GILL RESPIRATORY PARAMETERS DURING PROGRESSIVE HYPOXIA IN THE FACULTATIVE AIR-BREATHING FISH, HYPOSTOMUS-REGANI (LORICARIIDAE)

Oxygen consumption and gill respiratory parameters during progressive hypoxia were investigated in the facultative air-breathing loricariid fish, Hypostomus regani, without access to atmospheric air, after the determination of the threshold O-2 tension for aerial respiration in f ish with free access to water surface. In well-oxygenated water, H. re gani did not breathe air. The onset of aerial respiration occurred at inspired oxygen tensions (Pi(O2)) between 50 and 60 mmHg; there was no correlation between body mass and the O-2 threshold for air-breathing . During progressive hypoxia without access to atmospheric air, H. reg ani was found to be an oxygen regulator and maintaining aquatic oxygen consumption ((V) over dot(O2)) at 31 +/- 2 ml O-2 kg(-1) h(-1) at 25 degrees C. The critical oxygen tension (Pc-O2) was 34 mmHg. Gill venti lation, ventilatory volume and breath frequency increased during hypox ia and reached the highest values at a Pi(O2) range between 56 and 25 mmHg. The increment (percentage of normoxic values) of ventilatory vol ume was higher than the respiratory frequency in response to hypoxia. (C) 1998 Elsevier Science Inc. All rights reserved.