Mudskipper Periophthalmodon schlosseri can repay oxygen debts in air but not in water

We determined aerial and aquatic oxygen uptake rates (ṀO₂) of Periophthalmodon schlosseri at rest and after exhaustive exercise at 30°C. Resting ṀO₂ in air (3.16 ± 0.10 [SE] μmol/g/hr) was significantly higher than that in air saturated water (2.41 ± 0.06 μmol/g/hr). When the fish was placed in air after 2 min exhaustive exercise, ṀO₂. Immediately increased about 2.5 times, thereby repaying oxygen debt of 5.0 μmol/g. In contrast, ṀO₂ failed to show any significant elevation from the resting level as long as the fish was confined in water after exercise. However, when the fish was subsequently emerged, ṀO₂ did increase above the resting level, amounting to an excess oxygen uptake of 5.5 μmol/g. These results demonstrated that the gas transport system of P. schlosseri is better adapted to air breathing and the mode of adaptation limited the capability for water breathing.