osmoregulation

Osmoregulation in biology is the maintenance by an organism of an internal balance between water and dissolved materials regardless of environmental conditions.

In many marine organisms osmosis (the passage of solvent through a semipermeable membrane) occurs without any need for regulatory mechanisms because the cells have the same osmotic pressure as the sea. Other organisms, however, must actively take on, conserve, or excrete water or salts in order to maintain their internal water-mineral content.