The effects of dissolved oxygen levels on the metabolic interaction between digestion and locomotion in Cyprinid fishes with different locomotive and digestive performances
To test whether the effects of water oxygen concentration ([O2]) on the metabolic interaction between locomotion and digestion differ between fish species with different locomotive and digestive behaviours in normoxia, we investigated the swimming performance of fasted and fed fish at water [O2] of 1, 2 and 8 (normoxia) mg L−1 (2.5, 5 and 20 kPa) at 25°C in three juvenile Cyprinidae fish species: goldfish (Carassius auratus), common carp (Cyprinus carpio) and qingbo (Spinibarbus sinensis). Digestion, taxon and water [O2] all had significant effects on the pre-exercise oxygen consumption rate $$ (\dot{M}{\text{O}}_{{ 2 {\text{pre-exercise}}}} ) $$ and the swimming performance (P < 0.05). Among the three fishes, qingbo showed the highest swimming performance and the lowest feeding $$ \dot{M}{\text{O}}_{2} $$ at the saturated water [O2], and its active oxygen consumption rate $$ (\dot{M}{\text{O}}_{{ 2 {\text{active}}}} ) $$ and critical swimming speed (Ucrit) decreased the most with decreases in water [O2]. Qingbo exhibited a locomotion-priority metabolic mode at all three water [O2]. Digestion was sacrificed to locomotion in a postprandial swimming situation, but fed qingbo could not maintain their Ucrit at water [O2] of 2 and 1 mg L−1. Goldfish showed the lowest swimming performance and the highest feeding $$ \dot{M}{\text{O}}_{2} $$ at the saturated water [O2]. They exhibited a digestion-priority metabolic mode at high water [O2]. However, with a decrease in water [O2], the feeding $$ \dot{M}{\text{O}}_{2} $$ decreased more acutely than the respiratory capacity; thus, digestion and locomotion performed independently in a postprandial swimming situation (i.e., an additive metabolic mode) at a water [O2] of 1 mg L−1. The common carp showed moderate and balanced swimming performance and feeding $$ \dot{M}{\text{O}}_{2} $$ at the saturated water [O2], and exhibited an additive metabolic mode at all 3 water [O2], because digestion, swimming and respiratory capacities decreased in parallel with the decrease in water [O2].