Teleosts maintain body fluid ionic and acid-base homeostasis through energy-consuming ion transport mechanisms in the gills and other organs. Our serial studies identified a new gill cell type (GR cells) in zebrafish and tilapia, which are rich in glycogen and provide glucose or lactate/pyruvate to neighboring ionocytes. Different types of glucose and monocarboxylate transporters were found to facilitate energy deposition and supply between ionocytes and GR cells. On the other hand, ammonotelic fishes are suggested to rely on ammonia excretion to enhance net acid secretion because ammoniogenesis produces HCO3- to facilitate body fluid acid-base homeostasis. We recently extended our study to understand how ammoniagenesis occurs and is coordinated with ammonia excretion in fish gills. In medaka and other species, we have discovered an undiscovered ammonia-producing cell type that is rich in glutaminase (GLS cells) and located adjacent to the ammonia-excreting ionocytes (NHE cells) in the gills. The unique division of labor between GLS and NHE cells in the gills allows medaka to simultaneously upregulate GLS activity and ammonia excretion shortly after exposure to acidic environments. These findings provide fundamental information to enhance our mechanistic understanding of the energy metabolism underpinning gill ion transport functions and the related physiological adaptations of vertebrates during evolution.