Origin and genetic diversity of the invasive mussel Semimytilus algosus in South Africa, relative to source populations in Chile and Namibia

Worldwide, the spread of alien species in marine ecosystems has increased, with mussels frequently being involved. In South Africa, 91 alien and 39 cryptic marine and estuarine species, from 17 taxonomic groups, are known. Of these, three are associated with major invasions, including the Pacific mussel, Semimytilus algosus, which has rapidly spread and colonised the lower portions of rocky shores on the west coast. Semimytilus algosus is indigenous to Chile and has been introduced to Namibia. To determine the source and likely mode of introduction of S. algosus, we analysed the population genetic structure of eight populations on the west coast of South Africa and Namibia, as well as three populations in the native Chilean range. Using cytochrome c oxidase subunit I, we collected sequence data from 226 individuals sampled across the 11 localities. We detected no evidence for founder effects, as the South African and Namibian populations had comparable genetic variation to those sampled in Chile, pointing to large effective population sizes and high propagule pressure in the region. It is probable that the Namibian population originated from Chile, and that introduction took place from there to South Africa via a single and substantial natural larval dispersal that breached the biogeographic barrier created by the Lüderitz upwelling cell; but we cannot discount the possibility that introduction associated with oyster aquaculture played a role. Based on comparisons with Mytilus galloprovincialis, we predict S. algosus will continue to spread in South Africa but will be confined to temperate regions; reduce limpet abundance by monopolising primary substratum and because it is too small for them to occupy its shells; and supplement food sources for predators such as oystercatchers and dogwhelks.