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.

在世界范围内，外来物种在海洋生态系统中的扩散有所增加，贻贝也经常参与其中。在南非，已知来自17个生物分类群的91种外来物种和39种隐性海洋和河口物种。其中三起与重大入侵有关，包括太平洋贻贝，Semimytilus algosus，后者已迅速扩散并定居在西海岸多岩石的海岸下部。Semimytilus algosus是智利的土生土长，已被引入纳米比亚。以确定源和引入的可能的模式S. algosus，我们分析了南非和纳米比亚西海岸的8个种群以及智利本土的3个种群的种群遗传结构。使用细胞色素C氧化酶亚基I，我们收集了来自11个地区的226名个体的序列数据。由于南非和纳米比亚的人口与智利的样本具有相似的遗传变异，因此我们没有发现任何影响创始人的证据，这表明该地区的有效人口规模很大，繁殖压力很高。纳米比亚人口很可能起源于智利，并通过单一的，大量的自然幼虫扩散从那里传入了南非，从而突破了吕德里茨上升流道形成的生物地理屏​​障。但我们不能忽视与牡蛎养殖有关的引进发挥作用的可能性。根据与紫贻贝，我们预测S. algosus将继续在南非蔓延，但将仅限于温带地区; 通过垄断初级基质并降低其厚度，因为它们太小而无法占据其壳；并补充捕食者（如牡蛎捕捞者和狗窝）的食物来源。