The missing effect of temperature on branchial O2 loss in an air-breathing catfish.

Air-breathing fish risk losing aerially sourced O2 to hypoxic water during branchial passage. Two adaptations thought to mitigate this loss are reduced gill size and increased blood O2 affinity. Both are affected by temperature in the facultative air-breathing catfish Pangasianodon hypophthalmus, where increased temperature results in larger gills and reduced blood O2 affinity. Here, we tested whether branchial O2 loss increases with temperature, by measuring this loss and the aerial and aquatic gas exchange at 25°C and 33°C in near aquatic anoxia. Surprisingly, increasing temperature did not change the absolute O2 loss while metabolic rate increased by 75%. Hence, animals suffered a 10% loss of the aerial O2 uptake at 25°C compared with only a 5% loss at 33°C. Our results indicate an increased hypoxia-induced reduction in gill ventilation at 33°C, negatively affecting aquatic exchange of both CO2 and O2, resulting in unchanged O2 loss and a CO2 partitioning shift towards the air phase.