Body Fluids of Marine Teleost
The body fluids of marine teleost are hypotonic to seawater, so these fishes lose water to the environment especially across the gill epithelium. To replace the lost volume of water they drink seawater.
Figure: chloride cells
The ingested seawater along with NaCl and KCl is absorbed across the intestinal epithelium, 70 to 80% of it enters the bloodstream. Most of the divalent ions iuch as ca2+, Mg2+ and SO2-4 are expelled through the anus. The excess salt absorbed along with the water is subsequently eliminated from the blood by active transport of Na+ and Cl- and perhaps K+ across the gill epithelium- the seawater. Divalent salts are also secreted by the kidneys. Gills have special type of secretory epithelium known as chloride cells. These cells actively secrete chloride and probably sodium also into the seawater.