The major function of the contractile vacuole of amoeba is osmoregulation. The solute concentration found in the cell of amoeba's cytoplasm is more than the solute concentration in the freshwater that surround the external part of the organism, thus, water enter the cell through osmosis. The contractile vacuole collect the excess water and expel it through an opening in the cell membrane. By doing this, the contractile vacuole maintains the water balance in amoeba. This how the contractile vacuole normally operate.
In a situation where amoeba is placed in seawater, then water from the cell cytoplasm will rush out of amoeba cell, because of the higher salt content of the surrounding medium. The contractile vacuole will respond to the situation by increasing its contraction and pumping water out the cell in an accelerated manner, this will lead to the shrinking of the cell.
