Oxygen Acquisition of the Reedfish, Erpetoichthys Calabaricus

Abstract

In normoxic water at 25°C, the reedfish (Erpetoichthys calabaricus) can breathe in both air and water, although water is the primary source of oxygen. This pattern of oxygen partitioning is altered by a number of factors. Increases in metabolic costs (e.g. locomotor activity) and decreases in aquatic oxygen concentration are met by increases in aerial oxygen uptake. Reedfish acclimated to 33°C have increased lung breathing compared with fish acclimated to 25°C. Also, activity stimulates lung breathing more at 33°C than at 25°C. Unexpectedly, weight-specific oxygen uptake rates of reedfish at these two temperatures were not significantly different. Increased lung-breathing compensates for changes in locomotor activity, dissolved oxygen and temperature, allowing reedfish to meet their overall oxygen requirements. Exposure of reedfish to six different combinations of oxygen and carbon dioxide indicated that lung ventilation frequencies were more labile than gill ventilation frequencies. The maximum change in lung ventilation rates was 849% above controls whereas the largest change in gill ventilation frequency was 27%. Exposure of reedfish to hyperoxia significantly depressed both gill and lung ventilation. Hypercapnic gases stimulated gill ventilation at both low (0.5%) and high (5%) concentrations; however, lung ventilation was only stimulated at the higher carbon dioxide concentration. The observed depression of gill ventilation frequencies in reedfish exposed to combinations of hypercapnic and hypoxic gases indicates that the inhibitory effect of low oxygen on gill ventilation dominates the stimulatory effect of hypercapnia. Overall, these data suggest that oxygen exerts a stronger influence in the control of respiration than carbon dioxide in this air- and water-breathing fish. Reedfish survived out of water for 6 and 8h at 25°C without obvious ill effects. Oxygen consumption rates of these fish were significantly less than those of similar sized reedfish measured in water at either 25 or 33°C. The respiratory physiology of the reedfish frees it from aquatic oxygen constraints and allows it to tolerate at least short-term terrestrial exposures.