African lungfish, which are often found in small stagnant pools of fresh water, 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. e. Because lungfish breathe air, their blood is more oxygenated and prone to reacting with excess ammonia.

Amongst urinary waste products which are ammonia, urea and uric acid, ammonia is the most toxic, then comes urea which is mildly toxic while uric acid is the least toxic. Various organisms excrete these urinary wastes as per their habitat/adaptations. These waste products have some advantages and some disadvantages over each other. For example, ammonia is extremely toxic so it cannot be retained in the body for a longer time but it requires very less energy to produce ammonia than to produce urea that is why most of the aquatic animals excrete ammonia as urinary waste to save their energy and keep on urinating frequently because they live in water all the time. In a large water pool, excreting this ammonia does not cause toxicity because it gets diluted in the surrounding easily.

In contrast to this, land inhabiting animals like mammals prefer to excrete urea as urinary waste even if they have to spend more energy/ATP to convert ammonia into urea with the help of urea cycle. Land animals have to retain urinary waste products in their body for a longer time as compared to aquatic animals and this is the reason why they urinate less frequently because urea being less toxic can be retained in the body for a longer time.

But african lungfish is an exception to other aquatic animals when it comes to producing urinary waste because they live in small stagnant pools of fresh water and if they will excrete ammonia as waste it will make the habitat toxic leading to their death.