Co‐extinctions are increasingly recognized as one of the major processes leading to the global biodiversity crisis, but there is still limited scientific evidence on the magnitude of potential impacts and causal mechanisms responsible for the decline of affiliate (dependent) species. Freshwater mussels (Bivalvia, Unionida), one of the most threatened faunal groups on Earth, need to pass through a parasitic larval (glochidia) phase using fishes as hosts to complete their life cycle. Here, we provide a synthesis of published evidence on the fish–mussel relationship to explore possible patterns in co‐extinction risk and discuss the main threats affecting this interaction. We retrieved 205 publications until December 2015, most of which were performed in North America, completed under laboratory conditions and were aimed at characterizing the life cycle and/or determining the suitable fish hosts for freshwater mussels. Mussel species were reported to infest between one and 53 fish species, with some fish families (e.g., Cyprinidae and Percidae) being used more often as hosts than others. No relationship was found between the breadth of host use and the extinction risk of freshwater mussels. Very few studies focused on threats affecting the fish–mussel relationship, a knowledge gap that may impair the application of future conservation measures. Here, we identify a variety of threats that may negatively affect fish species, document and discuss the concomitant impacts on freshwater mussels, and suggest directions for future studies.

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Bivalve shells can potentially alter the structure of aquatic benthic communities. However, little is known about the effect that different shell morphologies have on their associated fauna. This study aimed to understand how empty shells, from four different freshwater bivalve species, affect macrozoobenthic communities, using the River Minho (Iberian Peninsula) as a study area. Three native (Anodonta anatina, Potomida littoralis, Unio delphinus) and one non-indigenous (Corbicula fluminea) species were used for this research. Comparisons among species and between scenarios (i.e. before and after invasion by C. fluminea) were performed. Our results suggest that macrozoobenthic community structure did not vary among treatments, with the exception of species richness, which was higher on shells of native species. Furthermore, little difference was detected when comparing scenarios with and without C. fluminea shells, despite dissimilarities in size and morphology between species. The empty shells of C. fluminea partially (in terms of density and biomass, but not in species richness) replaced the role of empty shells of native species as a physical substratum for the associated macrozoobenthic community.

C. fluminea influence on the mortality of native mussels glochidia was evaluated.• Glochidia mortality was highly dependent on C. fluminea density and time. • Glochidia mortality was influenced by C. fluminea activities. • High C. fluminea densities may be detrimental for freshwater mussel conservation.

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1. Freshwater mussels (Order Unionida) are highly threatened. Interspecific competition for food sources with invasive alien species is considered to be one of the factors responsible for their decline because successful invaders are expected to have wider trophic niches and more flexible feeding strategies than their native counterparts.2. In this study, carbon (δ 13 C: 13 C/ 12 C) and nitrogen (δ 15 N: 15 N/ 14 N) stable isotopes were used to investigate the trophic niche overlap between the native freshwater mussel species, Anodonta anatina, Potomida littoralis, and Unio delphinus, and the invasive bivalve Corbicula fluminea living in sympatry in the Tua basin (south-west Europe).3. The species presenting the widest trophic niches were C. fluminea and A. anatina, which indicate that they have broader diets than U. delphinus and P. littoralis.Nonetheless, all the species assimilated microphytobenthos, sediment organic matter, and detritus derived from vascular plants, although with interspecific variability in the assimilated proportions of each source. The trophic niche of the invasive species overlapped with the trophic niche of all the native species, with the extent varying between sites and according to the species. 4. From the three native species analysed, Potomida littoralis may be at a higher risk for competition for food with C. fluminea in the Tua basin, if food sources become limited, because this native mussel presented the narrowest trophic niche across sites and the highest probability of overlapping with the trophic niche of C. fluminea.Ester Dias has equal contribution as first author.

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To both my supervisors Martina Ilarri and Ronaldo Sousa, for giving me the opportunity to work with them, for all the guidance and support throughout the year, for all the effort to teach me the best they could, since they are the people that I admire the most. To Vanessa for all the help and guidance throughout the project, for the great companionship and support, for the endless good times spent at work making it easy. To Allan for all the advices, for being always ready to help and for the availability. Indeed, many thanks for this team that made every work day a joy. To all my Shark friends for all the friendship, all the trust, all the spontaneous coffees, for being there every time I needed, and for the best moments. To Amorim for the help, the friendship, the trust and the laughs that only you can give. A special thanks to Abel Sousa, Julio André and Luis Campos, for always being present, even though in the last couple of years we could not spend too much time together, for their faithful friendship, I wish you guys the best! To my brother Pedro, that has proudly become one of the best scientist I will ever know, thank you for all the advices, all the guidance, for following my course and watching over me, thank you for being my brother. To my parents that have supporting me and are always present. At the end of the day, everything that I became and achieved, I owe it to them. I am lucky to have you as my parents. Finally, to Vera, that has been the major support for me, for making me focus all the time, helping me achieve my goals, for all the love and care, for all the patience, for the happiness that follows you, I really cannot say enough. Thank you!