Changes in cardiac output during swimming and aquatic hypoxia in the air-breathing Pacific tarpon

Pacific tarpon (Megalops cyprinoides) use a modified gas bladder as an air-breathing organ (ABO). We examined changes in cardiac output (V̇_b) associated with increases in air-breathing that accompany exercise and aquatic hypoxia. Juvenile (0.49 kg) and adult (1.21 kg) tarpon were allowed to recover in a swim flume at 27 °C after being instrumented with a Doppler flow probe around the ventral aorta to monitor V̇_b and with a fibre-optic oxygen sensor in the ABO to monitor air-breathing frequency. Under normoxic conditions and in both juveniles and adults, routine air-breathing frequency was 0.03 breaths min^− 1 and V̇_b was about 15 mL min^− 1 kg^− 1. Normoxic exercise (swimming at about 1.1 body lengths s^− 1) increased air-breathing frequency by 8-fold in both groups (reaching 0.23 breaths min^− 1) and increased V̇_b by 3-fold for juveniles and 2-fold for adults. Hypoxic exposure (2 kPa O₂) at rest increased air-breathing frequency 19-fold (to around 0.53 breaths min^− 1) in both groups, and while V̇_b again increased 3-fold in resting juvenile fish, V̇_b was unchanged in resting adult fish. Exercise in hypoxia increased air-breathing frequency 35-fold (to 0.95 breaths min^− 1) in comparison with resting normoxic fish. While juvenile fish increased V̇_b nearly 2-fold with exercise in hypoxia, adult fish maintained the same V̇_b irrespective of exercise state and became agitated in comparison. These results imply that air-breathing during exercise and hypoxia can benefit oxygen delivery, but to differing degrees in juvenile and adult tarpon. We discuss this difference in the context of myocardial oxygen supply.