Inter- and intraspecies variation in hypoxia tolerance in lentic and lotic cyprinid fish species

Freshwater habitats display highly diverse flow regimes and oxygen tension (PO2) dynamics, driving lentic and lotic fish to evolve distinct physiological strategies for sustaining oxygen supply capacity (α) under hypoxic stress and high energy demand. We hypothesized associations of α with metabolic traits that differ between lentic and lotic species: lower PO2 thresholds for maximum metabolic rate (MMR) and other physiological functions in lentic fish, and higher MMR and metabolic rate (ṀO2) at given PO2 thresholds in lotic fish. We tested this by comparing MMR, hypoxia tolerance, and hypoxic tissue metabolite responses in the lentic Pseudorasbora parva and lotic Acrossocheilus fasciatus. P. parva had superior hypoxia tolerance (lower PCRIT-MMR, PCRIT and PLOE) via enhanced α, higher liver glycogen, and hypoxia-induced LDH upregulation; while A. fasciatus exhibited higher MMR, matching lotic aerobic demands. Within each species, elevated α correlated with lower PCRIT-MMR, PCRIT and PLOE. Unexpectedly, α was uncorrelated with MMR in both species. Biochemical analyses revealed brain LDH activity as a key determinant of hypoxia tolerance in P. parva, while interorgan transport inefficiency causing brain lactate accumulation and glucose depletion limited hypoxia tolerance in A. fasciatus. These findings demonstrate habitat-associated shifts in physiological and biochemical coping mechanisms, highlighting the complex environment-physiology adaptation interplay rather than categorical divergence.