All excrete water and transport salt into body actively. Steno restricted to living in a narrow salinity range. Eury animals can survive a wide range of salinity change. These animals need to have occasional access to saltwater. Active transport of water or salt.
Fish can transport salt ions back into body thru gill, get it from food.
Evolutinary history as freshwater spp. Reptiles, seabirds have salt glands to excrete salt actively. Sharks conserve trimethylamine oxide and urea in blood. Form metabolic water by food oxidation esp. Must be reused to build proteins or be eliminated.