Normoxia exposure reduces hemoglobin concentration and gill size in a hypoxia-tolerant tropical freshwater fish

Abstract Hypoxia is a widespread environmental stressor that shapes fish physiology and morphology. Plasticity in traits improve oxygen uptake delivery or reduce requirements may be critical for to cope with fluctuating dissolved (DO) conditions their natural habitat adapt new environments. In this study, we characterized suite of morpho-physiological respiratory naturally hypoxia-acclimated weakly electric mormyrid fish, Petrocephalus degeni , quantified plasticity response long-term normoxia exposure. We captured P. from hypoxic swamp (PO 2 = 2.43 ± 1.85 kPa) surrounding Lake Nabugabo, Uganda, acclimated them > 16 up 75 days. At various time points throughout exposure, measured blood hemoglobin lactate concentration, gill size, routine metabolic rate (RMR), regulation index (RI), tension ( P crit ). found 62–75 days exposure significantly reduced concentration (− 17%), filament length 14%), hemibranch area 18%), whereas RMR, RI, showed no significant change. Our results support earlier findings swamp-dwelling are well adapted life chronic severe hypoxia indicate they possess limited capacity phenotypic change DO environment.