Freshwater mussels of the Order Unionida provide important ecosystem functions and services, yet many of their populations are in decline. We comprehensively review the status of the 16 currently recognized species in Europe, collating for the first time their life-history traits, distribution, conservation status, habitat preferences, and main threats in order to suggest future management actions. In northern, central, and eastern Europe, a relatively homogeneous species composition is found in most basins. In southern Europe, despite the lower species richness, spatially restricted species make these basins a high conservation priority. Information on freshwater mussels in Europe is unevenly distributed with considerable differences in data quality and quantity among countries and species. To make conservation more effective in the future, we suggest greater international cooperation using standardized protocols and methods to monitor and manage European freshwater mussel diversity. Such an approach will not only help conserve this vulnerable group but also, through the protection of these important organisms, will offer wider benefits to freshwater ecosystems.

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Aim Given that riverine systems exhibit longitudinal environmental gradients from headwater to the mouth of a river, habitat heterogeneity appears to be a major driver of spatial variation in community composition among riverine localities. As freshwater ecosystems are amongst the most endangered ecosystems in the world, community‐based conservation and multiple‐species management are necessary to maintain ecosystem integrity. We used joint species distribution models (JSDMs) to investigate the relative importance of abiotic and biotic factors that are responsible for the distribution and co‐occurrence of species in riverine ecosystems. Location Central and northern Europe. Methods We examined the general patterns of species assemblage of two endangered freshwater mussel species (Margaritifera margaritifera and Unio crassus) and their associated fish communities. We examined the patterns of positive or negative co‐occurrence in mussel and fish species and identified shared abiotic responses between mussel–host pairs. Results We found that the relative importance of abiotic and residual factors and patterns of significant species correlations varied among taxa: significant residual correlations were prevalent among fish species, whereas mussel occurrences were exclusively explained by abiotic factors. Mussels and their fish hosts generally had shared abiotic responses with some mismatched responses between mussel–host pairs. Main conclusions Given that the composition of communities were tightly linked with abiotic factors and residual correlations, the results have significant implications for the conservation and restoration of aquatic communities. This study highlights the necessity to simultaneously consider environmental factors and species co‐occurrences in the modelling of species distributions and assemblages of riverine communities. Such a holistic community conservation approach can reveal ecological similarities and differences among species, which can help us avoid conflicts among target‐species conservation plans.

Recruitment and dispersal of freshwater mussels of the Unionoida is strongly linked to their host fishes on which the larvae develop into juvenile mussels. Host limitation has been suggested as one of the potential causes for the decline of the formerly widespread thick‐shelled river mussel (Unio crassus), although quantitative data analysing the importance of this factor are still missing. In this study, the status of host fish populations was assessed in nine U. crassus streams within the Danube drainage system situated in southern Germany. In particular, links between recruitment patterns of U. crassus populations, fish species composition and quantitative data of host fish densities in functional and non‐functional streams were established. Fish species composition and richness ranged between three and nine species with no difference in presence data between functional and non‐functional streams. Despite this high variability, functional U. crassus streams with high proportions of juvenile mussels had significantly higher (mean: 40 Ind. per 100 m2) densities of primary hosts (Phoxinus phoxinus, Squalius cephalus, Gasterosteus aculeatus) than non‐functional streams (mean: 8 Ind. per 100 m2). In contrast, no relationship was found between the numbers and densities of juvenile mussels and densities of poor hosts as well as of non‐hosts. The results of this study indicate that the recruitment status of U. crassus strongly depends on the density of primary host fish species. Host limitation is likely to play a major role in a lack of juvenile recruitment and consequently in the decline of central European U. crassus populations. Therefore, future conservation for this endangered mussel species should more thoroughly incorporate management of host fish species and consider both the requirements of mussels and their hosts in habitat restoration. Copyright © 2014 John Wiley & Sons, Ltd.

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