The arrival of a non-indigenous ecosystem engineer to a heavily invaded and flow-regulated estuary in Europe.

Ecosystem engineering bivalves can shape aquatic ecosystems because their high filtration capacity changes water quality and their shells increase the fractal dimension of benthic ecosystems with consequent abiotic and biotic effects. The Asian date mussel Arcuatula senhousia (W. H. Benson, 1842), native to East Asia between the South China Sea and Siberia, is one such bivalve that, despite its small size, can reshape a benthic ecosystem when forming dense, continuous mats. We describe here the first detected population of this non-indigenous species in southern Portugal. The Asian date mussel was found in the middle portion of the Guadiana estuary in 2022. There, river flow has been highly regulated since the construction of the biggest European reservoir in 2002, which may have been the precursor for the establishment of numerous non-indigenous species. We also discuss if this new non-indigenous species indicates an ongoing invasion meltdown process or if it can be framed under the empty niche or niche replacement hypothesis. So far, there is only circumstantial evidence supporting the niche theory hypotheses, but the interaction of several hypotheses promoting the spread and establishment of this species is also likely. Moving forward, better-informed management and conservation efforts should rely on new empirical and experimental evidence to understand the establishment mechanisms of nonindigenous species in the Guadiana estuary. [ABSTRACT FROM AUTHOR]