A contractile vacuole is an organelle in single-celled organisms that helps the cell remove wastes and excess water. It is found primarily in freshwater protists and algae. They are necessary because, in fresh water, the concentration of solutes inside a cell is greater than that outside the cell, so the cell constantly absorbs water through osmosis.
Vacuoles are usually organelles designed for the storage of wastes or water and other resources. The contractile vacuole, also known as the pulsing vacuole for its regular pumping action, is different. It stores its contents for only a few seconds before excreting them from the cell. They are crucial for the organisms that possess them, as without them the cells would expand with water, possibly even exploding.
While contractile vacuoles occur mainly in single-celled freshwater species, they do occur in a few marine and terrestrial organisms. They also occur in the early stages of some fungi, as well as in sponges.
Most marine species lack these structures, since the water around them has plenty of solutes to keep water from rushing into the cells. The species that possess contractile vacuoles in non-freshwater environments have extremely high levels of solutes in their cytoplasm, and always use the vacuoles.