The missing effect of temperature on branchial O₂ loss in an air-breathing catfish Available to Purchase

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.