Final answer:
Aquatic animals secrete ammonia as their waste product because it requires less metabolic energy to excrete. Ammonia is easily diffused in water, making it an efficient waste product for organisms living in aquatic environments.
Step-by-step explanation:
Excretion in Aquatic Animals
The substance that is secreted by most aquatic animals is ammonia, given that it requires less metabolic energy to excrete. Aquatic animals are typically ammonotelic, meaning they excrete ammonia directly into their surrounding water. This excretion method is efficient in aquatic environments because ammonia is highly soluble in water and can be removed by diffusion. Conversely, terrestrial animals have developed mechanisms to convert ammonia into less toxic compounds like urea or uric acid to conserve water and prevent toxicity. Mammals, including humans, convert ammonia into urea, which is less toxic and requires less water for excretion, becoming ureotelic animals.
Thus, the correct answer to the student's question is b) Ammonia, to reduce metabolic energy expenditure. Aquatic animals can easily rid themselves of ammonia due to its high solubility, requiring less energy as it diffuses directly into the water.
The correct answer is b)