The bizarre life of the pouched lamprey, a jawless parasite with a mysterious pouch

Nature · Published 3 October 2026 Swipe more like this →

In short: The pouched lamprey is a jawless, eel-like creature that spends its youth in freshwater before migrating to the sea to live as a parasite. Upon returning to spawn, males develop a strange, baggy pouch under their eyes, a feature that may assist in oxygenating their eggs or building nests.

The pouched lamprey (Geotria australis) is a unique species native to the Southern Hemisphere, found in regions including New Zealand, Chile, Argentina, and parts of Australia. As the sole member of the family Geotriidae, it exhibits a complex life cycle. Larvae, known as ammocoetes, spend about four years in freshwater before undergoing a six-month metamorphosis. During this transition, they develop blue-green stripes and significant neurological changes, specifically increasing the volume of the brain's visual areas, before migrating to the sea as large-eyed macropthalmia.

Adults live in the open ocean, where they survive as parasites by attaching to other fish and rasping at their tissues. After this marine phase, they return to freshwater to sexually mature, a process that can take up to eighteen months. Once they spawn, the adults do not immediately perish; they have been observed surviving for up to 105 days, during which they may wrap themselves around egg masses to provide parental care. This behavior is distinct from many other lamprey species.

The physical appearance of the lamprey is equally remarkable. While they are a striking silver color when caught fresh from the sea, their skin turns brown in freshwater due to the deposition of biliverdin. The male's signature pouch remains a subject of scientific interest. While its primary function is thought to be related to egg care, some researchers have hypothesized that similar structures in northern hemisphere species are used to gather stones for nest construction. Today, these creatures face potential threats from the degradation of water quality in lowland waterways, which may be contributing to an observed decline in their population numbers.

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