Final answer:
Aquatic organisms with lower salt concentrations than their surrounding water are hypotonic, with saltwater fish secreting concentrated urine and freshwater fish excreting dilute urine to maintain osmotic balance.
Step-by-step explanation:
Aquatic organisms that have a lower salt concentration in their bodies than the surrounding water are considered hypotonic. Organisms in hypertonic environments, such as saltwater, have adaptations to prevent water loss. For instance, saltwater fish excrete highly concentrated urine and secrete salt through their gills, an osmoregulatory mechanism to maintain salt and water balance. Freshwater fish, which live in a hypotonic environment, actively take in salt through their gills and excrete dilute urine to handle the excess water. The osmoregulation processes are vital for these organisms to survive in their respective environments.