Freshwater mussels are declining globally, and effective conservation requires prioritizing research and actions to identify and mitigate threats impacting mussel species. Conservation priorities vary widely, ranging from preventing imminent extinction to maintaining abundant populations. Here, we develop a portfolio of priority research topics for freshwater mussel conservation assessment. To address these topics, we group research priorities into two categories: intrinsic or extrinsic factors. Intrinsic factors are indicators of organismal or population status, while extrinsic factors encompass environmental variables and threats. An understanding of intrinsic factors is useful in monitoring, and of extrinsic factors are important to understand ongoing and potential impacts on conservation status. This dual approach can guide conservation status assessments prior to the establishment of priority species and implementation of conservation management actions.

The global decline of freshwater mussels is related with a great variety of factors, including the introduction of invasive species. However, the possible effects of other invasive bivalves, such as the Asian clam Corbicula fluminea (Müller, 1774), remain mainly unknown and highly speculative with very few manipulative experiments addressing this issue. In this study, field and laboratory experiments were conducted to assess the possible negative effects of C. fluminea on the native freshwater mussel Unio delphinus Spengler, 1783. Growth, physiological condition, and the locomotor activity were assessed in U. delphinus at increasing C. fluminea density. U. delphinus exhibited lower growth, lower physiological condition, and higher locomotor activity at higher C. fluminea density, which may suggest that this unionid is negatively affected by C. fluminea and may be displaced to less favorable habitats. Although we were not able to establish the main mechanism(s) responsible for these results, possibilities include competition for food resources, competition for space which may result in mussel displacements and/or changes in microhabitat features as a result of bioturbation activities, and production of feces and pseudofeces by C. fluminea.

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Temperature has a key impact on the physiology, food acquisition, metabolic rate, growth, and reproduction of organisms. Thus, exploring the effects of climate change on species physiology is important for the conservation and management of native and non‐native bivalve species. In this work, an integrative laboratory approach was used to compare the physiological, cellular, and molecular responses of the native Unio delphinus and the invasive non‐native Corbicula fluminea under a natural heat‐wave event. Whereas the filtration rate in C. fluminea was clearly affected throughout the whole heat‐wave event (from the heat peak onwards), it was only significantly affected during the heat‐wave climax in U. delphinus, and recovered afterwards. At the cellular level, lysosomal membrane stability was affected during the heat‐wave climax, but recovered afterwards, in both native and non‐native bivalve species. At the molecular level, hsp70 and hsp90 gene expression were increased and decreased (respectively) in C. fluminea after the heat wave. In contrast, only hsp90 gene expression decreased in U. delphinus during the climax and after the heat wave. The present work demonstrates that accurate monitoring of native and non‐native species using an integrated biomarker approach provides new insights into how both species might cope with the expected increase in frequency and intensity of heat events in a global warming context. Direct and indirect impacts of heat waves have been associated with massive mortalities of both native and non‐native species. Despite present results showing that C. fluminea are more sensitive to heat waves than U. delphinus, massive mortality events might act in favour of C. fluminea as it has a higher reproductive capacity. Although native freshwater mussel species conservation is already threatened by overexploitation, flow regulation and diversions, habitat destruction, and pollution, it is probably also adversely affected by global warming and the presence of invasive non‐native species, such as C. fluminea.

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