Water and salt regulation by isosmotic, hyperosmotic, and hypoosmotic aquatic organisms.

Aquatic organisms generally live in one of three environmental circumstances. Organisms with body fluids containing the same concentration of water as the external environment are isosmotic. Organisms with body fluids with a higher concentration of water (lower solute concentration) than the external medium are hypoosmotic and tend to lose water to the environment. Those with body fluids with a lower concentration of water (higher solute concentration) than the external medium are hyperosmotic and are subject to water flooding inward from the environment. In the face of these osmotic pressures, aquatic organisms must expend energy to maintain a proper internal environment. How much energy the organism must expend depends upon the magnitude of the osmotic pressure between them and the environment and the permeability of their body surfaces.