Sea lampreys (Petromyzon marinus) are eel-like fish that feed on larger fish. There are two types of sea lampreys. One is the anadromous and the other is those landlocked in the Great Lakes. Historically all sea lampreys were anadromous and would migrate from salt water to freshwater to spawn.
Landlocked lamprey inhabit the Great Lakes and are cutoff from the ocean, they are not native to the Great Lakes but were somehow introduced and are now a firmly entrenched inhabitant that has caused lots of damage to the native fisheries of the Great Lakes.
Sea Lamprey's attach to host fish by a sucking disk shaped mouth. They then suck the body fluids out of the host fihs that often leave hosts dead but not always.