The two species of Cercopagidae found in the Great Lakes (both exotics) are easily distinguished. While roughly the same overall length (7-8mm) the body of Cercopagis pengoi is much smaller than the body of Bythotrephes longimanus, a difference visible even to the naked eye with some practice. The tail spine of Cercopagis pengoi is more slender and has a definitive hook at the end (hence its common name fishhook waterflea).