Do air-breathing fish suffer branchial oxygen loss in hypoxic water?

In hypoxia, air-breathing fish obtain O₂ from the air but continue to excrete CO₂ into the water. Consequently, it is believed that some O₂ obtained by air-breathing is lost at the gills in hypoxic water. Pangasionodon hypophthalmus is an air-breathing catfish with very large gills from the Mekong River basin where it is cultured in hypoxic ponds. To understand how P. hypophthalmus can maintain high growth in hypoxia with the presumed O₂ loss, we quantified respiratory gas exchange in air and water. In severe hypoxia (PO₂: ≈ 1.5 mmHg), it lost a mere 4.9% of its aerial O₂ uptake, while maintaining aquatic CO₂ excretion at 91% of the total. Further, even small elevations in water PO₂ rapidly reduced this minor loss. Charting the cardiovascular bauplan across the branchial basket showed four ventral aortas leaving the bulbus arteriosus, with the first and second gill arches draining into the dorsal aorta while the third and fourth gill arches drain into the coeliacomesenteric artery supplying the gut and the highly trabeculated respiratory swim-bladder. Substantial flow changes across these two arterial systems from normoxic to hypoxic water were not found. We conclude that the proposed branchial oxygen loss in air-breathing fish is likely only a minor inefficiency.