Ostracod radiations

Ostracods presently occur in almost all aquatic habitats, be they freshwater, brackish or marine, surface or groundwater, stagnant or flowing, in large lakes and small phytothelmata, in permanent or temporary pools, as well as in (semi-) terrestrial habitats. Some of these habitats are species-poor, other habitats harbour large numbers of species. Some of these species can be wide-spread (cosmopolitan and even ubiquitous), but others are endemic to a single lake or watershed. Why did speciation in certain ostracod lineages lead to large numbers of species in certain habitats, while other “ostracod lineage vs habitat type” combinations did not lead to such, often spectacular, morphological speciation events? A speciose lineage of ostracods which derived from a single ancestor is called a species flock. In ancient lakes such as Lake Baikal (Siberia, Russia), Lake Tanganyika and Lake Malawi (East Africa), several such ostracod species flocks exist side by side. These ancient lakes are excellent examples where ostracod radiations, including several species flocks, can be studied in situ, in the cradle in which they originated, namely in ostracod lineages such as Cyprideis s.l.and Cytherissa (Cytherideinae) (in Tanganyika and Baikal, respectively) , Gomphocythere (Timiriaseviinae) and Cypridopsis s.l., (Cypridopsinae) (in both Tanganyika and Malawi) to name only a few. Other examples of ostracod radiations are in the subfamily Candonidae in groundwaters in Pilbara (Australia), Texas (USA) and in Benin (West Africa). Several lineages in the family Cyprididae radiated in temporary water bodies on various continents, for example the genus Bennelongia (Cyprididae) in Australia, the genus Sclerocypris in the Megalocypridjnae in Africa and Cypretta and Strandesia in the southern Hemisphere (excluding Antarctica) as a whole. A special case are the species of the genus Elpidium (Timiriaseviinae) occurring in bromeliad phytothelmata in South and Central America. A new field of diversity discovery is in the identification of genetic species, which might not be identifiable based on the phenotype, aka “cryptic species”. Various intrinsic (pertaining to the specific biology) or extrinsic (habitat-related) factors can contribute to large speciation events resulting in ostracod radiations and species flocks. Yet, in some habitats where at least some of the extrinsic factors are present, no vast ostracod species flocks can be found. This is mostly due to historic events, such as in the African lakes Kivu and Turkana.

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