Phylogenetic analysis of Prisodontopsis Tomlin, 1928 and Mweruella Haas, 1936 (Bivalvia: Unionidae) from Lake Mweru (Congo basin) supports a Quaternary radiation in the Zambian Congo

The freshwater mussels Prisodontopsis aviculaeformis (F.R. Woodward, 1991) and Mweruella mweruensis (E.A. Smith, 1908) are endemic to Lake Mweru and confluent rivers in Zambia and the Democratic Republic of Congo in southcentral Africa. Each species has traditionally been regarded as monotypic at the genus-level or above. We assessed the phylogenetic relationships of these two species together with species of Coelatura, Nitia and three outgroup species (35 terminals representing 11 species in total) using two gene fragments, 28S nuclear rDNA and cytochrome oxidase subunit I mtDNA. Maximum parsimony, maximum likelihood and Bayesian inference provided strong support for a sister relationship between P. aviculaeformis and M. mweruensis as well as evidence of incomplete lineage sorting or introgression between them. This Lake Mweru clade was resolved as sister to a clade composed of Coelatura choziensis (Preston, 1910) and C. Luapulaensis (Preston, 1913) from Lake Bangweulu and the adjacent Chambeshi River. These four species from the Zambian Congo were nested within a paraphyletic Coelatura Conrad, 1853. We advocate placing Mweruella Haas, 1936 as a subjective junior synonym of Prisodontopsis Tomlin, 1928, while maintaining Coelatura as paraphyletic until a more comprehensive phylogeny is available. We hypothesize that the endemic freshwater mussels of Lake Mweru in the Zambian Congo evolved within the lake during the Quaternary from an ancestor originating in the Chambeshi River following stream capture from the Zambezi Basin.