Modes of Nitrogenous Excretion

The chemical form in which an animal excretes its nitrogenous waste is not a matter of chance; it reflects a direct trade-off between the metabolic energy needed to convert highly toxic ammonia into a safer form and the volume of water needed to excrete whatever form is chosen without harm. Three broad patterns are recognised, named for the substance each involves.

Ammonotelism is the excretion of nitrogenous waste directly as ammonia, without any further chemical conversion. Ammonia is highly toxic even at low concentrations, but it is also extremely soluble in water, and this solubility is what makes ammonotelism workable: an ammonotelic animal simply allows ammonia to diffuse continuously out of its body, across the gills or general body surface, directly into the surrounding water, which dilutes it to harmless levels almost instantly and is available to the animal in effectively unlimited quantity. This is the excretory strategy of most bony (teleost) freshwater fish, of aquatic amphibians (both larval stages, such as tadpoles, and many adults that remain in water), of aquatic insects, and of many other aquatic invertebrates. Because no chemical conversion is needed, ammonotelism costs the animal essentially no metabolic energy — but it is only workable for an animal permanently surrounded by large volumes of water, since ammonia cannot be safely stored or concentrated within the body even briefly.

Ureotelism is the excretion of nitrogenous waste as urea, a compound formed from ammonia through a multi-step biochemical pathway called the ornithine cycle (urea cycle), which takes place chiefly in the liver. Urea is far less toxic than ammonia and only moderately soluble in water, which means it can be safely carried in the blood and even stored at a somewhat higher concentration for a time before being excreted, so that an animal need not be in continuous contact with water to void it safely — though a moderate supply of water is still needed to flush it out of the body as urine. This is the strategy used by mammals (including humans), by most adult amphibians once they take up a partly terrestrial existence, and, interestingly, by marine cartilaginous fish (elasmobranchs such as sharks and rays), which retain urea in their blood at a high concentration for the additional purpose of osmotic balance with the surrounding seawater. Converting ammonia into urea does cost the liver metabolic energy, a cost that ureotelic animals accept in exchange for greater independence from a constant water supply. …