The invasive aquatic macrophyte Hydrilla verticillata facilitates the establishment of the invasive mussel Limnoperna fortunei in Neotropical reservoirs

The effects of introduced species on native species have been widely studied, however, invader-invader interactions are still poorly explored. Two non-native invasive species, the bivalve Limnoperna fortunei and the submersed macrophyte Hydrilla verticillata, have infested many aquatic ecosystems worldwide, and they are rapidly becoming nuisances in several water bodies in Brazil. We tested the hypothesis that H. verticillata facilitates the establishment of L. fortunei more than do native macrophytes. We surveyed mussels attached to two native macrophytes and to this invasive macrophyte in three Brazilian reservoirs and compared the length, density and biomass of the attached mussels relative to the host macrophyte species. All of the values of these attributes in L. fortunei were higher in the mussels attached to H. verticillata than in those attached to the native macrophytes. These results supported our initial hypothesis because the settlement of L. fortunei appeared to be facilitated by the invasive macrophyte, allowing higher abundances of the mussels as well as the more effective establishment of the mussel population. Various mechanisms could interact to facilitate the superior performance of L. fortunei in the invasive plant. H. verticillata is more flexible and withstands more mussels without breaking. Furthermore, this plant has different attached algae, which may be more beneficial for mussels. Thus, as a response to the facilitation suggested by our work, the mussel has greater opportunities to succeed in habitats colonised by the non-native H. verticillata. Although we have not evaluated the effects of this facilitation on aquatic communities or the ecosystem, our results could represent the first step of an invasional meltdown.