Hormonal control of fluid intake and fluid balance

Fishes occupy a unique position among vertebrates for the control of water balance because of their aquatic life. In teleosts, whose plasma osmolality is about one third that of seawater, water moves through or between epithelial cells of the gills depending on environmental salinities. They gain water across the gills in freshwater and excrete it as dilute urine to maintain water balance. By contrast, water is lost by the gills in seawater, which is compensated by drinking and subsequent absorption of water by the intestine. Despite the importance of drinking and intestinal absorption of water in the marine environment, inhibitory hormones play critical roles in the regulation to prevent excess salt gain, which is opposite to terrestrial tetrapods in which stimulatory hormones are predominant.