Ammonotelic, Ureotelic, and Uricotelic Animals

Ammonia (NH₃) is highly toxic and requires a massive amount of water to dissolve and flush out of the body safely. Therefore, only aquatic animals can afford to be ammonotelic.

• Mechanism: Ammonia diffuses out across the body surface or through gills directly into the surrounding water.
• Examples: Bony fishes (teleosts), aquatic amphibians (tadpoles/salamanders), and aquatic invertebrates (sponges, aquatic insects).
• Water requirement: High (approx. 500 ml of water needed to excrete 1 gram of ammonia).

Terrestrial animals cannot afford to lose large amounts of water. To save water, their livers convert highly toxic ammonia into Urea, which is 100,000 times less toxic. Urea can be stored safely in the body and excreted in liquid urine.

• Mechanism: Ammonia is converted to urea in the liver via the Ornithine Cycle.
• Examples: Mammals (including humans), adult terrestrial amphibians (frogs, toads), and marine fishes (sharks/cartilaginous fishes).
• Water requirement: Moderate (approx. 50 ml of water needed to excrete 1 gram of urea).

Animals living in very dry environments need to conserve almost all their water. They convert ammonia into Uric Acid, which is virtually non-toxic and insoluble in water. It is excreted as a thick white paste or dry pellet.

• Mechanism: Highly energy-intensive process, but saves maximum water.
• Examples: Birds, reptiles (snakes, lizards), land snails, and terrestrial insects (cockroaches).
• Water requirement: Extremely low (approx. 10 ml of water needed to excrete 1 gram of uric acid).