Magellan mussels Aulacomya atra from the South African coast show high diversity within populations but a lack of geographic differentiation

The Magellan mussel Aulacomya atra is a bivalve mollusc found along parts of the South African and Namibian coastline. Its numbers were low historically compared with other indigenous species but have decreased further since the 1970s owing to habitat invasion by Mediterranean mussels Mytilus galloprovincialis. We studied sequences of the mitochondrial cytochrome oxidase subunit 1 (CO1) and the nuclear internal transcribed spacer (ITS1) of A. atra to determine patterns of differentiation among three localities on the South African coastline and the phylogenetic position of these populations relative to other populations of Aulacomya species in the Southern Hemisphere. Results from both mitochondrial and nuclear genes revealed a high level of diversity within South African populations from the west and south coast, with little to no geographic differentiation among these populations. Phylogenetic trees constructed using maximum likelihood and haplotype network analysis show that individuals from all three regions sampled are intermingled in groups with low bootstrap support. Our CO1 sequences exhibited a phylogeographic structure concordant with the spatial distribution in South Africa, Argentina, Chile and New Zealand. However, results for ITS1 showed a lack of differentiation over a large spatial scale stretching from South Africa to New Zealand. Future studies should include additional samples from across the species’ distributional range and the examination expanded to include genetic markers with adaptive significance.