The native smooth-shelled blue mussel Mytilus trossulus in the North Pacific: geographic patterns of hybridisation and the likelihood of multi-source invasions by Mytilus galloprovincialis lineages

To better understand the distribution of native smooth-shelled blue mussels (genus Mytilus ) in the North Pacific region (USA, Canada, Russia, China, Korea) and to quantify the geographic extent of invasion and the degree of hybridisation between native and non-native mussels we analysed mussels from 15 North Pacific sites and 16 global reference sites for variation at 59 single nucleotide polymorphism (SNP) loci. Samples were collected between 2006 and 2014, and some sample sizes were small (< 10 mussels) with the result that some site-specific interpretations are limited. Overall, our SNP-based results confirmed earlier findings based on other marker types and revealed (1) the likelihood of multiple independent invasion events of M. galloprovincialis , (2) the spatially patchy nature of bioinvasion in the USA, Canada, and Russia, (3) that sites in China and Korea exhibited only invasive M. galloprovincialis , and (4) the limited extent of hybridisation between M. trossulus × M. galloprovincialis beyond the F 1 generation. The regions of origin of invasive Mediterranean M. galloprovincialis are primarily the central Mediterranean Sea and secondarily the Black Sea and Sea of Azov. The source sites of origin of invasive Atlantic M. galloprovincialis cannot be determined. Whilst we cannot categorically rule out alternative hypotheses, these results support the interpretation of multiple independent invasion events. Our results provide the first large geographic scale SNP-based assessment of smooth-shelled blue mussel genetic variation in the North Pacific region and highlight the positive (development of aquaculture) and negative (hybridisation with the endemic species) impacts of invasive M. galloprovincialis in the North Pacific region.