The genus Daphnia shows a very high degree of intraspecific variation. Many studies have been carried out in an attempt to establish morphological characteristics that can be used to distinguish consistently between closely related species of this genus. Although many authors have suggested differences, few of these have withstood scrutiny when specimens are examined from a wide geographic area. Even genetic studies have failed to establish whether some species complexes contain many closely related species or a few extremely variable species. Breeding studies are difficult because parthenogenic reproduction means males are often unavailable, especially in larger lakes where seasonal variations in water conditions are not as radical as in smaller bodies of water.
Hybrids also present a problem.
Hybrids are usually not capable of breeding and producing viable offspring. Using asexual reproduction however, cladoceran hybrids can quickly produce a large population of animals with characteristics of both parent species, further muddying the already turbid waters of cladoceran taxonomy. Members of some families such as the Macrothricidae and Moinidae are rare, so collecting sufficient specimens from a wide geographic area, for taxonomic comparisons, can take years of intensive collecting. Museum collections are extremely valuable for the study of the taxonomy of rare or morphologically variable organisms. Using collections from the RBCM and museums throughout North America, a number of specimens can be assembled from a large geographic area in a relatively short period of time.