03 January, 2012

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.

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.

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!

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