Decapod crustaceans which successfully invade habitats with hypo-osmotic conditions must show strong hyper-osmoregulatory capacities, stable growth also at reduced salinities, strongly differentiated ion-transporting tissues, high activities of Na+-K+-ATPase, and an enhanced expression of mRNA encoding for the Na+-K+-ATPase -subunit. Ontogenetic changes in the patterns of osmoregulation occurring under constant laboratory conditions reflect characteristic changes in the environmental conditions faced by the successive developmental stages in nature.