Are protozoans presenting contractile, or pulsatile, vacuoles easily found in fresh or in salt water?
Fresh water is the less concentrated of solutes than sea water and it (fresh water) tends to be less concentrated than the intracellular environment making cells to swell. Sea water, on the other hand, as it is very concentrated tends to dehydrate the cell.
The vacuoles of protozoans are internal structures specialized in water storage that when essential liberate water to the cytoplasm. Vacuoles therefore can dilute the cytoplasm for it to enter into osmotic equilibrium with the environment. Protozoans of fresh water then require vacuoles more since their intracellular is hypertonic in relation to the exterior. Without the dilution mechanism given by the vacuoles, protozoans of fresh water would absorb too much water and would die.