SUMMARY
The effects of experimental alterations of aerial O2 partial pressure (PO2,air) on bimodal gas exchange and air-breathing behaviour were investigated in the aquatic air-breathing fish Trichogaster leeri in normoxic water. Fish responded to increasing PO2,air by decreasing air-breathing frequency,increasing aerial O2 consumption rate(V̇O2), increasing mean O2 uptake per breath(V̇O2/breath) and decreasing aquatic V̇O2 to maintain a constant total V̇O2. The rate of oxygen uptake from the air-breathing organ (ABO) during apnoea(V̇O2,ap) was derived on a breath-by-breath basis from VO2/breath and apnoea duration. V̇O2,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 V̇O2,ap increased with increasing PO2,air, indicating that the observed increase in V̇O2/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.
