Daphnia pulicaria is closely related to D. pulex, distinguishable primarily by its elongate polygonal structures on its rostrum. Although it is considered to be the “pulex” Daphnia in large lakes, and, presence in a lake habitat is a key character used by Hebert (1999), in the Northeast USA it is often difficult to differentiate D. pulicaria from D. pulex. Forms with intermediate characters are frequently encountered, suggesting hybridization of these two species.