Behavioural and ecomorphological responses of the neotropical pacu Piaractus mesopotamicus (Teleostei, Serrasalmidae) to oxygen-deficient waters.

The serrasalmid Piaractus mesopotamicus is an obligate gill-breather encountered in the floodplains of the Paraná-Paraguay River system, even when the O2 concentrations there are below 0.5 mg 1(-1). Similar conditions occur in fish ponds where this species is cultured. Locomotory activity is affected by both O2 concentration and surface access. Activity increases with hypoxia if fish have surface access, but decreases if surface access is denied by floating macrophytes. Below 0.4 mg 1(-1), larvae as well as larger fish are capable of utilizing the oxygen-rich surface layer of the water for respiration, in order to survive periods of habitat-induced hypoxia. This so-called aquatic surface respiration (ASR) entails an increase in locomotory activity and an ecomorphosis, involving the formation of a dermal extension on the lower jaw in larger fish, that apparently has a hydrodynamic funciton of using the surface layer for gill respiration. Although larvae display ASR, no ecomorphological adaptations were found. When the water is aerated, the extended lip of the larger fish regresses to its original size. Below a macrophyte cover fish are not able to survive O2 depletion. Histological examinations show that the extension is formed by edematous processes in the stratum spongiosum. Ventilation frequency increases with falling O2 and decreases during ASR, thus demonstrating a better O2 supply at the surface layer of the water.