Physiology of Finfish and Shellfish

2.4.6. Osmoregulatory problem in marine fishes

Marine fishes experience just the opposite effect in comparison to freshwater fishes because their internal salt content is less than that of the surrounding water. Marine teleosts are hypoosmotic (body fluids 1/3 of that seawater) and live in a medium ing high concentration of salts. The skin of marine fishes as in freshwater species is impermeable and osmotic exchanges occur through the gills and oral membrane. Or in other words a marine fish has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water and gain salt. It actively excretes salt out from the gills . Marine fishes are faced with the problem of continuous loss of water and gain salts. To prevent osmotic dehydration or to maintain their water balance marine fishes drink large quantities of seawater but this further increases salt content of the body. Urine output is considerably reduced to conserve water. The glomeruli are reduced or become non functional. The distal segment of the tubule is absent. Thus, only a small amount of concentrated urine is produced and most of the water is reabsorbed in the tubule.

As marine fish drinks seawater and Na and Cl ions are actively absorbed through the gut. Many species are reported to drink water >25% of their body weight/day, of which 80% is absorbed. This excess of salt ions is excreted out into the water by the special cells in the gills, oral membrane and inner surface of the operculum. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia.

A diagram showing the movement of water and ions in saltwater fish.