Which nitrogenous waste is primarily used by aquatic organisms?

Aquatic organisms primarily excrete ammonia as their nitrogenous waste because it is highly soluble in water and can be easily diluted and eliminated from their bodies. In aquatic environments, the concentration of ammonia can be maintained at low levels due to the vast volume of water available to disperse it. Ammonia is a byproduct of amino acid metabolism and is highly toxic, but its quick elimination in water allows aquatic animals to cope with its toxicity effectively.

In contrast, urea and uric acid are less toxic and are used by terrestrial organisms to conserve water, as they can be excreted with minimal loss of water. Creatinine, produced from muscle metabolism, is not the main nitrogenous waste product for excretion but rather a byproduct related to muscle activity. Thus, ammonia is indeed the primary nitrogenous waste utilized by aquatic organisms due to its properties and the nature of their environment.