Interspecific Variation in Oxygen‐Binding Properties of Scorpion Hemocyanin

Abstract

Though they don’t display much structural variation in their respiratory systems, scorpions exhibit variable lifestyles that may necessitate different aerobic capacities. These lifestyles are largely conserved at the family level – typically, members of the family Buthidae do not burrow, unlike most members of Scorpionidae. Among digging scorpions, burrows can be dug overnight and may reach depths of up to 1m. Scorpions, like many other arachnids, utilize hemocyanin as a respiratory pigment in the hemolymph. We therefore hypothesized that the oxygen‐binding affinity of hemocyanin would vary between buthids and scorpionids. To test this hypothesis, we used a Blood Oxygen Binding System (Loligo Systems) to determine the partial pressure of oxygen required to reach 50% oxygen‐saturation of hemocyanin (P50) among three buthids (Leiurus hebraeus, Buthus Israelis, and Hottentotta judaicus) and two scorpionids (Scorpio fuscus and Scorpio palmatus). We predicted that scorpionids would have higher P50 values, as lower O2 affinity would enhance oxygen release at the muscles during intense bouts of exercise. Our preliminary data supports this hypothesis ‐ buthids had lower P50 values than scorpionids. Mean (±SD) P50 values of buthids (12.34±1.83 Torr for L. hebraeus (n=8), 18.86±2.89 Torr for B. Israelis(n=13), and 19.78±6.38 Torr for H. judaicus (n=8)) were significantly lower than that of S. fuscus (46.11±10.57 Torr; n=9). S. palmatus had a lower P50 (25.73±8.64 Torr) than all three buthids, though the differences were not significant – perhaps due to small sample size (n=4). Taken together, these data suggest hemocyanins in scorpions have evolved to address the physiological demands associated with burrowing behavior.