Large freshwater mussels, also known as naiads, are important organisms that provide crucial ecosystem functions and services but are experiencing significant declines across Europe. To ensure effective conservation, it is essential to have a well-coordinated monitoring scheme. Therefore, we analyzed the functioning of naiad monitoring systems in 31 European countries. Monitoring of naiad populations is not coordinated in Europe, is largely unrelated to monitoring of water quality or host fish species, and receives irregular funding. Raw data on monitored species are typically unavailable, which hampers independent and/or large-scale analyses. The quality of EU monitoring schemes according to Article 17 of the Habitats Directive is poor for the most threatened species. To improve this, a new scheme is recommended with raw data stored in publicly available repositories to enable instant analysis and rapid, appropriate, evidence-based responses that can support the conservation of this imperilled taxon. 

Mass mortality events (MMEs) are decimating populations and compromising key ecosystem functions around the globe. One taxon particularly vulnerable to MMEs is freshwater bivalve mollusks. This group has important ecosystem engineering capacities and includes highly threatened and highly invasive taxa. Thus, MMEs of freshwater bivalves have important implications for conservation and ecosystems. Despite this, little is known about the magnitude, frequency, duration, distribution, and causes of freshwater bivalve MMEs. Using a questionnaire, we compiled data from 239 reports describing freshwater bivalve MMEs across 22 European countries since 1960. With these data, we analyzed trends in MME timing, location, and magnitude; identified the species affected; and evaluated the suggested causes (including reporter certainty). We found that the frequency of reports of MMEs increased each year, MMEs affected a broad range of species, clear geographical patterns linking certain causes to specific locations were lacking, factors related to drying and habitat destruction predominated suggested causes, and considerable uncertainty surrounded the causes of many MMEs, particularly those associated with potential pollutants and disease agents. Based on our findings, we recommend the standardization of many aspects of MME research (e.g., reporting and recovery assessment protocols), increased surveying for MMEs, further investigation into the causes of MMEs, especially those with significant uncertainty, and immediate actions to improve waterbody management, mitigate the effects of high temperatures, and further protect freshwater bivalves through the development and implementation of appropriate management actions and legislation. 

Ecosystem engineers modify habitats in many ways with consequences for other species. Aggregations of autogenic ecosystem engineers in lotic systems, such as unionid mussels, can increase habitat complexity and change water flow, affecting the diversity and abundance of other benthic species. The effects of unionid mussel aggregations on mobile species, such as their host fishes, are less well-studied, but the increase in habitat complexity caused by mussels may provide refuges from water flow and may change fish behaviour. Using stream flumes, we examined how the presence and density (zero/low density/high density) of the freshwater pearl mussel (Margaritifera margaritifera) and water flow (low/high) affected behaviour of three different size classes of young-of-the-year brown trout (Salmo trutta) in May, June and August 2023, respectively. We found that small-sized trout were more likely to reside on the bottom at high than at low water flow but increased the likelihood to swim when mussel density increased. Medium-sized trout were more likely to reside on the bottom during high flow at zero to low mussel densities, while the proportions of swimming individuals increased regardless of flow at high mussel density. Large-sized trout were instead more likely to reside on the bottom when mussel densities were high, and in contrast to the small- and medium-sized trout, positioned themselves within the mussel zone more often. Our results demonstrate that freshwater pearl mussels can influence juvenile trout behaviour, probably by modifying habitat structure and flow conditions in the downstream area nearest to the mussels, particularly under hydrological stress. These findings underscore the role of mussels as ecosystem engineers and highlight potential feedback loops between freshwater pearl mussel presence and the behaviour of juvenile brown trout, which may be highly relevant for conservation strategies targeting both species. 

The ecology and co-management of endangered host-parasite associations, particularly between unionid mussels and their host fish, are an important yet underexplored area of research and conservation. Unionid mussels are important ecosystem engineers and are amongst the most globally endangered taxa, reliant on a fish host to complete a generational turnover. Freshwater fish are themselves highly endangered, a noted reason for the decline in unionid populations. The European eel (Anguilla anguilla), one of Europe's most endangered fish, has experienced a 90% population decline in the past half century. Theoretically, eels should make ideal hosts for unionids due to their broad distribution, long upriver migrations, dispersal capabilities within and across catchments, resistance to gill damage and physical proximity to gravid mussels. We tested the function of eels as hosts to the six most widespread European unionid species to assess if there is significant merit in the co-management of these organisms. We demonstrate that eels can host all but one host specialist unionid, suggesting that declines in global eel populations are a likely factor behind the declines of unionid populations. As conservation actions for eels and mussels are broadly similar, and the conservation of one directly affects the conservation of the other, co-managing both could enhance recovery of both. This study further aims to improve unionid conservation, by pairing an uncharismatic mussels with a well-known and charismatic fish.

Habitat restoration followed by species reintroduction is a key strategy for biodiversity recovery. For species with parasitic life stages, such as the freshwater pearl mussel (Margaritifera margaritifera), host suitability is crucial. Before a planned reintroduction of mussels into a newly constructed nature-like fishway, we tested the compatibility of four brown trout (Salmo trutta) strains from local and foreign drainages as hosts for mussels from a nearby river. The strains included (a) a local sympatric wild strain, (b) a local allopatric wild strain from near the fishway, (c) a local allopatric hatchery strain used for stocking, and (d) a foreign allopatric hatchery strain. After forty days, infestation rates of the parasitic mussel glochidia did not differ significantly among strains, indicating that all could serve as hosts. Glochidia that developed on the local allopatric hatchery strain had the highest growth rate, suggesting the highest suitability for production under laboratory conditions. While stocking hatchery strains can have negative ecological impacts, local wild fish provide a sustainable option without continued introductions of hatchery strains. If wild fish are scarce, carefully chosen hatchery strains may support juvenile mussel production and reintroduction under controlled conditions. Our findings highlight the importance of evaluating host compatibility before mussel reintroduction and fish stocking. 

The freshwater pearl mussel (Margaritifera margaritifera) is among the most endangered unionids in Europe. Many populations are highly specialized and solely infest specific parapatric populations of either brown trout (Salmo trutta) or Atlantic salmon (Salmo salar). River fragmentation can prevent mussel populations from completing a generational turnover if their migratory hosts can no longer reach upriver mussel habitats. Here, we tested a seminatural reintroduction method and compared the mussel infestation rates on different salmonid ecotypes in an experimental field scenario where previously separated salmonid populations will be reconnected following river rehabilitation. In our study, upriver resident trout sympatric to the mussels and migratory trout and Atlantic salmon allopatric to the mussels (separated by several migration barriers) were cohabitated in in-field cages containing M. margaritifera allowing for a seminatural infestation. We demonstrate that all three salmonid ecotypes hosted a significantly different number of larval mussels: the resident trout was the most heavily infested, while the allopatric salmon hosted the fewest. This study demonstrates that, insofar as only the mussels are moved between locations, in-field cage infestation can be a successful method for mussel conservation as it requires significantly less investment than producing juvenile mussels in a dedicated facility. Further work should be conducted to evaluate the long-term success rate of this reintroduction method and on how to optimize the husbandry of the fish in the cages to minimize mortality. 

Most freshwater mussels (FM) in Europe have dramatically declined and are now facing extinction due to human disturbances. Given the recent EU initiatives to protect and restore freshwater ecosystems, public awareness should be considered in the implementation of possible management measures. With this aim, a total of 1102 interviews exploring FM knowledge and use were carried out in four European regions. Results evidenced differences linked to socio-economic metrics-e.g., human density, GDP, residency, and sex-though using FM is independent of being aware of threats they face. Low awareness of FM threats makes Southeastern Europe the most challenging region for conservation, while in Southwestern Europe, poor knowledge about their existence may hinder engagement. To address these challenges, targeted educational campaigns and engagement initiatives should be developed to increase awareness in both regions.

Freshwater mussels in the order Unionida are a highly endangered taxon of ecosystem engineers with a variety of conservation and reintroduction efforts around the world attempting to increase wild populations. The unionid lifecycle involves a parasitic larval life-stage (glochidia), primarily adapted for dispersal rather than growth. Little is known about the behavioural effects of the glochidia on their host fish. We investigated the shoaling behaviour and habitat preference of the Eurasian minnow (Phoxinus phoxinus) while infested with the glochidia of one of Europe’s most endangered unionid mussels, the thick-shelled river mussel (Unio crassus). Behavioural assays were carried out in a hydrologically complex flow-through aquaria at two time points of the infestation period, a carrying stage and an excystment stage (14 and 28 days post infestation). We demonstrate that glochidiosis reduces minnow shoaling frequency and increases the number of isolated individuals. However, minnow shoal area and nearest neighbour distance remained unaffected. Infested minnows demonstrated a significant preference for slower flowing and less turbulent habitat. These results suggest that glochidiosis reduces host swimming efficiency, which likely increases predatory pressure on infested minnows. Simultaneously, this preference for calmer habitats may benefit the encysted mussels, as these habitats are better suited for juvenile mussel excystment. Our results highlight the importance of low-flow and low-turbulence habitats for unionid recruitment, as parasitized host individuals tolerate these conditions well.

Freshwater mussels are among the most endangered animal groups, highly sensitive to climate change due to their strict dependence on freshwater habitats. While freshwater mussels are often considered ecologically strict, their distribution across broad environmental gradients raises the possibility of population-specific adaptations mediated by phenotypic plasticity. This study investigates whether geographically and climatically distinct populations of two freshwater mussel species (Unio pictorum and Unio delphinus) exhibit different transcriptomic responses to prolonged heat stress and whether these responses reveal signs of local adaptation. We exposed northern and southern populations of both species to gradually increasing temperatures in controlled laboratory conditions, simulating a prolonged thermal extreme event, and RNA-seq was used to quantify differential gene expression. Results showed strong differences between northern and southern populations of the two species, both in the magnitude and functional composition of transcriptomic responses. Southern populations exhibited intense expression shifts involving classical stress pathways, heat shock proteins, detoxification (cytochrome P450s), apoptosis, and energy metabolism, while northern populations, particularly U. delphinus, showed a markedly subdued response. Notably, U. pictorum's northern population relied heavily on the cytochrome P450 family even at moderate temperatures, while the southern populations of both species activated broader proteostasis and immune responses at higher stress thresholds. These findings demonstrate clear population-specific phenotypic plasticity, shaped by environmental conditions rather than phylogenetic proximity. They underscore the need for conservation strategies to move beyond species-level management, embracing intraspecific variation as a buffer against climate impacts. As climate change accelerates, safeguarding the evolutionary potential encoded within populations, not just species, is essential to preserving biodiversity resilience.

The freshwater pearl mussel Margaritifera margaritifera (L.) remains Critically Endangered in Europe and Endangered throughout its world range in spite of many years of conservation efforts. It has been legally protected since at least the 1990s in most countries it inhabits. It also benefits from international protection under the European Union Habitat's Directive, which has opened up over 30 years of funding to improve and ultimately restore its conservation status in Europe. This monograph brings together the research and conservation efforts to date that have been undertaken across most of its range, which has increased our understanding of the species' life history, interactions with host fish, its ecology and habitat requirements. Efforts to understand the catchment, water quality and flow changes needed to rehabilitate populations are ongoing. The juvenile stages are extremely sensitive, and the natural environment of M. margaritifera is oligotrophic. Many rivers have changed considerably since juvenile recruitment was last successful. However, adults are long lived and populations can recover if targeted catchment-level restoration is undertaken, providing benefits for climate action as well as mussels. This publication provides a synthesis to assist research and conservation management.