Question

African lungfish, which are often found in small stagnant pools of freshwater, produce urea as a nitrogenous waste. What is the advantage of this adaptation?

(A) Urea takes less energy to synthesize than ammonia.

(B) Small stagnant pools do not provide enough water to dilute ammonia, which is toxic.

(C) Urea forms an insoluble precipitate.

(D) Urea makes lungfish tissue hypoosmotic to the pool.

Short answer

(A) The statement “urea takes less energy to synthesize than ammonia” is false.

(B) The statement “Small stagnant pools do not provide enough water to dilute ammonia, which is toxic” is true.

(C) The statement “urea forms an insoluble precipitate” is false.

(D) The statement “urea makes lungfish tissue hypoosmotic to the pool” is false.

Step-by-step solution

Excretion in aquatic organisms

The primary excretory organ of most of the aquatic vertebrates comprises protonephric or mesonephric kidneys. Establishing osmoregulation in aquatic organisms is more problematic than the terrestrial organisms.

The water constantly enters the body through gills and proper balance of salts maintained by their various adaptions.

Explanation of option (A)

The conversion of urea from the ammonia takes much energy. The excretion of ammonia as nitrogenous waste does not require more cellular energy.

Therefore, the synthesis of urea needs more energy than ammonia.

Thus, the given statement is false.

Explanation of option (B)

Aquatic organisms generally excrete ammonia. However, African lungfishes are ureotelic and have adapted to excrete urea because of their habit. When the small stagnant pool accumulates with ammonia, it becomes toxic. During their aestivation, the ammonia detoxifies as urea.

Therefore, lungfishes have adapted to excrete urea as nitrogenous waste.

Thus, the given statement is true.

Explanation of option (C)

The urea produced as the excretory substance is soluble in water and also eventually mixes with the soil.

Therefore, urea does not have insolubility with water and soil.

Thus, the given statement is false.

Explanation of option (D)

The excretion of urea instead of ammonia in lungfishes brings advantages to them. The amount of water lost during the excretion of urea is much less. The soluble salts are maintained in the body.

Therefore, urea does cause hypoosmolality in African lungfishes.

Thus, the given statement is false.