Biogeographic Origins and Diversification Dynamics of Freshwater Molluscs in West African River Systems With Focus on the Niger Basin

The biogeographic origins and diversification history of freshwater molluscs in West African river systems remain poorly understood despite their ecological importance and their role as intermediate hosts of human parasitic diseases. This study reconstructs the biogeographic history and diversification dynamics of freshwater gastropod and bivalve assemblages in the Niger Basin and connected West African river systems using an integrative approach combining molecular phylogenetics, divergence time estimation, ancestral area reconstruction, and diversification rate analysis. Specimens of 68 freshwater mollusc species were collected across 34 sites in the Niger, Benue, Volta, and Senegal river systems. Multilocus phylogenies were inferred from mitochondrial cytochrome oxidase-1 and nuclear ribosomal markers using Bayesian and maximum likelihood methods. Divergence times were estimated using a relaxed molecular clock calibrated with fossil and biogeographic constraints. Ancestral area reconstructions using the BioGeoBEARS framework tested competing hypotheses about the role of Pleistocene river captures, lake systems, and coastal connections in structuring current distributions. Results reveal multiple independent colonisation events of the Niger Basin from both East African and coastal West African source areas, with major diversification pulses corresponding to Pleistocene climatic oscillations and associated river connectivity changes. Several species complexes showed cryptic diversity undetected by morphology alone, with up to four genetically distinct taxa masked under single nominal species. Disease-relevant Bulinus and Biomphalaria species showed signatures of recent range expansions consistent with irrigation-facilitated dispersal. The study substantially advances understanding of West African freshwater mollusc biogeography and provides a systematic framework for schistosomiasis risk assessment informed by mollusc evolutionary history. Keywords: freshwater molluscs, biogeography, Niger Basin, phylogenetics, diversification.