Why are contractile vacuoles common in fresh water but not in salt water?
Easy enough to answer! In fresh water, the organisms that have got contractile vacuoles are hyper tonic when compared to fresh water. Fresh water doesn’t have that many ions or molecules in it. Water keeps coming inside the organisms that have a higher osmolarity and so they need put that water out of them, least they explode. In salt water, the osmolarity is much, much higher outside the organisms that live in them. Water keep going out of the organisms by osmosis. They need to keep the water in, and not out, so they don’t need contractile vacuoles. They have different mechanisms that do the opposite of contractile vacuoles.
