Question

Why do all the freshwater organisms have contractile vacuoles whereas the majority of marine organisms lack them?

Answer 1

Freshwater organisms:

Freshwater organisms survive in water which has lower salt concentration compared to the body of the organism. Hence, there is a continuous inflow of water to balance its hypertonic internal body fluid (endosmosis). Therefore, such organisms need contractile vacuoles to excrete out excess water.

Marine organisms :

Marine organisms survive in sea water which has higher salt concentration as compared to the body of the organism. They continuously eliminate salts from the body since their internal body fluid is hypotonic and produce very small volumes of urine to conserve water. Thus, they do not need contractile vacuoles to remove excess water.