Origin of the invasive sea anemone Metridium senile in the southern cone of South America

The sea anemone Metridium senile was first reported from Central Patagonia in 2005, where it was already abundant, and has since expanded widely along the Chilean fjord region. Its proliferation has generated significant ecological impacts, particularly through the displacement of native benthic assemblages and the overgrowth of commercially important sea urchin beds. Despite its rapid spread, the introduction pathways and biogeographic origin of M. senile in the Southern Hemisphere remain uncertain.

Here, we examined both nuclear (ITS) and mitochondrial (COI, COIII) sequence data with genome-wide SNPs obtained using the hexacoral-v2-actiniaria baitset to investigate the origin and dispersal history of M. senile populations from Chilean Patagonia and Argentine coast. Phylogenetic and population structure analyses resolved three well-supported clades corresponding to major Pacific and Atlantic lineages and a significant split within one clade differentiated Northeastern Atlantic from Northwestern Atlantic populations. Chilean samples clustered predominantly within North Pacific groups while Argentinean samples were closely related to North Atlantic populations, indicating an independent introduction pathway.

Approximate Bayesian Computation (ABC) supports the Northeastern Atlantic source of the Argentinean population and one invasion of Chilean waters from the North Pacific, likely via ballast water transport, with stepping stone spread southward throughout the region. Our results provide evidence for multiple, regionally distinct introductions of M. senile into southern South America, most likely mediated by maritime traffic. These findings highlight the species' high invasive potential and emphasize the need for targeted monitoring to mitigate further spread across the Southern Hemisphere.