Ammonia is highly soluble in water and can be eliminated directly.
Explanation
The form in which animals excrete nitrogenous waste-Ammonia, Urea, or Uric Acid-depends primarily on the availability of water in their habitat and the toxicity of the waste product. This classification divides animals into ammonotelic, ureotelic, and uricotelic groups.
Detailed Analysis:
- Ammonia is a semi-solid compound that is easy to excrete. is Incorrect: Ammonia is not excreted as a semi-solid. It is excreted in a dissolved state or as a gas. Excretion in the form of a semi-solid paste or pellet is characteristic of uric acid (uricotelism), which is observed in birds, reptiles, and insects to minimize water loss.
- Ammonia is highly soluble in water and can be eliminated directly. is Correct: Ammonia is the most toxic nitrogenous waste and requires a very large volume of water for its dilution and elimination. However, it is highly soluble in water. Since aquatic animals (such as bony fishes) are surrounded by water, they can excrete ammonia directly into the environment through diffusion across body surfaces or gill surfaces (as ammonium ions). This method is energy-efficient as it avoids the metabolic cost of converting ammonia into urea or uric acid.
- Aquatic animals have limited access to water for excretion. is Incorrect: The premise is factually wrong; aquatic animals have abundant access to water, which allows them to flush out toxic ammonia. It is terrestrial animals that face limited water access and must convert ammonia to less toxic forms.
- The kidneys of aquatic animals are incapable of synthesizing urea. is Incorrect: The excretion of ammonia is driven by habitat suitability rather than a physiological inability to synthesize urea. For instance, cartilaginous fishes (like sharks) are aquatic but are ureotelic (excrete urea) to maintain osmotic balance with seawater.
Key Takeaway:
Ammonotelism is characteristic of aquatic animals because ammonia is highly soluble and can be easily diffused into the surrounding water, despite its high toxicity. Terrestrial adaptation requires the conversion of ammonia to less toxic Urea or Uric Acid to conserve water.