Osmoregulation

Water and salt move constantly across a fish's semi-permeable membranes - especially the gills. In freshwater, water floods in and salts leak out; the fish must actively excrete excess water and pump salts back in. In saltwater, the opposite - the fish loses water to the sea and must drink and excrete concentrated urine. Osmoregulation is the physiological machinery that manages this balance moment-to-moment.

Freshwater fish: gills actively absorb ions (Na+, Cl-, Ca2+); kidneys produce large volumes of dilute urine; fish rarely drink. Saltwater fish: gills excrete excess ions via specialized chloride cells; kidneys produce small volumes of concentrated urine; fish drink seawater constantly. Euryhaline fish (salmon, tilapia, killifish) switch between the two modes via hormonal cascades - primarily cortisol and prolactin.

Chloride cells (ionocytes), gill epithelium, cortisol, prolactin, urea vs ammonia excretion, hypertonic vs hypotonic environments, euryhaline vs stenohaline.

Atlantic salmon undergo smoltification - a hormonal reorganization that flips the fish from freshwater to saltwater osmoregulation over 2-4 weeks. Bull sharks and green sawfish are among the few marine fish that can tolerate freshwater indefinitely.

Aquaculture: understanding osmoregulation guides brackish-water farming of tilapia, milkfish, and salmon smolts. Aquarium hobby: acclimation of fish to new water requires osmoregulatory adaptation.

Bull sharks reduce the salt content of their blood by roughly 50% when swimming up the Amazon - the only shark that can genuinely live in freshwater indefinitely.