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Why do all the freshwater organisms have...

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

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The osmotic concentration of fresh water organisms (e.g. Amoeba, Paramecium etc.) is hypertonic to surrounding water so water enters inside them by endosmosis. This excess of water is expelled with the help of contractile vacuoles so these help in osmoregulaiton . Marine protozoans are isotonic to sea water, so do not have contractile vacuole.
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