Contractile Vacuoles - Excretion

Contractile vacuoles are always found in freshwater protozoans and sponges, but are frequently absent in their marine counterparts. Since a freshwater animal is hyperosmotic to the medium (freshwater) and its surface is permeable to water, it should continually endeavour to pump out the excess water that enters osmotically. When a freshwater protozoan is transferred to dilute sea water, the vacuolar activity decreases. In those marine species that have a contractile vacuole (e.g., some ciliates), vacuolar output increases with dilution of the sea water.

The fluid present in the contractile vacuole of freshwater protozoans is hypoosomotic to the surrounding cytoplasm, though still hyperosmotic to freshwater. These observations are consistent with the hypothesis that contractile vacuoles are primarily organs of water balance and function as pumps to remove excess osmotic water. Any role they might play h the excretion of nitrogenous wastes, which is mainly ammonia-in protozoans and sponges, is considered to be secondary.