Daphnia spp are representatives of the Cladocera, and are common in the plankton of shallow and deep lakes. They may be up to 4mm long and are strong swimmers. Daphnia are ecologically important as grazers of the smaller phytoplankton species, and feed very efficiently on small flagellates. These organisms are themselves an important food source for both adult planktivorous fish, and juvenile fish of a number of species. Though a large number of different species are recognised, morphological variation and hybridisation may make it difficult to identify particular species.