Persistent dominance of aerial oxygen uptake during recovery from exercise and air exposure in Arapaima gigas.

The pirarucu Arapaima gigas is an obligate air-breathing fish with a massive internal air-breathing organ, yet it also retains substantial gills. In this study, Arapaima were exercised for 10 min either in water or in air, or were exposed to air for one hour. During subsequent recovery from exercise or air exposure aerial and aquatic oxygen uptake and ventilatory activity were recorded for two hours to assess the contribution of aerial respiration to repayment of an oxygen debt. Compared to the initial control period, oxygen uptake (MO2) was almost doubled during air exposure. A doubling of total MO2 was also recorded during the first 30 min of recovery in all three experimental series. Thereafter MO2 returned to resting values, indicating a quick recovery from exercise as well as from air exposure. Aerial and aquatic ventilation were significantly elevated during initial recovery from all treatments. Air-breathing frequency returned to resting values more quickly after air exposure than after exercise in water or air. Relating oxygen uptake from air and water to ventilatory activity revealed that breathing air was about 200-fold more effective than breathing water. Aerial oxygen uptake contributed between 69% and 81% to total MO2 irrespective of total MO2, and this did not change during recovery from the different treatments. Total oxygen debt encountered after air exposure was about half the size encountered after swimming exercise. Although breathing air was much more effective for the uptake of oxygen than breathing water, the relative contribution of air-breathing to repayment of the oxygen debt remained stable at about 80%.