Population genetics of the brown mussel Perna perna in southern Africa

We studied geographic variation in the allozyme frequencies of Gpi, Mpi, Pep-1 and Pgd in 14 samples of Perna perna collected over 3700 km from Swakopmund, Namibia to Cape Vidal, South Africa. Unlike other species of mussels, Perna shows a general lack of heterozygote deficits. If heterozygote deficits in other mussels are due to Wahlund's effect on a microgeographic scale, then the lack of deficits in Perna suggests that there is a lack of microgeographic heterogeneity in this mussel. Since this mussel is short-lived and since recruitment is sporadic, settling larvae probably originate from only a few randomly mating subpopulations. There was no significant allele-frequency heterogeneity over several hundreds of km on the south and east coasts of South Africa. There were, however, strong allele-frequency differences for Gpi and Mpi at the western edge of the geographic distribution of P. perna. This clinal variation may be due to founder effects among distantly spawned recruits or to temperature selection in the colder upwelled waters of the west coast. The small amount of genetic divergence between Namibian populations of P. perna and more southern populations suggests that there has been gene flow along the west coast during recent periods of oceanic warming.