Dependence on body size of respiratory function in Hoplias malabaricus (Teleostei, Erythrinidae) during graded hypoxia.

Oxygen uptake and ventilatory responses to environmental hypoxia were assessed for two groups of Hoplias malabaricus of nearly tenfold different body weights (mean weights, group I = 39 g; group II = 365 g). The main purpose of the study was to evaluate the relationships between body weight and the critical O2 tension (PCO2) for maintenance of normal resting O2 uptake. The smaller specimens had a threefold larger weight-specific O2 uptake compared to that of the larger specimens. Moreover, smaller size coincided with a higher PCO2, i.e., a more limited range for maintenance of O2 uptake during hypoxia. Likewise, at any inspired PO2 the weight-specific gill ventilation was largest for group I. This was most pronounced during severe hypoxia where both tidal volume and respiratory frequency increased markedly. In addition, the oxygen cost of breathing tended to be highest in the smaller individuals. The results open questions as to correlations between development, weight-dependent O2 demands, and tolerance to hypoxia on an intraspecific level.