If an animal is an osmoregulator and they live in a hypotonic environment they must discharge excess water. Or if they are an inhabitant of a hypertonic environment they take in water to offset the osmotic loss. The main movement of the water occurs in an osmotic gradient, therefore the water of the lower osmolarity moves toward the region of higher osmolarity. To be able to move the water in and out of the osmotic gradient must be maintained by having the osmoregulators expend energy. To do this the body fluids must be maintained through the manipulation of the concentration solute.
Animals are able to live because they are osmoregulators.
This allows them if they are in freshwater to adjust where the osmolarity is too low for support of cellular life or on land where the water is in a shorter supply. Thus the osmoregulation is costly, based on how many different animals osmolarity are in their surroundings. Also it depends on how much of the membrane transport work is required to actively transport solutes. The osmoregulation makes up almost five percent of the resting metabolic rate of freshwater bony fish and many marine animals. Some animals use up to thirty percent of their resting metabolic rate.