Rivers are increasingly fragmented and degraded, yet the European Union Nature Restoration Regulation calls for restoring at least 25,000 kilometres of free-flowing rivers by 2030. Translating this ambition into effective implementation remains challenging because restoration priorities differ across ecological, social, economic, and governance contexts. Here, we synthesize expert knowledge from 45 countries through a structured, multi-step prioritization process to identify research priorities for restoring free-flowing rivers in Europe. We identified 27 priorities and analysed how expert background and spatial context influenced their ranking. Restoration priorities differed systematically depending on whether experts emphasized ecological integrity, community engagement, economic considerations, or governance capacity, revealing clear patterns in how disciplinary and professional perspectives shape implementation pathways. This demonstrates that restoration strategies cannot be universal but must be adapted to local and regional political, institutional, and ecological conditions. Building on these findings, we propose a structured prioritisation framework that links barrier removal, connectivity restoration, governance mechanisms, and policy instruments to context-specific needs. Together, our results provide an empirically grounded and implementation-oriented roadmap to support European Union Member States in delivering ambitious river restoration targets in a context-sensitive and socially robust manner.

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.

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 bivalves are globally distributed, diverse, and common in benthic communities. Many taxa, particularly in the most species-rich order, Unionida, are declining due to anthropogenic stressors, while a small number of non-native species have become increasingly abundant and widespread, commonly replacing native bivalve assemblages. To understand how these global changes may impact ecosystems and people, we conducted a meta-analysis of existing literature quantifying the ecological functions (= supporting or intermediate ecosystem services) and regulating ecosystem services of freshwater bivalves (hereafter “ecosystem services”). Random effects meta-analysis modeling across 447 case studies revealed a positive effect on human health, safety, or comfort of freshwater bivalve ecosystem services overall and specifically, via effects on native macrofauna, microorganisms, wastes, and pollutants, and the physico-chemical condition or quantity of sediments. Generally, effects of native species and species within the orders Unionida and Venerida were more significant and positive than those of other freshwater bivalves. No significant overall effect was found for ecosystem services related to zooplankton, algae, invasive species, and the physico-chemical condition of ambient water. Moreover, a significant bias toward publication of positive results existed for studies quantifying ecosystem services related to algae. These findings illustrate the global importance of the ecosystem services of freshwater bivalves and highlight the need for large-scale conservation and restoration efforts for their species and populations globally, including those of common species. Our findings also question common assumptions of strong and ubiquitous effects of freshwater bivalves on algae and water condition, cautioning against extrapolating observations across systems. 

Parasitic freshwater mussels are endangered ecosystem engineers with an array of impacts on multiple trophic levels and life stages. While the impacts of adult mussels on separate trophic levels have been studied, few have directly tested how adult mussels can impact trophic interactions, or investigated the impacts of the parasitic mussel larvae (glochidia) on such interactions. We present a laboratory study which mimics two-stream substrates for the endangered thick-shelled river mussel (Unio crassus): one dominated by gravel and one by cobbles. First, the preference of a gammarid (Gammarus pulex) for mussel-dominated habitats was tested in the presence/absence of chemical cues from the predator bullhead (Cottus gobio). Second, the preference of bullhead for mussel-dominated habitats was tested under or without glochidia infestation. Third, the effect of infestation on bullhead predation on gammarids was assessed in the presence of adult mussels. Gammarids only significantly preferred mussel habitats in the absence of predator cues, whereas infested bullhead tended to prefer mussel habitats in cobble substrates. The presence of adult mussels only significantly reduced bullhead predation on gammarids in the gravel habitat, whereas infestation did not affect bullhead predation. Despite gammarids not preferring mussel habitat in the presence of predator cues, mussel beds represent valuable habitat to gammarids as mussel presence can facilitate a reduction of predation by bullhead. Infestation did not affect the rate of bullhead predation on gammarids but did attract bullhead to mussel habitat in cobble substrates. Our results suggest that mussel beds may be valuable habitat for both their host fish and the prey of their hosts, attracting both and increasing predator–prey interactions. This study highlights the cross-trophic mechanisms by which multiple life stages of parasitic mussels can impact the interactions of their surrounding benthic community, underscoring their importance as ecosystem engineers.

Unionid mussels are a distinct order possessing a parasitic life history stage known as a glochidium that generally infests the gills of fish. Upon contacting the host tissue, the glochidium ‘bites’ down causing minor surface trauma but leaving most structural tissue unharmed. Host tissue immediately reacts and encompasses the larval mussel in a cyst where, if able to survive, the glochidia will develop and ultimately excyst as free-living mussels; in the case of the freshwater pearl mussel (Margaritifera margaritifera) adults can live in the sediment for up to 200+ years. While many histological studies have detailed both the encystment process and larval development with a fair degree of detail, few have utilized scanning electron microscopy to add further prospective. Few have investigated the effects of juvenile mussel excystment on host tissue. The freshwater pearl mussel is the longest encysting unionid mussel, remaining on their hosts for close to a year. Here, we investigate three stages of freshwater pearl mussel glochidia development on two host salmonid species (Salmo trutta and S. salar). Our survey supports previously published results and suggests that juvenile mussel excystment causes significantly more harm to host tissue than initial encystment. We provide a large library of images as supplements to this survey for both researchers and educators to use as references, either for educational purposes or out of general interest.

Freshwater mussels in the order Unionida are a highly endangered and globally distributed taxon of parasitic bivalves. Consequently, many conservation and reintroduction projects have emerged to protect this valuable order. Adult mussels release parasitic larvae (glochidia) that affix themselves to their hosts, typically the gills of fish, where they become encysted in the tissue and metamorphose into juveniles; later, excysting as juveniles to develop into free-living adults in the sediment, completing the lifecycle. As Unionids are well-known ecosystem engineers, reintroduction efforts are generally met with support. However, members of the public unfamiliar with the scientific literature often have reservations when learning that the restoration project aims to introduce parasites to their local waterways. Little is known about the impacts of glochidiosis on host fishes; however, most effects are negative. With the growth of mussel conservation projects, more fish will be exposed to these negative impacts. 

In this dissertation, I explore the parasitic interactions of two of Europe’s most endangered mussels with some of their respective host fishes. I initially produced a general literature review to summarize all previous work on the topic and better understand which aspects needed the most attention (Paper I). From this, I surmised that the behavioural and molecular impacts of glochidiosis were the least studied and that there was a general over-representation of the interaction between the freshwater pearl mussel (Margaritifera margaritifera) and its salmonid hosts, a highly specialized interaction not well-suited for cross-species generalizations. I followed Paper I with an investigation into the impacts of infestation by the thick-shelled river mussel (Unio crassus) on the schooling behaviour of its host minnow (Phoxinus phoxinus; Paper II), and the impacts of both adult and larval life stages of U. crassus on the predator-prey interactions of its host bullhead (Cottus gobio) with a prey gammarid (Paper III). The results of Papers II and III suggested that unionid mussels may manipulate host behaviour; to test this hypothesis, a year-long field study was conducted on the movement patterns and habitat choice of brown trout (Salmo trutta) when infested with glochidia from M. margaritifera, the species most likely to express an extended phenotype (Paper IV), as speculated in Paper I. This investigation provided evidence in support of an expressed extended phenotype by a unionid mussel on its host fish.

Unionid mussels are a unique order of bivalves characterised by the presence of a parasitic larval life stage, known as a glochidium. In this dissertation, I explore the behavioural effects of glochidiosis (the condition of bearing glochidia) on common hosts of Europe’s most endangered unionids. The dissertation includes a general review of the effects of glochidiosis, two laboratory studies, and a year-long field study. The results presented here demonstrate that glochidia induce effects on their host fish similar to those caused by other parasites, including reduced growth, decreased swimming performance, and a general preference for calmer water and more structurally complex habitats. Together, the behavioural alterations induced by glochidiosis may benefit unionid recruitment, as infested host fish may transport excysting juvenile mussels to habitats favourable for later development. This provides evidence for the presence of an extended phenotype in unionids.

Parasites often have a large impact on their hosts and can alter host phenotype to increase their own fitness, a phenomenon known as extended phenotype. Studies demonstrating extended phenotype for non-trophically transmitted parasites are scarce. Unionid mussels have a parasitic life stage adapted to parasitize fish which can affect host behavior, habitat use and growth rates, raising the question if parasitic freshwater mussels can also manipulate their host fish to compensate for downstream dispersal and to reach habitats favorable for newly excysted juvenile mussels. Wild-caught, parasite-na & iuml;ve juvenile brown trout (Salmo trutta) were PIT-tagged, and half of the individuals were infested with parasitic larvae from the freshwater pearl mussel (Margaritifera margaritifera), all individuals were then returned to their home stream. During the following year, trout were tracked to investigate movement and habitat use, and also periodically recaptured to measure growth and body condition factor. The infested trout showed significantly higher upstream movement than non-infested trout and were more often recaptured in stream sections with slow-moving shallow water, particularly during the parasite excystment period (270 d post infestation). These data suggest that the juvenile mussels were successfully transported an average of 170 m upstream from the host trout release points to stream sections favorable for adult mussels. Infested trout survived as well as the non-infested, but had a significantly lower specific growth rate than non-infested trout. These results indicate a first example of extended phenotype in unionid mussels and highlight the importance of understanding glochidia-induced changes to host fish behavioral ecology. Parasites often manipulate host behavior to increase their reproductive success, but studies on this phenomenon typically focus on parasites which lead to the eventual death of their host, an effect that parasitic freshwater mussels generally do not have. Here, we show that parasitic freshwater mussel larvae manipulate host fish into swimming further upriver to habitats well suited for the next stage of their lifecycle.