The lampreys comprise 6 genera and 41 extant species (Nelson 1994). These eel-like, jawless fishes occur in cool zones of the world including temperate marine and freshwaters. Most species are found in the Northern Hemisphere; of these, 19 are native to North America north of Mexico (Page and Burr 1991). Four species are known from southern South America and southeastern Australia, although these are usually considered to represent two additional families, the Geotriidae and the Mordaciidae (Berra 1981). Some species are parasitic as adults, and they use their suction disk mouths and rasping teeth to attach to other fish, thereby feeding on the body fluids. Free-living species use their suction disk mouths to hold onto rocks and move stones during spawning. All species require freshwater to breed; the resulting young are called ammocoetes. Ammocoetes filter feed and take three to eight years to change into adults.