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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]

Several non-native invertebrate and vertebrate species have been detected in the Guadiana Estuary (SW-Iberian Peninsula, Europe) during the 21st century. In June 2015, the non-native hydroid Cordylophora caspia (Pallas, 1771) was detected for the first time in this estuary, which motivated an assessment of its distribution during late Spring and Summer 2016. The main goals of this paper were to: i) report the presence of Cordylophora caspia and its distribution in the Guadiana Estuary, ii) record the substrates colonized, salinity, and water temperatures at locations where the species was detected, iii) evaluate possible introduction vectors and invasion routes; and iv) discuss the potential impacts and management options. Cordylophora caspia occupied a 25-km stretch of the estuary with salinities between 0.2 and 13.8 and occupied a variety of human-made substrates. Shipping was the most likely introduction vector of C. caspia, which might have originated from populations in the Atlantic Ocean or the Mediterranean Sea. Currently, the potential ecological impacts are likely low since the population size is small due to an apparent shortage of suitable habitat. Economic effects are minimal at present because there are no major industries along the basin extracting water from the estuary. An integrated ecohydrological approach—i.e. freshets released from dams to control the populations of Cnidaria—was proposed to minimize or mitigate the potential negative effects of this species in the Guadiana Estuary. [ABSTRACT FROM AUTHOR]