Freshwater Molluscs of the Nile Basin, Past and Present

Abstract

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 — and ipso facto also fish — 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...

Modern and early Holocene mollusc fauna of the Ounianga lakes (northern Chad): implications for the palaeohydrology of the central Sahara

“…Light and oxygen availability are evidently less favourable beneath and between the floating Phragmites mats covering much of the current lakes at Ounianga Serir. Elsewhere in (sub)tropical Africa, standing waters covered for a large part by floating vegetation such as Phragmites or Papyrus are typically poor in molluscs (often only Pila and Biomphalaria ), owing to the unfavourable combination of flocculent mud/detritus bottoms, the lack of light and periodic water column anoxia due to the immobilisation of water among and below swampy vegetation (Beadle, 1981; Van Damme and Van Bocxlaer, 2009). However, also the Ounianga lakes which are only partly reed‐covered have open surfaces of clear and oxygenated water (Fig.…”

“…Trou au Natron), the Wadi Teshuinat in the Libyan desert and piedmont lakes in the Hoggar. Even during the early Holocene humid period, these waters lacked gastropods belonging to the Ampullariidae and Viviparidae families as well as bivalves of the Unionidae and Iridinidae, which all require good hydrological connectivity of surface waters for their dispersal (Gautier, 1980; Van Damme, 1984; Girod, 1998; Van Damme and Van Bocxlaer, 2009). Given that favourable mollusc habitat prevailed at Ounianga Serir in the early Holocene (see above), the absence of such molluscs at Ounianga Serir, while present in Lake Mega‐Chad (Table 7), likely indicates the lack of a surface water connection allowing upstream colonisation from Lake Mega‐Chad.…”

Disjunct distributions of freshwater snails testify to a central role of the Congo system in shaping biogeographical patterns in Africa

“…To implement this calibration point, we used a uniform prior over the root of the phylogeny spanning 23–13 My to account for dating uncertainty and the various published hypotheses as to when the Bellamyinae invaded Africa from Asia, e.g. [66-68]. The oldest Neothauma fossil ( Neothauma hattinghi Van Damme & Pickford, 1999) is reported from the Upper Miocene Kakara Formation from the western branch of the East African Rift System and is 10–11 My old [15,69].…”

Ichthyofauna of the Itimbiri, Aruwimi, and Lindi/Tshopo rivers (Congo basin): Diversity and distribution patterns

“…Before the tectonic uplifting in the area during the Late Miocene/Pliocene (ca. 7.5-2.5 MYA), a paleo-lake Obweruka existed in the Albertine rift, roughly where now lakes Albert and Edward (currently draining into the Nile system) are situated, and which was connected with the proto-Aruwimi of the Congo basin Pickford 1999, Van Damme andVan Bocxlaer 2009). This ancient connection between the Congo and the Northern Albertine Rift Lakes may still be reflected in the present ichthyofauna.…”