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.

Europe has a long history of human pressure on freshwater ecosystems. As pressure continues to grow and new threats emerge, there is an urgent need for conservation of freshwater biodiversity and its ecosystem services. However, whilst some taxonomic groups, mainly vertebrates, have received a disproportionate amount of attention and funds, other groups remain largely off the public and scientific radar. Freshwater mussels (Bivalvia, Unionida) are an alarming example of this conservation bias and here we point out six conceptual areas that need immediate and long-term attention: knowledge, threats, socioeconomics, conservation, governance and education. The proposed roadmap aims to advance research, policy and education by identifying the most pressing priorities for the short-and long-term conservation of freshwater mussels across Europe.

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.

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!

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