Metabolic scaling is a well-known biological pattern. Theoretical scaling exponents near 0.67 and 0.75 are the most widely accepted for aerobic metabolism, but little is known about the scaling of anaerobic metabolism. Furthermore, metabolic scaling has been mainly evaluated in organisms primarily relying on aerobic pathways. Here we evaluate both aerobic and anaerobic metabolic scaling in Parastacus pugnax, a burrowing freshwater crayfish endemic to Chile, which inhabits waters with low pO2 (~ 1 mg O2 L-1, measured in this study). We determined the metabolic rate, total oxidative capacity (Electron Transport System: ETS), critical oxygen tension (Pcrit) and muscular Lactate dehydrogenase (LHD) and Malate dehydrogenase (MDH) enzymatic activities (proxies of anaerobic metabolism) over a wide range of P. pugnax sizes (0.24-42.93 g wet mass). Aerobic metabolism scaled with crayfish size with an exponent of 0.78, remarkably similar to the 0.73 which scaled the ETS, the enzymatic complex behind respiration. Critical partial pressure of oxygen (Pcrit) was calculated as 15.6 ± 2.9 mmHg, showing that aerobic metabolism was efficiently maintained until ~ 10% air saturation. Below this threshold, P. pugnax switched to anaerobic metabolism, evidenced by a reduction in aerobic metabolism and ETS activity under chronic low oxygen conditions. None of the activities of MDH, LDH, their ratio (MDH/LDH), nor Pcrit scaled with crayfish size, indicating that these animals are equally adapted to hypoxic environments throughout their whole ontogeny. Given the particularities of its habitat, the information presented here is valuable for a proper management and successful conservation.
Keywords: Burrowing crayfish; Burrows water conditions; Freshwater; Hypoxia; Metabolic scaling.