Why do freshwater protozoa have a contractile vacuole while marine ones do not?

It’s all about one physical phenomenon called osmosis. This term explains that fresh water tends to liquids with a high salt content. In freshwater protozoa, there is more salt inside the membrane than in water, and water constantly flows inside, and a contractile vacuole is needed to filter it. In the seas, there is no such problem, since the water contains a large amount of soluble salts, and the simplest take only the required amount of water.