Pacific lamprey are dark blue or brown in color and grow to about 30 inches (76 cm) as adults. They are anadromous and semelparous. Although the adult and juvenile stages are more noticeable, lamprey spend the majority of their lives as larvae (ammocoetes). Ammocoetes live in fresh water for many years (usually 3-7 years, but at least one species has been recorded for +17 years). Ammocoetes are filter feeders who draw overlying water into burrows they dig into soft bottom substrates. After the larval period the ammocoetes undergo metamorphosis and take on the juvenile/adult body morphology. Juveniles/adults have a jawless sucker-like mouth that allows them to become parasitic on other fish. The adults live at least 1-2 years in the ocean and then return to fresh water to spawn. It is not well understood if Pacific lamprey return to their natal streams or seek spawning areas based on other cues. They typically spawn in similar habitat to Pacific salmon and trout. Lamprey construct a nest (redd) in small gravel and females can lay over 100,000 eggs, which are fertilized externally by the male. After spawning the adults usually die within 4 days. Also, like salmon, the Pacific lamprey does not feed while migrating to spawn.