question_answer 60)

  Why do all the freshwater organisms have contractile vacuoles whereas majority of marine organisms lack them?

Answer:

                  Contractile vacuole helps in osmo regulation. Because of the cellular environment of afresh water 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, while in case of marine organisms, this does not occur due to high salinity, therefore no need of contractile vacuole.