Why can aquatic animals excrete ammonia rather than urea?
We will consider the elimination of nitrogen first, a process technically known as excretion. Excretion is a necessary consequence of protein breakdown; when proteins are converted to carbohydrates to provide energy, the amino group is removed and must be dealt with. In the body, the amino group is quickly oxidized to form ammonia (or, at high body pH the ammonium ion). Ammonia is highly toxic and highly soluble in water. If the organism has a sufficient source of water, ammonia can simply excreted in the water. This is the course taken by many (if not most) aquatic organisms, particularly those in freshwater. In any event, ammonia must be dealt with quickly because of its toxicity. Ammonia will diffuse passively out of respiratory structures such as gills. It takes a lot of water to dissolve and flush ammonia, however, and each ammonia molecule carries only one nitrogen. Organisms with less fresh water available, such as some marine organisms and all terrestrial organisms, are not as
