Lineage distribution and barriers to gene flow among populations of the globally invasive marine mussel Musculista senhousia

Introduced species seem to rapidly adapt to non-native conditions although founder events may decrease genetic variance and impede adaptation. Cryptic introductions of an invader from different parts of its native range may allow colonization of habitat patches with compatible conditions, or admixture may restore genetic diversity lost during invasion bottlenecks. We present the first genetic study of global invasion pathways for the Asian date mussel, Musculista senhousia, comparing three putative source populations from Asia with six US estuaries and one invaded site in New Zealand and the Mediterranean Sea, totaling 117 individuals. Distribution of mitochondrial DNA and alleles of a nuclear intron revealed two genetically divergent populations in the native range. A warm-water lineage from Tokyo Bay and southwestern South Korea predominated in southern California and New Zealand, while a cold-water haplotype group sampled in northeastern Korea was more abundant at invaded sites in the northwestern US and Mediterranean Sea. The duration of high summer water temperatures accounted for substantial variance in the worldwide distribution of common haplotype groups, suggesting thermal pre-adaptation may influence invasion success for differentiated source populations. There was no gene flow across central California, but on either side of this phylogeographic barrier there was high exchange among estuaries within the northern and the southern US regional populations. Gradients in water temperature may therefore constrain post-introduction spread by limiting dispersion along a coast.