Human Introductions and Migration Shape the Genetic Population Structure of Invasive Pacific Oysters in Northern Europe

Invasive species threaten biodiversity and ecosystems and require global monitoring and management efforts. One such species is the Pacific oyster (Magallana gigas), originally introduced to Europe for aquaculture purposes, and after several decades of establishment, it has become a widespread invasive species in marine ecosystems. In this study, we used genetic tools to investigate the current genetic landscape of M. gigas in Northern Europe. Using a newly developed SNP panel based on a previous study on the species, we analyzed gene flow, genetic diversity and potential local adaptation at both micro- and macro-ecological scales at different sites within Limfjorden and single locations in the Netherlands, Wadden Sea, United Kingdom, Sweden, Norway and Isefjord. Our findings reveal a generally well-connected population across northern European waters, yet with detectable genetic signatures of early establishment events, i.e., Isefjord, and directional gene flow patterns, notably from Denmark to Sweden and Norway. While effective population sizes were large at broader geographic scales, localized cohorts exhibited smaller Ne values, suggesting genetic founder effects at the microgeographical level. No strong evidence of local adaptation was found, though some outlier loci in Limfjorden hint at possible early-stage adaptation to local environments. Our study provides a current genetic snapshot of the invasive Pacific oyster in Northern Europe, highlighting its ongoing expansion and emphasizing the importance of up-to-date genetic monitoring to guide effective population management and mitigate ecological impacts.