Oxygen consumption of desert pupfish at ecologically relevant temperatures suggests a significant role for anaerobic metabolism.

Oxygen consumption is oftentimes used as a proxy for metabolic rate. However, pupfish acclimated to ecologically relevant temperatures may employ extended periods of anaerobism despite the availability of oxygen-a process we called paradoxical anaerobism. In this study, we evaluated data from pupfish exhibiting stable oxygen consumption. Routine oxygen consumption ([Formula: see text]) of a refuge population derived from Cyprinodon spp. acclimated to 28 and 33 °C was evaluated at the ecologically relevant assay temperatures of between 25 and 38 °C. Different interpretations of the data are available depending on normalization. For instance, [Formula: see text] of smaller fish, measured per fish, was remarkably stable over a wide range of assay temperatures and was not different between acclimation groups. However, when measured on a mass-specific basis, [Formula: see text] in these same smaller fish increases more predictably as temperature increased. [Formula: see text] of refuge fish and the closely related pupfish, C. nevadensis mionectes, measured near their respective acclimation temperatures, were essentially identical. However, [Formula: see text] of 28 °C acclimated fish of both species, when measured at 34 °C, was greater than that of the 33 °C acclimated fish measured at 28 °C. We suggest that this observed 'efficiency' may result from significant anaerobic metabolism use. Experiments investigating factorial aerobic scope ([Formula: see text]/[Formula: see text]) yielded values less than 1 in 21-36% of the 33 °C acclimated fish. These values indicate a substantial contribution of anaerobic metabolism to energy utilization by these fish. However, muscle lactate levels are not elevated in exercising fish-a result that is consistent with paradoxical anaerobism use.