Although some populations exist where the entire life cycle takes place in freshwater, generally the river lamprey is anadromous; the adults migrate to the sea and return to freshwater to spawn. Upon reaching a suitable breeding area, a male wriggles around and creates a depression on the riverbed. He then attaches to the back of a female's head with his mouth and wraps around her body. The eggs are deposited, fertilised externally and covered with sand, a succession of males are involved and one female can lay up to 25,000 eggs in this way (6). After spawning the adults die. The blind juveniles hatch and then spend four to six years buried in the substrate, filtering organic particles from the water (6). After they mature the adults travel to the sea where they live for about 2 summers before returning to spawn. Whilst at sea, they feed on fish flesh and bodily fluids by attaching to small fishes with the sucking mouth and rasping away at the flesh (4). Roman, Viking and Medieval Britons regarded river and sea lampreys as delicacies (7).