Global species delimitation of smooth-shelled blue mussels in the Mytilus edulis complex

The species is often described as the basic unit of biology, yet defining what constitutes a species has been a long-standing challenge. The advent of modern molecular techniques in conjunction with advanced analytical approaches now provide new opportunities for the robust and repeatable delineation of taxa from previously difficult to assess species complexes. Using marine mussels of the genus Mytilus (smooth-shelled blue mussels of the globally distributed Mytilus edulis species complex) we tested regionally differentiated putative taxa from the Northern and Southern hemispheres using a Bayesian species delimitation model that infers species trees. Using a multilocus panel of 54 single nucleotide polymorphic loci (SNPs) we tested four alternative hypotheses against the hypothesis of the currently recognised taxonomy to better understand the evolutionary history and the contemporary species of this complex. Only one model provided a better fit than the contemporary taxonomy model: this best fit model included the three reference Northern hemisphere taxa (M. edulis, M. galloprovincialis, M. trossulus) plus M. chilensis (Chile), M. platensis (Argentina), M. planulatus (Australia) and the newly recognised M. aoteanus (New Zealand). Phylogenetic reconstruction based on our nuclear DNA-based SNP data suggests that M. trossulus is the oldest of the modern smooth-shelled blue mussels, that a first migration event from the north to the south occurred that gave rise to M. platensis and M. chilensis in South America, subsequently that M. edulis and M. galloprovincialis diverged in the Northern hemisphere, and that subsequently again there was a second migration event from the north that gave rise to M. planulatus in Australia and M. aoteanus in New Zealand. Our findings provide very strong support for earlier mitochondrial DNA-based phylogenetic findings for globally distributed blue mussels and also help to clarify uncertainty about the number of north-to-south migration events that gave rise to important Mytilus speciation events in South America and in Australasia.