Osmotic and ionic regulation

Abstract The contents of ions and organic compounds in freshwaters vary to a great extent. In turn, the body fluids of freshwater fish are hyperosmotic in relation to their habitat, making osmoregulatory mechanisms a key feature of their general well-being and survival. This chapter discusses the structure and function of osmoregulatory organs—epithelia, gills, gastrointestinal tract, kidney, and urinary bladder—of Neotropical fish, addressing effects of hypoxia, pH, water hardness, and dissolved organic matter on osmoregulation. Even though a consistent core of knowledge on the subject is recognizable, studies regarding osmoregulatory mechanisms in Neotropical fish remain necessary, given the scarcity of reports on species dwelling in alkaline waters and hard waters, independently of pH range. Studies on the ionic dynamics of the gastrointestinal tract of feral fish can add to the knowledge on osmoregulation of freshwater fish, especially in ion-poor waters, an environment in which it is assumed that a considerable part of the ion uptake derives from ingested food.