Invader in Disguise for Decades: the Plumose Sea Anemone Metridium Senile in the Southwestern Atlantic Ocean

The attention towards non-native sea anemone introductions has been steadily increasing as multiple species are reported from new locations each year. However, a lack of monitoring efforts and difficulties associated with the detection and identification of these species may result in overlooking introductions in certain areas. In the southern hemisphere, one of these non-native species is Metridium senile, whose current taxonomic and invasion status in Argentina is unclear. Here, we pooled scientific and community records to shed light on the past, current and future invasion scenario of this species. First, we clarified the taxonomic and invasion status of suspected M. senile populations from Argentina by revising available morphological descriptions and attributes associated with non-native species. Then, we inferred and described the potential dispersal pattern in Argentina from the late 1890s to present day. Finally, we provided a forecast of the species potential distribution range in the Southwestern Atlantic Ocean, along the coast and in off-shore areas. Enough evidence suggests that M. senile is a non-native species that has successfully established in Argentina. This species has been mistakenly considered as native for decades due to taxonomic errors and historical and geographical gaps (i.e., pseudoindigenous species). Its current distribution range in Argentina covers over 2000 km of coastline with a southward expansion in recent years. Moreover, according to our forecasted potential distribution, this species can spread further and reach novel areas in coastal and off-shore locations. As M. senile combines multiple traits of a successful invader, monitoring efforts should be implemented to early detect or prevent its establishment in areas recently colonized or at risk.