Effect of aerial O-2 partial pressure on bimodal gas exchange and air-breathing behaviour in Trichogaster leeri

The effects of experimental alterations of aerial O-2 partial pressure (PO2, air) on bimodal gas exchange and airbreathing behaviour were investigated in the aquatic airbreathing fish Trichogaster leeri in normoxic water. Fish responded to increasing PO2, air by decreasing air-breathing frequency, increasing aerial O2 consumption rate (VO2), increasing mean O2 uptake per breath (VO2/ breath) and decreasing aquatic VO2 to maintain a constant total VO2. The rate of oxygen uptake from the air-breathing organ (ABO) during apnoea (VO2, ap) was derived on a breath-by-breath basis from VO2/ breath and apnoea duration. VO2, ap and estimates of ABO volume were used to calculate the PO2 in the ABO at the end of apnoea. This increased with increasing PO2, air, suggesting that ABO-PO2 is not regulated at a constant level by internal chemoreceptors. Furthermore, mean VO2, ap increased with increasing PO2, air, indicating that the observed increase in VO2/ breath with increasing PO2, air was facilitated not only by an increase in apnoea duration but also by an increase in the air-blood PO2 gradient.