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Freshwater Molluscs of the Nile Basin, Past and Present

Book Contribution - Chapter

The malacofauna of the Nile is poor compared to that of the Congo and its degree of endemicity is lower. While the highest species richness of the Congo Basin is in stenotopic taxa that live in the rivers and lakes, the highest diversity in the Nile Basin occurs in eurytopic taxa living in fringe habitats such as temporary pools. The paucity of endemics that need perennial waters as well in the Lower Nile as in the White Nile confirms the geological evidence indicating instability and discontinuity in water supply during Plio-Pleistocene times. The fauna of the Nile is predominantly Afrotropical in the Lower Nile and exclusively Afrotropical south of the junction of the White Nile and Blue Nile. Of all sub-basins, the degree of endemicity (either zero or two species) is lowest in the Equatorial Nile, indicating that the perennial aquatic environment in this sub-basin is young (probably Holocene) and lending support to the idea that the Bahr el Arab and White Nile Transcontinental Rift System were hydrologically unstable, with endorheic, alkaline lakes during most of the Plio-Pleistocene (Salama, 1997). In the Lower (Egyptian) Nile and in the Ethiopian Highlands palaearctic faunal components occur, consisting of widespread species and of a limited number of endemics of palaearctic origin, related to Levantine species. Most of these taxa first appear in the fossil record around 2.5 Ma. There is no evidence that the Nile functioned as an invasion route for Eurasiatic species prior to that time. Only Theodoxus niloticus is possibly of Paratethyan origin and may have invaded in the Late Miocene. The main invasion of freshwater molluscs into Africa appears to have occurred via the Horn of Africa in Middle and Late Miocene times. In the East African Rift, a diversified fauna occurs that only shares a fraction of species with the Nile Basin north of the Albert Nile (> ~5° N). In the equatorial headwaters, some species from southern Africa reach their northern limit, but the main community consists of species that are either endemic to one of the rift lakes or endemic to the region stretching from Turkana to the Kivu Basins. There hence exists a distinct East African bio-province, that was already recognisable in Pliocene assemblages and that had developed before the present lakes formed in the Late Pleistocene. Concerning the young age of Lake Victoria and the occurrence of an endemic cichlid superflock in the lake, the fossil molluscan record points to a persistence in the East African Rift of freshwater bodies sufficiently extensive to harbour a molluscan community U+2014 and ipso facto also fish U+2014 during periods of aridity/hyperaridity. The hypothesis of Verheyen et al. (2003, 2004), that the stem lineages of the East African haplochromines, presently inhabiting Lakes Victoria, Kyoga, Rukwa, Albert, George, Edward and surrounding waters, already existed during the Late Pliocene is consistent with the fossil evidence. The location of the area(s) of refuge (around Mount Elgon?) remains hypothetic, however. The fossil records of the Turkana Basin, Tugen Hills and Albertine Basin show that around the Miocene/Pliocene boundary, two distinctive tropical molluscan palaeo-bioprovinces existed in equatorial Africa: an East African and a Congolian province. Regular incursions in both provinces during the Pliocene by elements of a third group, the so-called U+2018NiloticU+2019 community, indicate the presence of a third distinctive Afrotropical palaeo-bioprovince situated more to the north. The molluscan community of the Chad Basin contains the same U+2018NiloticU+2019 species (or closely related forms) as found in the Egyptian and Sudanese Nile but is more diverse, particularly in unionoids. Considering the climatic instability in the region of the different Nile Basins north of ~5° NL, the Neogene Chad-Eosahabi/Palaeosahabi system sensu Griffin (2006) is considered the most likely region of origin of the U+2018NiloticU+2019 malacofauna, while the southern part of the Chad Basin (Chari-Logone system) may have acted as a refuge of this U+2018NiloticU+2019 fauna during arid phases. During Late Pliocene and Pleistocene wet phases the White Nile may have contained a perennial hydrological system, creating opportunities of exchange of aquatic faunas with Chad and the East African Rift System.
Book: The Nile: Origin, environments, limnology and human use
Series: Monographiae Biologicae
Pages: 585 - 629
ISBN:978-1-4020-9725-6