Final answer:
Marine invertebrates like sea squirts are isotonic with seawater and don't require active osmoregulation. Freshwater and saltwater fish must expend energy to maintain osmotic balance. Ureotelic animals like sharks store urea, and euryhaline organisms can tolerate various salinities.
Step-by-step explanation:
Osmoregulation in Marine Organisms
Marine invertebrates that are isotonic with their environment, such as sea squirts, do not need to actively regulate the osmotic pressure, as their internal salt levels match those of the seawater. However, not all aquatic organisms are osmoconformers. Some, like freshwater fish, live in a hypotonic environment and must actively regulate their internal salt concentration by taking in salts through their gills and excreting diluted urine. Conversely, saltwater fish live in a hypertonic environment and maintain osmotic balance by drinking seawater and excreting the excess salts through their gills and in concentrated urine. Euryhaline organisms, like salmon, can tolerate a wide range of salinities and thus have evolved sophisticated osmoregulatory mechanisms to survive in various aquatic environments. Ureotelic animals, such as sharks, store large concentrations of urea to achieve isotonicity with seawater. Their bodies have specific structures, like the rectal gland in sharks, which aids in the excretion of salts.
Some terrestrial animals, such as birds and reptiles, convert ammonia to uric acid or guanine to handle nitrogenous waste. If these terrestrial animals were to evolve back into water-dwellers, their excretory systems would also likely evolve to favor excretion mechanisms that are less energy-intensive and better suited to an aquatic environment.