 The skin of a teleost is largely impermeable to water and the main interface between the fish 's body and its surroundings is the gills . In freshwater , teleost fish gain water across their gills by osmosis while in seawater , they lose it . Similarly , salts diffuse outwards across the gills in freshwater and inwards in salt water . The European flounder spends most of its life in the sea but often migrates into estuaries and rivers . In the sea it can gain in an hour , Na + ions equivalent to forty percent of its total free sodium content , with 75 percent of this entering through the gills and the remainder through drinking . By contrast , in rivers there is an exchange of just two percent of the body Na + content per hour . As well as being able to selectively limit salt and water exchanged by diffusion , there is an active mechanism across the gills for the elimination of salt in sea water and its uptake in fresh water . 
