Hypoxia tolerance in elasmobranchs. I. Critical oxygen tension as a measure of blood oxygen transport during hypoxia exposure

The critical O(2) tension of whole-animal O(2) consumption rate (M(O2)), or P(crit), is the water P(O2) (Pw(O(2))) at which an animal transitions from an oxyregulator to an oxyconformer. Although P(crit) is a popular measure of hypoxia tolerance in fishes because it reflects the capacity for O(2) uptake from the environment at low Pw(O(2)), little is known about the interrelationships between P(crit) and blood O(2) transport characteristics and increased use of anaerobic metabolism during hypoxia exposure in fishes, especially elasmobranchs. We addressed this knowledge gap using progressive hypoxia exposures of two elasmobranch species with differing hypoxia tolerance. The P(crit) of the hypoxia-tolerant epaulette shark (Hemiscyllium ocellatum, 5.10±0.37 kPa) was significantly lower than that of the comparatively hypoxia-sensitive shovelnose ray (Aptychotrema rostrata, 7.23±0.40 kPa). Plasma [lactate] was elevated above normoxic values at around P(crit) in epaulette sharks, but increased relative to normoxic values at Pw(O(2)) below P(crit) in shovelnose rays, providing equivocal support for the hypothesis that P(crit) is associated with increased anaerobic metabolism. The M(O2), arterial P(O2) and arterial blood O(2) content (Ca(O(2))) were similar between the two species under normoxia and decreased in both species with progressive hypoxia, but as Pw(O(2)) declined, epaulette sharks had a consistently higher M(O2) and Ca(O(2)) than shovelnose rays, probably due to their significantly greater in vivo haemoglobin (Hb)-O(2) binding affinity (in vivo Hb-O(2) P(50)=4.27±0.57 kPa for epaulette sharks vs 6.35±0.34 kPa for shovelnose rays). However, at Pw(O(2)) values representing the same percentage of each species' P(crit) (up to ∼175% of P(crit)), Hb-O(2) saturation and Ca(O(2)) were similar between species. These data support the hypothesis that Hb-O(2) P(50) is an important determinant of P(crit) and suggest that P(crit) can predict Hb-O(2) saturation and Ca(O(2)) during hypoxia exposure, with a lower P(crit) being associated with greater O(2) supply at a given Pw(O(2)) and consequently better hypoxia tolerance. Thus, P(crit) is a valuable predictor of environmental hypoxia tolerance and hypoxia exposures standardized at a given percentage of P(crit) will yield comparable levels of arterial hypoxaemia, facilitating cross-species comparisons of responses to hypoxia.