Oxygen consumption from air and water, ammonia and urea‐<scp>N</scp> excretion, and <scp>Na</scp>⁺ fluxes during progressive aquatic hypoxia in <scp>Amazonian</scp> armoured catfish <i>Pterygoplichthys gibbiceps</i> and <i>Pterygoplichthys

The Amazonian loricariid fish Pterygoplichthys gibbiceps, from the Rio Negro, and Pterygoplichthys pardalis, from the Rio Solimões, are facultative air-breathers that can use the stomach as an air-breathing organ. Measurement of oxygen uptake under progressive aquatic hypoxia revealed a relatively high hypoxia resistance of both species. In both species, air-breathing was initiated at aquatic PO₂ values below 3 kPa. In hypoxia, aerial oxygen uptake was dominant, but in P. gibbiceps total oxygen uptake was reduced to 55 ± 5% of the normoxic values, and in P. pardalis to only 43 ± 4% of the normoxic value. P. pardalis took a greater percentage of its total O₂ consumption from air (92 ± 2%) than did P. gibbiceps (85 ± 3%). Air-breath volume increased with body mass in P. gibbiceps, whereas in P. pardalis air-breathing frequency increased with body mass. The minimal breath volume required to account for aerial oxygen uptake was calculated as 24.9 ± 2.1 mL*kg^-1 for P. pardalis, and 17.3 ± 1.1 mL*kg^-1 for P. gibbiceps. In both species, ammonia and urea-N excretion were not significantly modified under hypoxic conditions, and urea-N excretion contributed a relatively high percentage (23%) to total nitrogen excretion. Measurement of unidirectional and net Na⁺ flux rates during normoxia, hypoxia and subsequent normoxic recovery in P. gibbiceps revealed a significant decrease in Na⁺ influx rate under hypoxic conditions, followed by a significant increase during recovery compared to the control period, with no changes in net Na⁺ balance. The data suggest that a reduction in energy-consuming processes may contribute to the observed hypoxia resistance.