Reliance on aquatic respiration across different life stages of the facultative air-breathing catfish, Clarias gariepinus.

Understanding ontogenetic shifts in bimodal respiration is essential for elucidating how air-breathing fishes optimise physiological performance, yet whether individuals at different life stages sustain aerobic function without aerial access is poorly understood. This study investigated ontogenetic shifts in reliance on aquatic respiration and hypoxia tolerance in African catfish (Clarias gariepinus) across five size classes, fry (< 1 g), fingerlings (1-5 g), juveniles (20-40 g), and sub-adults (100 g, 300 g), under submergence at normoxia (~ 17 kPa), moderate hypoxia (8.7 ± 0.8 kPa), and extreme hypoxia (2.2 ± 0.1 kPa). Ventilation frequency (fV) and time to loss of equilibrium (LOE) served as indices of respiratory effort and tolerance. All size classes maintained equilibrium for 48 h under normoxic submergence, confirming facultative air-breathing capacity, though the accompanying fV rise was uniform across sizes. Under hypoxia, fV responses were strongly size-dependent: fry upregulated fV by ~ 90% under moderate hypoxia versus 62% in 300 g sub-adults, a divergence more pronounced under moderate than extreme hypoxia, consistent with shifts in ventilatory strategy and metabolic scaling, though the roles of gill efficiency, ventilation depth, and anaerobic capacity remain unresolved. Time to LOE increased with body mass under moderate hypoxia (R2 = 0.37, p < 0.001), while under extreme hypoxia the relationship weakened and became non-linear (R2 = 0.14, p = 0.0018), with intermediate sizes showing peak tolerance. These findings show that aquatic respiration reliance strengthens with ontogeny in C. gariepinus, contradicting the hypothesis of increasing dependence on air-breathing organs with size.