Genome sequence analysis provides evidence that a boreal crustacean colonised Svalbard well before the ongoing Atlantification of the Arctic.

The study of present-day species distributions often raises questions about historical demography. A particularly interesting phenomenon to put in historical context is contemporary human-induced atlantification and its role in reshaping Arctic ecosystems. Despite this, the colonisation history of the Arctic remains generally understudied. In this study, we investigated the demographic history of the northern acorn barnacle, Semibalanus balanoides, a typically boreal species on the Svalbard Archipelago. Our focus was to determine the source and timing of its colonisation of this Arctic archipelago. Using low-coverage whole-genome sequence data, we evaluated two competing hypotheses: whether S. balanoides populations colonised Svalbard through ancient natural processes before the Anthropocene, or if their appearance is more recent, either natural or a consequence of growing anthropogenic influences, such as increased connectivity and global warming. Our results suggest that this boreal species expanded into the Arctic during the later phase of the Holocene Thermal Optimum, well before human-induced climate change.