Continuous measurement of oxygen tensions in the air-breathing organ of Pacific tarpon (Megalops cyprinoides) in relation to aquatic hypoxia and exercise

The Pacific tarpon is an elopomorph teleost fish with an air-breathing organ (ABO) derived from a physostomous gas bladder. Oxygen partial pressure (PO2) in the ABO was measured on juveniles (238 g) with fiber-optic sensors during exposure to selected aquatic PO2 and swimming speeds. At slow speed (0.65 BL s-1), progressive aquatic hypoxia triggered the first breath at a mean PO2 of 8.3 kPa. Below this, opercular movements declined sharply and visibly ceased in most fish below 6 kPa. At aquatic PO2 of 6.1 kPa and swimming slowly, mean air-breathing frequency was 0.73 min-1, ABO PO2 was 10.9 kPa, breath volume was 23.8 ml kg-1, rate of oxygen uptake from the ABO was 1.19 ml kg-1 min-1, and oxygen uptake per breath was 2.32 ml kg-1. At the fastest experimental speed (2.4 BL s-1) at 6.1 kPa, ABO oxygen uptake increased to about 1.90 ml kg-1 min -1, through a variable combination of breathing frequency and oxygen uptake per breath. In normoxic water, tarpon rarely breathed air and apparently closed down ABO perfusion, indicated by a drop in ABO oxygen uptake rate to about 1% of that in hypoxic water. This occurred at a wide range of ABO PO 2 (1.7-26.4 kPa), suggesting that oxygen level in the ABO was not regulated by intrinsic receptors. � 2007 Springer-Verlag.