There are four main types of lamprey:
1) Arctic Lamprey (_Petromyzon marinus_): Arctic lampreys are anadromous, meaning they migrate from the ocean to freshwater to spawn. They are found in the North Atlantic Ocean and freshwater rivers and lakes of the northern United States, Canada, and Europe. Arctic lampreys are the largest lamprey species, reaching lengths of up to 100 cm (39 inches).
2) American Brook Lamprey (_Lampetra appendix_): American brook lampreys are non-parasitic lampreys found in freshwater streams and rivers of eastern North America. They are small, reaching lengths of up to 15 cm (6 inches).
3) Pacific Lamprey (_Lampetra tridentata_): Pacific lampreys are anadromous, like Arctic lampreys, and are found in the North Pacific Ocean and rivers and lakes of western North America. They are also relatively large, reaching lengths of up to 90 cm (35 inches).
4) Western Brook Lamprey (_Lampetra richardsoni_): Western brook lampreys are non-parasitic lampreys found in freshwater streams and rivers of western North America. They are small, reaching lengths of up to 12 cm (5 inches).
In addition to these four main types of lamprey, there are also several other species of lamprey found around the world. Lampreys are an ancient group of vertebrates, and they have been around for over 500 million years. They are a fascinating and unique group of animals that play an important role in the aquatic ecosystem.