Then they return to rivers, reproduce and generally die. Many lampreys are parasites. They attach on other fishes by means of their sucker, scrape their skin with their rasping tongue, and suck their blood. All lampreys, however can also feed on small invertebrates. The sucker is also for them a means to travel upstreams in rivers.
Lampreys have an amphitropical distribution and are restricted to relatively cold waters.
Geotria and Mordacia are the only lampreys of the southern hemisphere, all other genera live in the northern hemisphere. A large sucker surrounding the mouth, strengthened by an annular cartilage. It is therefore not diagnostic of lampreys only. Lampreys are devoid of a mineralized skeleton, although traces of globular calcified cartilage may occur in the endoskeleton. The head of adult lampreys has relatively large eyes, followed posteriorly by a series of seven, rounded gill openings.