Is hypoxia vulnerability in fishes a by-product of maximum metabolic rate?

The metabolic index concept combines metabolic data and known thermal sensitivities to estimate the factorial aerobic scope of animals in different habitats, which is valuable for understanding the metabolic demands that constrain species' geographical distributions. An important assumption of this concept is that the O-2 supply capacity (which is equivalent to the rate of oxygen consumption divided by the environmental partial pressure of oxygen: (M) over dotO(2)/PO2) is constant at O-2 tensions above the critical O-2 threshold (i.e. the PO2 where O-2 uptake can no longer meet metabolic demand). This has led to the notion that hypoxia vulnerability is not a selected trait, but a by-product of selection on maximum metabolic rate. In this Commentary, we explore whether this fundamental assumption is supported among fishes. We provide evidence that O-2 supply capacity is not constant in all fishes, with some species exhibiting an elevated O-2 supply capacity in hypoxic environments. We further discuss the divergent selective pressures on hypoxia- and exercise-based cardiorespiratory adaptations in fishes, while also considering the implications of a hypoxia-optimized O-2 supply capacity for the metabolic index concept.

Automated summary

The metabolic index concept combines metabolic data and known thermal sensitivities to estimate the factorial aerobic scope of animals in different habitats, providing insight into the metabolic demands that constrain species' geographical distributions. However, research shows that the oxygen supply capacity is not constant in all fish species, challenging the assumption that hypoxia vulnerability is a by-product of selection on maximum metabolic rate.