Daphnia pulex in the context of Model species

Daphnia is a genus of small planktonic crustaceans, 0.2–6.0 mm (0.01–0.24 in) in length. Daphnia are members of the order Anomopoda, and are one of the several small aquatic crustaceans commonly called water fleas because their saltatory swimming style resembles the movements of fleas. Daphnia spp. live in various aquatic environments ranging from acidic swamps to freshwater lakes and ponds.

The two most commonly found species of Daphnia are D. pulex (small and most common) and D. magna (large). They are often associated with a related genus in the order Cladocera: Moina, which is in the Moinidae group instead of the Daphniidae, and is much smaller than D. pulex (roughly half the maximum length).

Moina is a genus of crustacean within the family Moinidae, which are part of the group referred to as water fleas; they are related to the larger-bodied Daphnia species such as Daphnia magna and Daphnia pulex, though not closely. This genus was first described by W. Baird in 1850.

Moina demonstrates the ability to survive in habitats with adverse biological conditions, these being waters containing low oxygen levels and of a high salinity and other impurities; these habitats include salt pans but also eutrophic water bodies. An example of such an extreme habitat is the highly saline Makgadikgadi Pans of Botswana, which support prolific numbers of Moina belli. The genus is also known from bodies of water across Eurasia, where research indicated previously unknown species diversity in Northern Eurasia, including Japan and China; the newly described groups includes a number of phylogroups were discovered from Northern Eurasia, four new Moina species from Japan, and five new lineages in China. According to genetic data, the genus Moina is divided into two genetic lineages: the European-Western Siberian and Eastern Siberian-Far Eastern, with a transitional zone at the Yenisei River basin of Eastern Siberia.