Gill Design in Freshwater Fishes: Interrelationships among Gas Exchange, Ion Regulation, and Acid-Base Regulation

This study employed three freshwater fish species (rainbow trout, yellow perch, and smallmouth bass) to examine the interrelationships among gas and ion exchange and acid-base regulation by the gills. Interspecific differences in critical swimming speed (U̇crit), electrolyte and nonelectrolyte permeability, O₂ consumption, and rates of net branchial transfer of acid-base equivalents were assessed. The most active species was the rainbow trout, and it ranked highest for all of the above measures. However, the extent of interspecific variation in these parameters was not the same. There were, for example, much larger interspecific differences in U̇crit and electrolyte permeability than in gas-exchange capacity. We conclude that the gills of less active species conserve ions by reducing electrolyte permeability, and that this is achieved with relatively little sacrifice of gas-exchange capacity. However, there is a penalty in diminished capacity for acid-base regulation.