Sprinting performance is linked to surface activity in scorpions

Aerobic capacity in arachnids is closely linked with considerable structural variation in their respiratory systems. However, although all scorpions are non-tracheated and possess four pairs of book lungs, they vary greatly in their locomotor activity patterns. Many non-burrowing species express short bursts of activity whereas other species dig deep burrows within hours, suggesting more aerobically fueled exercise. We hypothesized that locomotion in surface dwellers is more dependent on anaerobic ATP synthesis, and that this would be reflected in their exercise performance and functional adaptation to potential disturbance of resting-state homeostasis. We used an experimental design consisting of two surface-dwelling Buthidae species (Hottentotta judaicus and Leiurus hebraeus), a burrowing buthid (Buthus israelis) and two burrowing Scorpionidae (Scorpio fuscus and Scorpio palmatus). Maximum running speeds were higher for buthids, which were also more prone to fatigue than scorpionids. Higher respiratory exchange ratios recorded for buthids during activity and subsequent recovery indicated a higher reliance on anaerobically fueled locomotion compared with scorpionids. Our data show that quicker removal of excess CO2 resulting from anaerobic exercise and hemolymph buffering in buthids is associated with significantly higher carbonic anhydrase activity in their hemolymph, compared with that of scorpionids. Efficient CO2 emission may also contribute to the lower respiratory water losses of buthids, and thus assist in facing both biotic and abiotic challenges of surface existence, from which the scorpionid burrows provide refuge.