Nitrogen excretion

Ammonia is a highly toxic waste product derived mainly from amino acid and nucleic acid catabolism that is usually not detoxified into less toxic compounds but excreted as-is in all aquatic invertebrates. Due to the structure and size of its phyllobranchiate gills that allow easy perfusion of this excretory organ, the European green crab has served for many decades as a model to investigate ammonia excretion mechanisms. Accordingly, numerous important discoveries have been made. Amongst them are (a) the first direct evidence of an active branchial ammonia excretion process in an aquatic animal, (b) the first evidence of a branchial-expressed Rhesus-like protein, now known as one of the key ammonia transporters in the animal kingdom, (c) the first evidence that an intact microtubule network is often required in transepithelial ammonia transports, and (d) the discovery in the animal kingdom of a novel and highly conserved ammonia transporter, HIAT1. This chapter will describe the pathways of ammonia synthesis in crustaceans, the toxicity of ammonia, the mechanism of branchial ammonia excretion, and our most updated knowledge of the participating transporters and enzymes. Branchial excretion processes will be summarized in a current comprehensive working model that incorporates all findings that have been made on this most extensively investigated organ. The information on ammonia excretion/transport mechanisms, as well as the described investigative techniques that have been used to explore these mechanisms, will be an excellent source for animal and plant physiologists alike.