Gill respiratory parameters during progressive hypoxia in the facultative air-breathing fish, Hypostomus regani (Loricariidae)

Abstract

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 fish with free access to water surface. In well-oxygenated water, H. regani did not breathe air. The onset of aerial respiration occurred at inspired oxygen tensions ( Pi O 2 ) 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. regani was found to be an oxygen regulator and maintaining aquatic oxygen consumption ( V ̇ O 2 ) at 31±2 ml O 2 kg −1 h −1 at 25°C. The critical oxygen tension ( Pc O 2 ) was 34 mmHg. Gill ventilation, ventilatory volume and breath frequency increased during hypoxia and reached the highest values at a Pi O 2 range between 56 and 25 mmHg. The increment (percentage of normoxic values) of ventilatory volume was higher than the respiratory frequency in response to hypoxia.