Which group of organisms primarily uses ammonia for nitrogenous waste?

The correct answer is that aquatic organisms primarily use ammonia for nitrogenous waste. Aquatic organisms, such as many fish and amphibian larvae, excrete ammonia directly into the water, where it can be quickly diluted. Ammonia is highly toxic, but because these organisms live in an environment where water is abundant, they can afford to eliminate ammonia immediately without the need for additional energy to convert it into less toxic forms.

This direct excretion of ammonia is efficient for aquatic life as it allows for rapid removal of waste products from their bodies, relying on the surrounding water to help disperse the ammonia quickly. Since ammonia is water-soluble, it easily diffuses into the surrounding water, reducing the concentration of this toxic compound in their bodies.

The other groups mentioned, such as terrapins, mammals, and insects, tend to convert ammonia into less toxic forms, such as urea or uric acid, before excretion. This conversion is energetically costly, but it is necessary for these organisms to conserve water and reduce toxicity given their terrestrial environments.