A contractile vacuole is found in eukaryotic cells, particularly protozoa and certain unicellular algae. It is involved in osmoregulation. The contractile vacuole contracts to expel water out of the cell
Related Theory
The marine protozoa do not have contractile vacuole because the marine protozoa live in a hypertonic environment. Marine protozoa do not need to remove excess water out of the cell, instead, they need to conserve water from getting lost in the hypertonic environment. Hence, they do not need contractile vacuole.