Sea lamprey (Petromyzon marinus) are parasitic fish not-native to the Great Lakes. Sea lamprey parasitize other fish by sucking their blood and other body fluids. Sea lampreys are unique from many other fishes in that they do not have jaws or other bony structures, and instead possess a skeleton made of cartilage. While sea lamprey resemble eels, they are not related and are set apart by their unique mouth: a large oral sucking disk filled with sharp, horn-shaped teeth surrounding a sharp rasping tongue.

There are four native lamprey species in the Great Lakes: the American brook, northern brook, silver and chestnut lamprey. All of the native lamprey species are much smaller than sea lamprey and are part of a balanced fish community. The largest of the native lamprey is the silver lamprey, which only reaches about half the size of an adult sea lamprey.

Of the native lamprey species, only two are parasitic: the chestnut and silver lamprey. The silver and chestnut lamprey have a similar life cycle as sea lamprey – the big difference is that, as parasites, silver and chestnut lamprey typically do not kill their fish hosts. Thus, unlike sea lampreys, which are considered both parasites and predators, the native lamprey species are only considered parasites.

The other two native lamprey species, the American brook and northern brook (the “brook” species), are non-parasitic. The brook species experience a metamorphosis similar to the parasitic species (developing eyes, fins, and a toothed mouth); however, the non-parasitic species skip the parasitic phase and instead immediately spawn after metamorphosis. The non-parasitic “brook” lamprey spend their entire life cycle in streams.