Abstract
The life cycle of lampreys consists of two distinct and divergently specialized phases. During the larval or ammocoete stage, the lamprey is relatively sedentary, remaining burrowed in the silt deposits of streams and rivers from which it derives its microphagous diet. After metamorphosis, several species enter a parasitic phase in either a marine, lacustrine, or large fluviatile environment where they feed voraciously, mainly on the blood and tissue of host fishes. However, the majority of lampreys do not feed at all after they complete the larval period and reach sexual maturity, 6 to 10 months after the onset of metamorphosis. These so-called nonparasitic, or brook, lampreys are believed to have been derived from parasitic lampreys, and in many cases there is convincing morphological, cytogenetic, and biochemical evidence that pairs of ancestral and derivative species can be recognized in the contemporary fauna.