Why do sharks and rays not need to drink seawater to prevent water loss?

Sharks and rays do not need to drink seawater because they have evolved a unique physiological strategy to maintain their internal osmotic balance. Unlike most marine bony fish, which must drink seawater to compensate for water loss, sharks and rays retain high concentrations of nitrogenous waste products—specifically urea and trimethylamine oxide—within their blood and tissues.12

By accumulating these organic molecules, these fish elevate their internal osmotic concentration to a level that is slightly higher than that of the surrounding seawater. Because their internal fluids are more concentrated than the external environment, water naturally and passively enters their bodies through osmosis rather than being lost to the environment. This adaptation eliminates the need for them to swallow seawater to stay hydrated. Any excess salt that enters their bodies is efficiently removed through a specialized organ known as the rectal gland.312