11 April, 2014

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.

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.

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]

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