These tapeworms use only a few species of freshwater fish in northern waters as their definitive host. The adults have hooks in addition to bothria on their scoleces, and can cause considerable localized damage to the intestine of their definitive host. A wide variety of fish can be a second intermediate host. One commercially important species invades the musculature of fish such as whitefish. When the larvae are present in numbers as few as 1 cyst per fish, the fish are unmarketable, not because the parasite can infect humans, but because the cysts in the flesh are large and unappealing.