Many animals can tolerate only a narrow range of external osmotic environments.
These organisms are known as stenohaline. In contrast, other organisms can function well over a wide range of external saline concentrations including freshwater and seawater . These animals are known as euryhaline. You will find in your studies of Zoology, that these definitions may not be rigid and that many animals can tolerate varied changes in their external environment with little apparent adverse physiological effects. An example of a true euryhaline species is the blue crab, Callinectes sapidus .