How do sharks and rays use urea to maintain water balance?

Sharks and rays maintain water balance through a process known as the urea retention habitus. Unlike most marine vertebrates, which must constantly drink seawater to compensate for water loss, these cartilaginous fishes reabsorb nitrogenous waste products—specifically urea and trimethylamine oxide—in their renal tubules.12

By accumulating these substances in their blood and tissues, sharks and rays elevate their internal osmotic concentration to a level slightly higher than that of the surrounding seawater. This adaptation allows water to enter their bodies passively through osmosis, eliminating the need to drink seawater or expend significant energy to maintain osmotic balance. Any excess salt that enters the body is subsequently removed via the rectal gland.312

This physiological strategy is highly flexible; when these fish move into freshwater environments, they increase their urine output and decrease the concentration of urea in their blood to adjust to the changing osmotic gradient. Purely freshwater species, however, generally lack this urea retention habitus.1