Why aquatic animals favor ammonia excretion?

Aquatic animals excrete ammonia because it’s the simplest and most energy-saving way to get rid of nitrogen waste. Since they live in water, they can easily diffuse and dilute the toxic ammonia without harming their bodies.

Ammonia forms when proteins and amino acids break down. Even though it’s poisonous, releasing it directly—known as ammonotelism—is an efficient adaptation that suits aquatic life perfectly.

How Aquatic Animals Use Their Environment

• Plenty of water around:
  Aquatic animals, especially freshwater ones, are surrounded by water all the time. They release ammonia directly through their gills or skin, and the surrounding water quickly dilutes it, making it harmless.
• Saving energy:
  Turning ammonia into safer compounds like urea or uric acid takes a lot of energy. By excreting ammonia directly, aquatic animals save energy for other vital functions like growth and movement.
• Ammonia dissolves easily:
  Ammonia mixes well with water. This means it quickly dissolves and gets carried away, preventing any toxic buildup inside the animal’s body.

Why This Doesn’t Work for Land Animals

Terrestrial (land) animals can’t use this method because they don’t have unlimited water. Excreting ammonia directly would cause too much water loss, so they’ve evolved different waste systems.

• Urea (used by mammals and adult amphibians): They convert ammonia into urea through the urea cycle. Urea is much less toxic and needs less water to be removed.
• Uric acid (used by birds and reptiles): These animals turn ammonia into uric acid, which is the least toxic form. It’s excreted as a thick paste, helping them conserve water—especially useful for desert animals and those that develop inside eggs.