Why do all the freshwater organisms have contractile vacuoles whereas majority of marine organisms lack them?
Contractile vacuole helps in osmoregulation. Because of the cellular environment of a freshwater organism being hypertonic, the water diffuses inside the cell constantly and gets collected in the contractile vacuole, which squeezes the extra water out of the cell periodically. Thus, keeping the internal environment constant. In case of marine organisms, this does not occur due to high salinity, therefore, there is no need of contractile vacuole.