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The pouched lamprey, Geotria australis, also known as wide-mouthed lamprey, is the only species in genus Geotria, which is in turn the only genus in the Geotriinae subfamily of family Petromyzontidae. It is a species of lamprey that occurs throughout much of the southern hemisphere. It has a thin eel-like body up to 60 centimetres long, with two low dorsal fins on the back half. The skin is a striking silver in adult lampreys caught fresh from the sea but soon changes to brown after they have been in freshwater for some time, due to deposition of biliverdin. The freshwater ammocoetes or larvae stage of the life cycle are a dull brown in colour for most of their lives, changing to silver with blue-green stripes during their six-month metamorphosis into the adult stage, after which they migrate downstream to the sea. Adults' eyes are relatively small, and located on the side of the head. When fully mature, males develop a baggy pouch under their eyes, the function of which is unknown. However there have been suggestions that the pouch has been used by males dureing breeding times for gathering stones to make a nest.
The pouched lamprey is widespread in the Southern Hemisphere, occurring in the southwest and southeast corners of Australia, and New Zealand, Chile and Argentina. It spends some of its adult life in the open sea, where it lives as a parasite on other fish. It returns to fresh water to breed, spending up to 18 months sexually maturing before spawning and dying shortly after. The ammocoetes remain in fresh water until reaching metamorphosis, a period of around four years.
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