THE OCCURRENCE OF AERIAL RESPIRATION IN RHINELEPIS-STRIGOSA DURING PROGRESSIVE HYPOXIA

Rhinelepis strigosa did not surface for air breathing in normoxic or m oderate hypoxic water. This species initiated air breathing when the P (i)o(2) in the water reached 22 +/- 1 mmHg. Once begun, the air-breath ing frequency increased with decreasing P(i)o(2). Aquatic oxygen consu mption was 21.0 +/- 1.9 mi 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 ve ntilation increased until close to the P(c)o(2) (7.9-fold) as a conseq uence of a greater increase in ventilatory volume than in breathing fr equency. Gill oxygen extraction was 42 +/- 5% and decreased with hypox ia, but under severe hypoxia returned to values characteristic of norm oxic. The critical threshold for air breathing was coincident with the P(c)o(2) during aquatic respiration. This suggests that the air-breat hing response is evoked by the aquatic oxygen tension at which the res piratory mechanisms fail to compensate for environmental hypoxia, and the gill O-2 uptake becomes insufficient to meet O-2 requirements. (C) 1998 The Fisheries Society of the British Isles.