Polyphemus is a genus of water fleas, and the only genus in the family Polyphemidae. There are two species, P. exiguus and P. pediculus, although allopatric speciation has resulted in a number of cryptic species of P. pediculus. Polyphemus exiguus inhabits open zones in the Caspian Sea, while Polyphemus pediculus exists throughout the Holarctic. It lives in diverse conditions, from small ponds to lakes and estuaries such as the Gulf of Saint Lawrence and the Gulf of Finland. It can be found quite far offshore.