More on scorpion fluorescence

Is scorpion fluorescence just an evolutionary accident or a genius adaption to scorpion life?

The mystery of scorpion fluorescence under UV-light have been under research in the last decade. Theories range from having no behavioral purpose to causing scorpions to be "living eyes". I have written several posts on the blog in the last couple of years on these theories.

This time Douglas Gaffin and Tristan Barker present a study on how scorpions react to different levels of UV irradiance. The study demonstrates that scorpions respond differently to different UV levels experienced during normal activity time and that locomotor activity was strongest when at UV irradiance levels that corresponded to sunset (the time scorpions move to the threshold of their burrows).

This study does indicate that scorpions are able to detect UV-levels, but more research is needed to demonstrate whether scorpion fluorescence has an adaptive function in UV-detection.

Abstract:
Scorpions are nocturnal arachnids that fluoresce a bright cyan-green when exposed to UV light. Although the function of this fluorescence remains unknown, some authors have suggested that it may aid the scorpions’ light detection. Taking advantage of scorpions’ negatively phototactic behavior, we tested the responses of desert grassland scorpions, Paruroctonus utahensis (Williams 1968), to 395 nm UV light at irradiances corresponding to an hour before sunset (0.15 mW/ cm2), sunset (0.01 mW/cm2), and moonlight (0.0001 mW/cm2), as well as no light. We found that animals showed the strongest responses to UV light levels equivalent to sunset. The animals moved more quickly and sporadically under the higher light levels. In addition, animals were less likely to complete a trial under highest light conditions, suggesting that UV light may inhibit normal scorpion locomotion. Finally, this study resulted in several methodological refinements, including automated tracking of the subjects’ movements that should prove useful in future behavioral studies of scorpion phototactic behavior.

Reference:
Barker TN, Gaffin DD. Comparison of scorpion behavioral responses to UV under sunset and nighttime irradiances. Journal of Arachnology. 2014;42(1):111-8. [Free Open Access Article]

Scorpions in the family Chaerilidae do not fluorescence under UV light

Scorpions fluorescence when they are exposed to ultraviolet light (in the range 320 - 400 nm). Several authors have discussed possible functions for this phenomena or if it has no function at all (previous blog posts on fluorescence).

Wilson Lourenco now reports that species in the family Chaerilidae do not fluorescence when exposed to UV light (Nine species were tested). This surprise finding is very interesting in the quest for any function of the fluorescence in scorpions. Interestingly, professor Lourenco also tested four species in the enigmatic family Pseudochactidae and they all did fluorescence. Pseudochactidae is considered to be the oldest and most primitive scorpion family and the presence of fluorescence may indicate that this is an early adaption in scorpions.

Abstract:
The fluorescence of scorpions in ultraviolet light, a well-known phenomenon, was discovered more than 60 years ago. Its possible function remains, however, a matter of discussion. Even during very recent studies, no conclusion has been reached. As suggested in these recent publications, the lack of or reduction of fluorescence could be a useful tool to explain the phenomenon. It is suggested here that, in at least some species of the family Chaerilidae Simon, this phenomenon is absent. This new discovery may initiate important comparative eco-physiological studies.

Reference:
Lourenço WR. Fluorescence in scorpions under UV light; can haerilids be a possible exception? Comptes Rendus - Biologies. 2012;335:731-4. [Subscritpion required for full text]

Thanks to professor Wilson Lourenco for sending me his paper!

Scorpions - Walking eyes?

Happy New year to you all!

The reason for the cryptic title of this post is a recent paper by Douglas Gaffin and co-workers in Animal Behaviour on the mystery of scorpion fluorescence.

There have been many theories on why scorpions fluorescence when exposed to UV-light. One theory is that this phenomena has no behavioral function and that the fluorescing components are just metabolic byproducts. Alternatively, scorpion fluorescence has a function (e.g. as a prey lure, as an aposematic signal or as an aid in the recognition of conspecifics (mate finding)). Some theories has been tested and rejected, but fluorescence in scorpions is still a mystery.

Gaffin and co-workers have tested scorpion activity under different light wavelengths (including UV-light) with eyes covered or not. The results suggest that scorpion cuticular fluorescence actually may be involved in their perception of light and contributes to orientation and light-avoidance behavior (e.g. helps the decision to stay in their burrow until outside light conditions are optimal for avoiding predators etc.).

This paper says that in addition to medial and lateral eyes, the metasomal elements that are sensitive to green light, the skin of the scorpion actaully works as an extra gigant eye helping in sensing light and detecting shadows. More studies are of course necessary before this theory is confirmed, but this is another proof of how fantastic animals scorpions are.

Abstract:
Scorpions are largely solitary, nocturnal arachnids that glow a bright cyan-green under UV light. The function of this fluorescence is a mystery. Previous 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 (350e400 nm). Scorpions are negatively phototactic, and we used this behaviour to assay the responses of desert grassland scorpions, Paruroctonus utahensis, to 395 nm UV light, 505 nm cyan-green light, 565 nm green light and no light within small, circular arenas. Based on the eye sensitivity data, we predicted maximal response to 505 nm, followed by lower responses to 395 and 565 nm. In our experiments, however, scorpions responded most intensely (abrupt bouts of locomotory activity) to 395 nm and 505 nm. Next, we ran trials under 395 and 505 nm on scorpions with their eyes blocked. Scorpions with blocked eyes were much less likely to move under 505 nm than under 395 nm and were much less likely to move under 505 nm than were control animals (those without their eyes blocked). These results suggest an active role for fluorescence in scorpion light detection. Other studies indicate that photosensitive elements in scorpion tails are sensitive to green light. We therefore propose 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. Scorpions may use this information to detect shelter, as blocking any part of the cuticle could diminish the signal.

Reference:
Gaffin DD, Bumm LA, Taylor MS, Popokina NV, Mann S. Scorpion fluorescence and reaction to light. Anim Behav. 2012. 83(2):429-436. Doi: 10.1016/j.anbehav.2011.11.014. [Subscription required for fulltext]

An interview with first author Douglas Gaffin about the paper can be found in Wired Magazine.

Thanks to Graeme Lowe for telling me about this paper!

Scorpion fluorescence - a way to avoid moonlight?

Scorpions fluorescence under UV-light. This is a great phenomenon for scorpion scientists and enthusiasts wanting to find and study scorpions, but its function has not been known so far. Suggested functions have been uv-light detection, mate identification, species identification, luring of prey, light amplification and that the fluorescence has no fuction at all.

Carl Kloock has written several papers on scorpion fluorescence (previous blog post), and in his latest paper (authored together with Abraham Kubli and Ricco Reynolds) it is concluded that their experimental data support the hypothesis that scorpion fluorescence serves as a means for the detection of UV-light at very low levels. Further, it is suggested that the UV-light acts as a cue for moonlight avoidance (fluorescence is a way to detect the presence of uv-light which is correlated to a moonlit night and this cause the scorpions to stay hidden in their hidings).

This is the first experimental evidence for a potential function for scorpion fluorescence, but further studies are necessary before final conclusion can be made on this phenomenon.

Abstract:
The hypothesis that fluorescence in scorpions functions in the detection of ultraviolet light was tested. We reduced the fluorescence of scorpions by prolonged exposure to ultraviolet light on a 16:8 h light:dark cycle and compared their activity levels and light environment choices to unmodified scorpions in simple arenas that were half in shadow and half exposed to light. Three different lighting conditions were tested: infrared (IR) light only, IR + ultraviolet light and IR + white light. Treatments were illuminated by infrared light for videotaping. Activity level was measured by the number of transitions from the exposed to shadowed regions, and choice was measured by the percentage of time spent in the shadowed portion of the arena. Under IR + ultraviolet light, fluorescent scorpions reduced their activity levels and the variance in habitat choice increased, compared with reduced-fluorescence scorpions. There were no differences between fluorescent and non-fluorescent scorpions in the IR only condition or in the IR + white light condition. This is interpreted as evidence that fluorescence aids in the detection of and response to ultraviolet light, and possible implications of this result in natural habitats are discussed. This is the first experimental demonstration of a possible function for scorpion fluorescence.

Reference:
Kloock CT, Kubli A, Reynolds R. Ultraviolet light detection: A function of scorpion fluorescence. Journal of Arachnology. 2010;38(3):441-5. [Subscription required for fulltext, but free fulltext after 12 months]

Prolonged exposure to uv light reduces scorpion fluorescence

It is well known that scorpions fluorescence under ultraviolet light exposure. No function for this phenomena has yet been proven. It is possible that there is no biological function and that the fluorescence is a byproduct of a molecule whose primary function is unrelated to fluorescence. More research is necessary to unravel the mystery of scorpion fluorescence.

Anecdontal reports have suggested that long-term exposure to uv light may reduce scorpion fluorescence. Carl Kloock has recently published a paper showing that this is indeed true. Interestingly, Kloock's work also show that fluorescence in live specimens recovered after one week without uv exposure. This is an indication that active metabolic processes are responsible for the recovery. Preserved specimens with reduced fluorescence showed no recovery after termination of uv exposure.

Abstract:
No abstract available

Reference:
Kloock CT. Reducing scorpion fluorescence via prolonged exposure to ultraviolet light. Journal of Arachnology. 2009;37(3):368-70. [Free fulltext - Open Access article]