1. Metabolic energy fuels all biological processes, and thus, energy metabolism is vitally important for organisms. It has been suggested that interspecific variation in physiological functions is associated with the energetic metabolism of organisms, which varies considerably due to selection, adaptation and evolution.
2. Thus, the present study aimed to clarify the relationship between the interspecific diversification of maintenance energy expenditure and two forms of anaerobic capacity (in terms of exercise and hypoxia tolerance) in freshwater fishes. Moreover, in the context of environmental adaptation, we hypothesized that selection for increased locomotor performance in species with a fast-flow lifestyle favours a high aerobic and anaerobic capacity and, consequently, a high maintenance metabolism, whereas selection for strong hypoxia tolerance in species with a slow-flow lifestyle favours a reduced maintenance metabolism and a reduced aerobic capacity.
3. We examined these topics among 30 species of freshwater fish with different lifestyles based on the flow regimes of their habitats (fast, intermediate and slow flow). The resting metabolic rate of each species was measured, and then, the fish were subjected to two treatments: (i) exercise to exhaustion, after which we measured the maximum metabolic rate (MMR), excess post-exercise oxygen consumption (EPOCe) and metabolic recovery rate (MRR); and (ii) hypoxia to the point of loss of equilibrium (LOE), after which we measured the metabolic response to recovery from hypoxia, including the peak metabolic rate (PMRh) and the excess post-hypoxia oxygen consumption (EPOCh).
4. We found that the resting metabolic rate was positively correlated with both the aerobic and anaerobic capacity of the fishes and that species from fast-flow habitats have higher aerobic and anaerobic capacity with concomitantly increased resting metabolism. However, neither EPOCh nor LOE was correlated with resting metabolic rate.
5. In summary, we suggest that selection for increased aerobic and anaerobic capacity to facilitate the demands of exercise has occurred in fast-flow species, that selection for hypoxia tolerance may have favoured a reduction in this capacity in intermediate and slow-flow species, and that specialized adaptations (such as flexibility in gill morphology and altered metabolic pathways) play important roles in hypoxia tolerance in some species.