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Distribution

Ecologically, marine ostracods can be part of the zooplankton or (most commonly) are part of the benthos, living on or inside the upper layer of the sea floor. Ostracods has been found as deep as 9,307 m (genus Krithe in family Krithidae). Subclass Myodocopa and the two podocop orders Palaeocopida and Platycopida are restricted to marine environments (except for Platycopida which have a few brackish species), but we find non-marine forms in the four superfamilies Terrestricytheroidea, Cypridoidea, Darwinuloidea, and Cytheroidea in the order Podocopida. Terrestricytheroidea is semi-terrestrial and usually found in brackish and marine-influenced environments such as salt marshes, but not in freshwater. The other three superfamilies also live in freshwater (Darwinuloidea is exclusively non-marine). Of these three, only Cypridoidea have freshwater species able to swim. Representatives living in terrestrial habitats are also found in all three freshwater groups, such as genus Mesocypris which is known from humid forest soils of South Africa, Australia and New Zealand.

As of 2008, around 2000 species and 200 genera of non-marine ostracods are found. However, a large portion of diversity is still undescribed, indicated by undocumented diversity hotspots of temporary habitats in Africa and Australia. Non-marine species have been found to live in sulfidic cave ecosystems such as the Movile Cave, deep groundwaters, hypersaline waters, acidic waters with pH as low as 3.4, phytotelmata in plants like bromeliads, and in temperatures varying from almost freezing to more than 50 °C in hot springs. Of the known specific and generic diversity of non-marine ostracods, half (1000 species, 100 genera) belongs to one family (of 13 families), Cyprididae. Many Cyprididae occur in temporary water bodies and have drought-resistant eggs, mixed/ parthenogenetic reproduction, and the ability to swim. These biological attributes preadapt them to form successful radiations in these habitats.