The occurrence of aerial respiration in Rhinelepis strigosa during progressive hypoxia

Rhinelepis strigosa did not surface for air breathing in normoxic or moderate hypoxic water. This species initiated air breathing when the P i o 2 in the water reached 22 ± 1 mmHg. Once begun, the air‐breathing frequency increased with decreasing P i o 2 . Aquatic oxygen consumption was 21·0 ± 1·9ml O 2 kg −1 h −1 in normoxic water, and was almost constant during progressive hypoxia until the P i o 2 reached 23·9 mmHg, considered the critical oxygen tension (P c o 2 ). Gill ventilation increased until close to the P c o 2 (7·9‐fold) as a consequence of a greater increase in ventilatory volume than in breathing frequency. Gill oxygen extraction was 42 ± 5% and decreased with hypoxia, but under severe hypoxia returned to values characteristic of normoxic. The critical threshold for air breathing was coincident with the P c o 2 during aquatic respiration. This suggests that the air‐breathing response is evoked by the aquatic oxygen tension at which the respiratory mechanisms fail to compensate for environmental hypoxia, and the gill O 2 uptake becomes insufficient to meet O 2 requirements.