Ionoregulation in Tropical Fishes from Ion‐Poor, Acidic Blackwaters

Abstract

The understanding about the mechanisms of ion regulation in freshwater fishes comes from a wealth of studies on a relatively small number of model species from temperate climates, primarily from the Family Salmonidae and to a lesser extent Cyprinidae. The results suggest that to maintain internal Na+ and Cl- levels higher than the surrounding fresh waters, fish must balance diffusive losses with active uptake and that the gills are the main site for both of these processes, although the gut and kidney play roles too. Recent studies on the cyprinodont killifish in freshwater indicate that the ionic transport systems are fundamentally different from those of the model species. Extremely ion-poor, acidic waters, such as those found in the Rio Negro, pose a variety of challenges for ion regulation in freshwater fish. The fish of the Rio Negro pose a superb opportunity to explore the range of physiological adaptations for ion regulation in ion-poor, acidic waters as well as the evolution of these specializations within a phylogenetic framework. To illustrate this potential, the chapter describes the current understanding regarding mechanisms of ion regulation in freshwater fishes and what is known about the effects of ion-poor, acidic waters on ion regulation.