Cercopagis, known in North America as the “fishhook water flea,” is an aggressive, predatory zooplankton that preys on smaller zooplankton. It belongs to the same family as Bythotrephes (Cercopagididae), and, like Bythotrephes, has a long caudal process (“tail”) with up to three pairs of barbs near its end. Also like Bythotrephes, Cercopagis is a native of the Ponto-Caspian region of eastern Europe/western Asia (the area of the Caspian, Azov, and Aral seas). As with the spiny water flea, the fishhook water flea is believed to be an international shipping ballast water introduction.