The oxygen uptake capacity of freshwater fish evolves to favour hypoxia tolerance in lentic species but locomotion capacity in lotic species

Oxygen supply capacity (α) is a physiological trait hypothesized to shape the metabolic niche and biogeography of aquatic breathers, yet how natural selection targets α remains unclear. Two key fitness‐related traits—locomotor ability and hypoxia tolerance—both depend on α, yet whether this shared physiological basis leads to a functional coupling between these traits, or instead allows them to evolve independently under habitat‐specific selection, remains an open question. We therefore hypothesized that divergent selection pressures across flow regimes decouple these two traits, leading to habitat‐specific associations between α and each trait. To test this hypothesis, we measured α, hypoxia tolerance and swimming performance in 32 freshwater fish species from lentic and lotic habitats. Our results showed that the relationship between α and each trait varied with habitat type: in lotic species, α was strongly associated with enhanced swimming performance, reflecting selection for supporting the high metabolic demands of prolonged locomotion; in lentic species, α was linked to improved hypoxia tolerance, driven by adaptations to function effectively under low oxygen tensions. Overall, these findings provide insights into the evolutionary lability of α and validate it as a highly flexible, fitness‐related trait shaped by habitat‐specific selection. This highlights the role of interactions between shared physiological foundations and habitat‐specific selection in driving trait divergence, thereby offering a broader framework for understanding how physiological flexibility promotes biodiversity across heterogeneous environments. Read the free Plain Language Summary for this article on the Journal blog.