Population expansion of the invasive sea anemone Metridium senile in the spatial mesoscale of a sea urchin bed in north-western Patagonia

This article analyses the detection and expansion of the invasive sea anemone Metridium senile at spatial meso- and microscale in an exploited bed of the edible sea urchin Loxechinus albus, which is part of the population monitoring of its fishery in southern Chile. By analysing images recorded from video transects between 2014 and 2021, and applying geostatistical and multidimensional methods, changes in the presence and expansion of M. senile, the biodiversity of the associated community, and changes in the sea urchin bed were evaluated. Metridium senile was first recorded at this site in 2016 and since then it increased in coverage (up to 39%), presence by area (75%) and patch diameter (up to 343 m). The Shannon diversity index of the benthic community decreased from 1.43 to 0.016, while non-metric multidimensional scaling showed a gradient in species composition and relative year-to-year importance, suggesting a transition towards M. senile dominance in the community studied. Both the density and patches size of the studied sea urchin bed decreased heavily down to 1 sea urchin/10 m² and 20 m diameter, respectively, between 2019 and 2021. This suggests a shrinking subpopulation that may have been displaced due to the expansion of M. senile. Based on information available on M. senile in the northern hemisphere, it is suspected that its expansion in north-western Patagonia is still ongoing. Our results suggest a serious threat to the biodiversity of local benthic communities, including species that are commercially important to small-scale fisheries. Thus, further research on the invasive M. senile expansion, its effects and its potential management is a priority.