Impoverished mobile epifaunal assemblages associated with the invasive macroalga Asparagopsis taxiformis in the Mediterranean Sea

There is an increasing concern about the ecosystem consequences of altering macroalgal assemblages. Many macrophytes are foundation species in coastal habitats, supporting much of the biodiversity of these ecosystems by providing essential resources such as food and habitat. The addition of invasive species strongly contributes to habitat modification, but the bottom-up impacts of non-native macroalgae on higher trophic levels remains difficult to predict. The main aim of this study was to evaluate the effects of the invasive macroalga Asparagopsis taxiformis on biodiversity by comparing the mobile macrofauna inhabiting this species to the dominant native species Halopteris scoparia. This is the first comprehensive study of the possible effects of this widespread invasive species on higher trophic levels. A hierarchical sampling design with two different spatial scales was conducted to explore the consistency of the patterns observed. Fifty-nine species belonging to superorder Peracarida were found, accounting 90% of all organisms. A. taxiformis hosted an impoverished epifaunal assemblage in comparison to that associated with the native seaweed, showing significantly lower values of diversity, abundance and number of epifaunal species across study locations. The structure of the associated macrofauna (both in terms of species composition, variability among samples and relative abundance of the species) was also different. Our results highlighted the strong influence of A. taxiformis in the resident community, with differences among the two macroalgae in all the parameters considered. Finally, our results also reflect a biotic homogenization of the epifaunal assemblages associated to A. taxiformis, a scarcely explored consequence of invasive processes in marine environments. Future studies exploring the cascading effects of the observed changes in the epifaunal assemblages would be necessary in order to estimate system responses to macroalgal invasions.