Aerobic scope falls to nil at P crit and anaerobic ATP production increases below P crit in the tidepool sculpin, Oligocottus maculosus .

The critical oxygen tension of whole-animal oxygen uptake rate, or P crit , has historically been defined as the oxygen partial pressure ( P O 2 ) at which aerobic scope falls to zero and further declines in P O 2 require substrate-level phosphorylation to meet shortfalls in aerobic ATP production, thereby time-limiting survival. Despite the inclusion of aerobic scope and anaerobic ATP production in the definition, little effort has been made to verify that P crit measurements, the vast majority of which are obtained using respirometry in resting animals, actually reflect the predictions of zero aerobic scope and a transition to increasing reliance on anaerobic ATP production. To test these predictions, we compared aerobic scope and levels of whole-body lactate at oxygen partial pressures ( P O 2 s) bracketing P crit obtained in resting fish during progressive hypoxia in the tidepool sculpin, Oligocottus maculosus . We found that aerobic scope falls to zero at P crit and, in resting fish exposed to P O 2 s < P crit , whole-body lactate accumulated pointing to an increased reliance on anaerobic ATP production. These results support the interpretation of P crit as a key oxygen threshold at which aerobic scope falls to nil and, below P crit , survival is time-limited based on anaerobic metabolic capacity.