Oxygen consumption from air and water, ammonia and urea-N excretion, and Na+ fluxes during progressive aquatic hypoxia in Amazonian armoured catfish Pterygoplichthys gibbiceps and Pterygoplichthys pardalis.

Abstract

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.