Structure and function of ionocytes in the freshwater fish gill

Freshwater fishes lose ions to the external medium owing to the steep concentration gradients between the body fluids and the water. To maintain homeostasis, they use ionocytes to actively extract Na+, Cl−, and Ca2+ from the dilute external medium and excrete acidic (H+) or basic (HCO3−) equivalents by specialized cells termed ionocytes that are responsible for transport of ions. Freshwater fishes have evolved diverse approaches to solving these similar ionic and acid–base problems. In the few well-studied species, there are clearly different patterns in the physiology and morphology for ionocytes in the gill. In this review, we describe the varying nomenclature of ionocytes that have been used in the past 80 years to allow direct comparison of ionocytes and their common functions in different species. We focus on the recent advancement in our understanding of the molecular mechanisms of ion and acid–base regulation as represented by ionocyte subtypes found in rainbow trout, killifish, tilapia and zebrafish gill.

► We compare the historical nomenclature for differing types of ionocytes in FW fish.
► We compare different strategies for ion regulation by 4 species of FW fish.
► We identify the known molecular mechanisms for FW ion transport in each species.
► We highlight the gaps for future research in FW transport physiology.