A long single or double dorsal fin extends from the mid-body to the tail. About half the 19 species in North America are nonparasitic. According to Dr. Carl L. Hubbs, a former curator of fishes at the University of Michigan who founded the Institute for Fisheries Research in Ann Arbor in 1930, the two nonparasitic species of brook lampreys likely evolved from parasitic forms. Modern brook lampreys have fewer and smaller teeth than the other species, which Hubbs believed was the result of biological degeneration, and which suggests that brook lamprey once were probably parasitic. Lampreys also are unique among fishes because they have a complex life cycle that includes a larval stage followed by a metamorphosis into the adult form.
All lampreys spawn in the spring on shallow gravel beds in freshwater streams.
A male builds the nest by using its suction mouth to move the stones to form a horseshoe-shaped pit. The male discharges sperm as the female deposits eggs into the nest. Both adults soon die after spawning. The eggs hatch in about two weeks, and the larvae will remain in the nest until they are about a half-inch long. The blind larvae then leave the nest and burrow into sandy or silty areas of the stream bottom where they feed on bacteria and organic filtered from the stream water.