Osmoregulation in Protozoans

A freshwater protozoan such as Amoeba or Paramecium lives in a hypotonic medium. Water continuously enters its body by osmosis.
The contractile vacuole collects this excess water and pumps it out at intervals (osmoregulation).
Marine water is hypertonic (or nearly isotonic) to the protoplasm. In it, water no longer enters the cell; instead the cell tends to lose water.
With no excess water to remove, the contractile vacuole is no longer needed. It stops functioning and disappears (marine protozoans generally lack contractile vacuoles).
So the vacuole will disappear.

Freshwater protozoans such as Amoeba and Paramecium live in a hypotonic medium, so water keeps entering their cells by osmosis.
The contractile vacuole collects the excess water from the cytoplasm and discharges it to the outside at intervals, so the cell does not swell and burst. This is osmoregulation.
Mitochondria produce energy, the nucleus controls cell activities, and food vacuoles digest food.
So the organelle for osmoregulation is the contractile vacuole.