Why do all the freshwater organisms have contractile vacuoles whereas the majority of marine organisms lack them?
Contractile vacuole helps in maintaining salt and water level called osmoregulation. Because of the cellular environment of a freshwater organism such as Amoeba, Paramecium etc. 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. While in the case of marine protozoans organisms, this does not occur due to high salt concentration. These organisms live in isotonic conditions in seawater. Thus there is no need for contractile vacuole.
