Adults of this anadromous species migrate up rivers in March and April, but spawning actually takes place the following year between May and July (4). Mating occurs in pairs, unlike the other lampreys in which a female is mated by a succession of males (4). The female lays up to 300,000 eggs into a depression in the river bed created by the male. After hatching, the larvae, known as ammocoetes burrow into the sediment where they live for three to five years, feeding by filtering organic particles from the water (4). During metamorphosis, the eyes and the sucker-like mouth develop and the adults then migrate to the sea where they adopt a parasitic lifestyle, feeding by attaching to the bodies of large fish with the mouth and rasping away at the flesh. They remain in the sea for a few years and then return to freshwater in order to spawn. They do not feed during this return trip because the digestive organs degenerate, and shortly after spawning they die (4). Roman, Viking and Medieval Britons regarded river and sea lampreys as delicacies (2).
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