Organelo que regula el equilibrio hídrico en protistos de agua dulce.

These organelles are contractile vacuoles

Explanation:

Contractile vacuoles are intercellular pumps that are slowly filled with fresh water and eventually expel their contents outside the cell. Freshwater protists maintain the hydric equilibrium by contractile vacuoles that pump out the water of the cell and thus prevent the cell swelling induced by osmotic stress.