinating trait?
Unfortunately, it is still a bit of an evolutionary puzzle.
In an attempt to solve this puzzle, scientists have put forth various theories for decades. However, upon closer examination, most of these theories have proven to be invalid or just wrong. Nevertheless, it’s interesting to look at them.
1. Luring Prey
Some experts believed that the glow in the night might attract unsuspecting prey. For example, insects could be drawn to the scorpion’s glow, thinking it’s a potential food source.
Probability: Low.
Experiments revealed that the fluorescence of scorpions did not enhance their hunting efficiency. More other, in some cases, it had quite the opposite effect.
- Mate Attraction
Scorpions use their glow to find and attract mates in the darkness. Additionally, each scorpion species may have a unique glow pattern used for identification.
Probability: Limited evidence.
The main problem of this theory is that scorpions, despite having multiple pairs of eyes, have very poor vision. At the same time, scientists believe that it may be possible for scorpions to detect low intensities of light at certain wavelengths. Studies of four species from three families have shown that scorpion lateral and medial eyes are maximally sensitive to green light (around 500 nm) and secondarily to UV (350–400 nm).
3. Warning Sign
The bright glow can serve as a warning signal to potential predators, deterring them.
Probability: Average, but requires further investigation.
4. Camouflage
Fluorescence may help scorpions blend into their environment, making them less visible to predators.
Probability: Low.
To begin, it’s important to note that not many animals can see in the UV light spectrum. Secondly, in order to blend with their surroundings, scorpions would need to be among objects that also fluoresce, which is not typically the case.
5. Thermoregulation
It might assist in regulating their body temperature in cooler nights.
Probability: Limited evidence.
6. Anti-Cannibalism
Glow helps avoid cannibalism by signaling to other scorpions that they are not prey.
Probability: Low.
I have already mentioned that vision is not a strong trait of scorpions. In addition, they do not have any problems hunting and eating each other. According to the study, there is no evidence to show that scorpions react to one another visually.
7. Communication
They may use the glow for communication within their own species.
Probability: Limited evidence.
8. Initial Environmental Adaptation
Fluorescence may have evolved as an adaptation to specific environmental conditions.
For example, the intensity of radiation at the beginning of the Paleozoic era, when the first scorpions appeared, was different. Scorpions may have initially evolved to perfectly adapt to that time. However, even millions of years later, when environmental conditions changed, they were still relatively unaffected by it since they had evolved to be so efficient.
Probability: Limited evidence.
9. Byproduct of Metabolic Reactions
The idea that bioluminescence is a consequence of metabolic processes is an additional intriguing theory. Therefore, it merely lacks a distinct purpose.
Probability: Limited evidence.
10. Shelter Detection
Some other studies indicate that the cuticle may function as a whole-body photon collector, transducing UV light to cyan-green before relaying this information to the central nervous system. They can use this information to detect shelter, as blocking any part of the cuticle could diminish the signal.
Interesting fact: The tails of scorpions are sensitive to green light.
Probability: High.
Scorpions are negatively phototactic animals. It means that means that they tend to move away from or avoid light.
The results of experiments indicate that the amount of time that scorpions spent on the light-exposed side varied depending on the treatment and that avoidance was greatest for UV light followed by green light.
Who Else Can Glow in the Dark?
Fluorescence is observed in various animals, including some insects, spiders, fish, and even turtles. However, in most cases, the mechanisms behind its occurrence are still unknown.
In Conclusion
Scorpions possess the intriguing ability of biofluorescence, making them glow in the dark.
Over the years, lots of hypotheses have been made to explain it. Among these theories, some have shown limited evidence, while others have a higher probability.
As for me, the shelter detection theory looks more probable since it aligns with their negative phototactic behavior.
Related articles:
Related articles:
- Kloock, C. T. 2005 . Aerial insects avoid fluorescing scorpions. Euscorpius, No. 21: 1-7.
- LOURENÇO, W. R. & J. L. CLOUDSLEYTHOMPSON. 1996. The evolutionary significance of colour, colour patterns and fluorescence in scorpions. Revue suisse de Zoologie, Vol. hors série, 449–458.
- Shawn J Stachel, Scott A Stockwell, David L Van Vranken, The fluorescence of scorpions and cataractogenesis, Chemistry & Biology, Volume 6, Issue 8, 1999, Pages 531-539.
- Filshie, Barry K., and Neil F. Hadley. “Fine structure of the cuticle of the desert scorpion, Hadrurusarizonensis.” Tissue and Cell11, no. 2 (1979): 249-262.
- Douglas D. Gaffin, Lloyd A. Bumm, Matthew S. Taylor, Nataliya V. Popokina, Shivani Mann, Scorpion fluorescence and reaction to light, Animal Behaviour, Volume 83, Issue 2, 2012, Pages 429-436.
- Gregory R.C. Blass, Douglas D. Gaffin, Light wavelength biases of scorpions, Animal Behaviour, Volume 76, Issue 2, 2008,Pages 365-373.
- Frost, Leslie M., David R. Butler, Brian O’Dell, and Victor Fet. “A coumarin as a fluorescent compound in scorpion cuticle.” Scorpions(2001): 363-368.