Population-level effects of parasitism on a freshwater ecosystem engineer, the unionid mussel Anodonta anatina

Funder: Woolf Fisher TrustAbstract: Parasites can negatively affect hosts at individual, population, and species‐level scales. However, the link between individual‐ and population‐level impacts is often poorly understood. In particular, the population‐level response to parasitism may alter wider ecosystem dynamics if animals with ecosystem engineering capabilities are infected. Here, we examine the effects of parasitism on a freshwater ecosystem engineer, the unionid mussel Anodonta anatina, at two different sites. We study three common parasites: the digenean trematode Rhipidocotyle campanula; the unionicolid mite Unionicola intermedia; and the ectoparasitic invasive zebra mussel Dreissena polymorpha. As well as demonstrating the individual‐level effects of parasitism on the native host mussel, we construct a simple model to estimate the reduction in population‐level reproductive output caused by parasites. We show that both infection prevalence and intensity were population‐specific, with one site having more than three times as many native mussels infected by trematodes and mites than the other, but more than four times fewer mussels afflicted by invasive zebra mussels. Negative reproductive consequences for individual host mussels were documented as a result of parasitism, with trematodes causing castration at both sites. Mites were also correlated with a reduction in the viability of larval offspring (glochidia) by more than 25%, but only at one site, suggesting some potential impacts of parasitism may be population specific. The population‐level model shows that parasitism alone reduces larval output of the two populations by 10% and 13%, respectively. Our study takes the important step of scaling individual‐level effects of parasitism to population‐level processes, and highlights the influence that parasites may have in the population dynamics of unionid mussels. Given the ecosystem engineering capabilities of A. anatina, such effects may have important impacts on the wider biota. Even at relatively low prevalences, the observed effects of parasites on native mussel populations suggests that parasitism must be considered in the conservation of freshwater mussels, one of the world's most globally imperilled faunal groups. Further, understanding how the effects of parasitism on individual hosts scales to the ecosystem level is a crucial and unaddressed question in freshwater biology

Enigmatic freshwater mussel declines could be explained by the biodiversity-disease relationship

Abstract The biodiversity‐disease relationship states that increased species richness leads to lower pathogen pressure (i.e. the dilution effect), an effect that erodes with biodiversity loss. However, whether losses of the dilution effect can trigger extinction cascades remains largely unexplored. To explore this idea, we consider declines in freshwater mussels, on average the most threatened non‐marine group of organisms in the world. We argue that anthropogenically stimulated declines in mussel richness amplify pathogens in remaining species. Pathogen amplification triggers further local losses in both mussel abundance and richness, explaining the observed so‐called “enigmatic” declines in freshwater mussels. Vulnerable communities could become trapped in cycles of pathogen amplification and host decline. We highlight knowledge gaps and provide key steps to assess the likelihood of this occurring; these key steps are applicable to any host group. Policy implications. Our argument constitutes a testable hypothesis that may explain richness or abundance declines in previously intact communities. We provide further impetus for the consideration and preservation of diversity at a local scale and show that effective conservation requires integration of both host and parasite ecology. </jats:p

The Role of Freshwater Mussels in River Bed Dynamics and Sediment Flux

Freshwater mussels have been found capable of improving water quality in river environments by filtering particles from the water column and depositing them on the river bed as pseudofaecal pellets. Growing recognition of the ecosystem services that freshwater mussels provide in river habitats has led to their inclusion in several river restoration projects. The focus of many of these projects in Europe has been on the freshwater pearl mussel, Margaritifera margaritifera with attempts to repopulate rivers with this critically endangered species. However, the very specific habitat requirements of M. margaritifera mean that many European rivers are not suitable environments for this species. In comparison to M. margaritifera, very little conservation effort has been directed at some of our more common, yet declining freshwater mussel species, and little is known about how these species influence river sediments and habitat conditions. Some of the more common European freshwater mussel species, such as Anodonta anatina and A. cygnea are capable of living in a much broader range of habitat conditions compared with M. margaritifera, meaning they could potentially be of benefit in river remediation projects not suitable for M. margaritifera. To improve understanding of how Anodonta species may influence river environments through bioturbation, filtration, and biodeposition, their influence on river sediment characteristics, sediment dynamics, and habitat conditions was investigated in two lowland English rivers and a laboratory-based flume environment. Anodonta anatina and A. cygnea from Markeaton Brook, Derbyshire and the River Sence, Leicestershire were translocated from mussel-dense reaches to locations within each river where mussels were absent. In both rivers, quadrats where mussels had been removed were compared with control quadrats where mussels were present, whilst quadrats at the sites where mussels had been introduced were compared with control quadrats where mussels were absent. At sites in both rivers where mussels had been removed from the river bed, significant decreases in hyporheic oxygen saturation were found in all quadrats, compared with the control quadrats that contained mussels. Half of all quadrats where mussels had been introduced to river sites showed significantly increased hyporheic oxygen saturation, with the remaining quadrats showing non-significant increases compared with the control quadrats without mussels. Grain-size distribution patterns of sediment cores taken from the stream bed indicated that the introduction of mussels to the River Sence increased the textural heterogeneity of the river bed sediment, whereas removal of mussels reduced textural heterogeneity of the river bed sediment. No significant differences in river bed textural heterogeneity were found in Markeaton Brook. Mean percentages of organic matter, inorganic carbon, and fine-grained sediment were not significantly affected by the presence of mussels in the majority of quadrats. Significant reductions in water turbidity were found in Markeaton Brook, where mussels were introduced, but no significant differences in turbidity were found in the River Sence. Significantly higher BMWP scores were found where mussels were present in the River Sence but differences in Markeaton Brook were non-significant. The variation in results between the river sites suggest that the extent to which freshwater mussels influence river bed conditions and water quality may be mussel density-dependent and site-specific. A recirculating flume-based study using fifty A. anatina investigated the impact of this species on substrate characteristics, hydrological conditions, and particle flux of a polymodal substrate. River seston was added to the flume at weekly intervals to provide food for the mussels, and water and substrate conditions were monitored for the eight-week duration of the study. A control experiment was also set up with mussels absent from the flume. It was found that the presence of A. anatina led to reduced near-bed, 0.6 depth and 0.4 depth velocities, and reduced suspended and dissolved solids in the water column. Anodonta anatina reduced the entrainment of fine and organic material but increased the entrainment of sand and gravel compared with the control study. Although water velocities were reduced with mussels in the flume, calculations based on the grain-sizes entrained into the flume’s sediment trap indicated that critical boundary shear stresses were significantly increased with mussels in the flume. Additionally, sediment grain-size distribution patterns and topographical measurements of the substrate surface indicated that the mussels increased the heterogeneity of the substrate. The results of the research described in this thesis indicate that bioturbation, filtration and biodeposition by Anodonta species may positively influence hyporheic oxygen saturation levels, water quality and habitat heterogeneity in river environments. Increased mixing and mobilisation of river bed sediment, and the transferral of material from the water column to the substrate by mussels implies they constitute a critical element in the sediment and nutrient dynamics of fluvial systems

Pre‐ and postinfection priority effects have contrasting outcomes for parasite prevalence in host populations

AbstractParasite species frequently co‐occur more or less than expected by chance. These nonrandom co‐occurrence patterns can be driven by pre‐ or postinfection priority effects: parasites are more or less likely to attempt infection in a host already infected by another parasite species or may have higher or lower establishment and survival in hosts already infected by another parasite species. How these two types of priority effects differentially affect parasite distributions at the host population level remains unexplored. Using a probabilistic state transition diagram parameterized with field data and analyzed with two simulation methods, I show that the two types of priority effects lead to contrasting outcomes at the host population level. As preinfection priority effects transition from facilitation to inhibition, uninfected hosts decrease by up to 39%, and hosts infected by both parasites decrease by up to 84%, with concomitant increases in single infections by either parasite species. In contrast, as postinfection priority effects transition from facilitation to inhibition, the proportion of uninfected hosts remains unchanged, but the proportion of hosts infected by both parasites decreases by up to 89%, with increases in hosts singly infected by the first‐arriving parasite. Interactions between parasites at the within‐host level and the specific nature of those interactions alter infection patterns at the host population level.</jats:p

Pre- and postinfection priority effects have contrasting outcomes for parasite prevalence in host populations

Parasite species frequently co-occur more or less than expected by chance. These nonrandom co-occurrence patterns can be driven by pre- or postinfection priority effects: parasites are more or less likely to attempt infection in a host already infected by another parasite species or may have higher or lower establishment and survival in hosts already infected by another parasite species. How these two types of priority effects differentially affect parasite distributions at the host population level remains unexplored. Using a probabilistic state transition diagram parameterized with field data and analyzed with two simulation methods, I show that the two types of priority effects lead to contrasting outcomes at the host population level. As preinfection priority effects transition from facilitation to inhibition, uninfected hosts decrease by up to 39%, and hosts infected by both parasites decrease by up to 84%, with concomitant increases in single infections by either parasite species. In contrast, as postinfection priority effects transition from facilitation to inhibition, the proportion of uninfected hosts remains unchanged, but the proportion of hosts infected by both parasites decreases by up to 89%, with increases in hosts singly infected by the first-arriving parasite. Interactions between parasites at the within-host level and the specific nature of those interactions alter infection patterns at the host population level.<p/

Freshwater mussel conservation: A global horizon scan of emerging threats and opportunities

We identified 14 emerging and poorly understood threats and opportunities for addressing the global conservation of freshwater mussels over the next decade. A panel of 17 researchers and stakeholders from six continents submitted a total of 56 topics that were ranked and prioritized using a consensus-building Delphi technique. Our 14 priority topics fell into five broad themes (autecology, population dynamics, global stressors, global diversity, and ecosystem services) and included understanding diets throughout mussel life history; identifying the drivers of population declines; defining metrics for quantifying mussel health; assessing the role of predators, parasites, and disease; informed guidance on the risks and opportunities for captive breeding and translocations; the loss of mussel-fish co-evolutionary relationships; assessing the effects of increasing surface water changes; understanding the effects of sand and aggregate mining; understanding the effects of drug pollution and other emerging contaminants such as nanomaterials; appreciating the threats and opportunities arising from river restoration; conserving understudied hotspots by building local capacity through the principles of decolonization; identifying appropriate taxonomic units for conservation; improved quantification of the ecosystem services provided by mussels; and understanding how many mussels are enough to provide these services. Solutions for addressing the topics ranged from ecological studies to technological advances and socio-political engagement. Prioritization of our topics can help to drive a proactive approach to the conservation of this declining group which provides a multitude of important ecosystem services.This publication is based upon work from COST Action CA18239, supported by COST (European Cooperation in Science and Technology). DCA was supported by Corpus Christi College and a Dawson Fellowship at St. Catharine's College, Cambridge. MLL was supported by FCT-Fundacao para a Ciencia e a Tecnologia (2020.03608.CEECIND). ISO was supported by a Whitten Studentship. INB was supported by the Russian Science Foundation (grant no. 21-17-00126). YVB was supported by RSF project no. 21-14-00092. KD was supported by the Czech Science Foundation (19-05510 S). TZ was supported by statutory funds of IOP PAN. MK was supported by funding through the Australian National Environmental Science Program. For the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising from this submission

Hydrogeomorphic stressing and the response of endangered freshwater pearl mussels, Margaritifera margaritifera: a trait-based approach to inform conservation management

The freshwater pearl mussel, Margaritifera margaritifera, is considered vital to conserving the ecological integrity of freshwater systems, fulfilling the criteria of a keystone, flagship, indicator and umbrella species. Yet populations of this rare, long lived freshwater bivalve have witnessed substantial declines across the species’ Holarctic range. River systems in the Scottish Highlands continue to support large reproductively viable populations; many of which inhabit regulated rivers, managed for hydroelectric energy production. However, there has been limited study concerning the response of M. margaritifera to alterations in habitat characteristics resulting from dam operation. Utilising a combination of field and laboratory experiments, this study aimed to address knowledge gaps regarding interactions between M. margaritifera and the hydrogeomorphological processes occurring within their habitat, providing urgently needed empirical evidence to drive future conservation strategies implemented by government (NatureScot, SEPA) and utilised by the hydroelectric industry (SSE). Initial lab-based work sought to derive methods to non-invasively quantify mussel stress. Here, variation in the expression of particular behavioural metrics was examined in accordance with measurements of oxygen consumption, across environmental stressors, and between two freshwater mussel species. Results from this study revealed an increase in the presence of behavioural traits associated with valve activity in response to stress exposure, in accordance with substantial deviations in metabolic functioning of corresponding individuals. This study corroborates previous work highlighting the potential of bivalve filter feeding organisms as indicator species for alterations in habitat conditions. Furthermore, results exhibit the applicability of these techniques to non-invasively quantify physiological stress in Unionid mussels, towards understanding thresholds in response to environmental stressors across individuals, populations and species. To understand the response of M. margaritifera to alterations in flow regime, resulting from drought conditions and dam operation, flume experiments examined the responses M.margaritifera from two different populations, to three different rates of drawdown, using two different spatial arrangements. Results demonstrated a propensity of M. margaritifera to detect alterations in flow depth, utilising vertical and horizontal movements to avoid prolonged aerial exposure. Data from this study foregrounded intraspecific variation between populations, indicating potential variation in behavioural phenotypic traits. Results from a field trial in a regulated system, using a subset of the corresponding M. margaritifera population, endorsed findings from the flume study. Evidence presented in this study advocates forcontrolled drawdowns in regulated rivers to assist in reducing mortalities associated with receding water levels, during periods of drought. Finally, this study developed and tested novel smart-sensors housed within mussel shells to provide an affordable, accurate and accessible tool to record near-bed flow dynamics in aquatic systems. The resulting instrumented shells were found to accurately detect, and potentially predict entrainment events in M. margaritifera. Entrainment risk was dependent on the flowrate, shell orientation and size; consequently, highlighting the importance of vertical movements in mitigating flow forcing, and the vulnerability of smaller mussels to high flow events. This tool could assist in identifying suitable habitat for M. margaritifera, guiding reintroduction and translocation efforts, and identifying at risk populations to surges in flow discharge. Recommendations are made towards future conservation management of M. margaritifera in Scotland, with a focus on adopting a context-driven approach at the population level. Work in this thesis has provided a foundation for the development of future monitoring techniques to improve understanding of M. margaritifera habitat requirements, in consideration of hydrogeomorphological processes

Interactions between freshwater mussels and non-native species

Biological invasions contribute to ecosystem change globally, with a disproportionate and intensified impact in freshwaters. This process is exacerbated in modified systems such as hydrogeneration reservoirs that promote favourable conditions for non-native species proliferation. One of the major threats from non-native species is the introduction of novel interactions that may be particularly impactful on species in affiliate (dependent) relationships and that have narrow habitat requirements during early life-stages. Freshwater mussels (Bivalvia: Unionida) are sessile benthic organisms in affiliate relationships with host fish on which they complete their ectoparasitic life-stage. Attached larvae (glochidia) transform on suitable fish hosts before dropping off as juveniles on surficial sediments. Significant disruption to such interactions may lead to local extinction if affiliate partners are unable to be replaced (i.e., by non-native fish) or the availability of critical life-supporting habitats is reduced (e.g., by non-native macrophytes). Non-native species may play a role in reducing recruitment leading to the adult-skewed mussel population size-structures commonly observed. Accordingly, this thesis contributes knowledge of the interactions between unionid mussels and non-native species in modified freshwater ecosystems, and provides information to assist in species and reservoir management for unionid mussel conservation. The thesis outputs are presented as chapters that have been published in, submitted to, or prepared for scientific journals. A general introduction (Chapter 1) provides context for a global meta-analysis of non-native species and unionid mussel interactions that highlighted non-native fish and macrophytes as potential threats to New Zealand mussels (Chapter 2). Accordingly, a laboratory experiment on three non-native fish (brown bullhead catfish, Ameiurus nebulosus; rudd, Scardinius erythrophthalmus; and goldfish, Carassius auratus) found mussel glochidia were not transformed in ecologically viable numbers compared to a known host fish (Gobiomorphus cotidianus) for a New Zealand unionid (Echyridella menziesii; Chapter 3). This finding suggested that shifts towards fish communities dominated by non-native species have potential to disrupt the obligate glochidial life-stage of unionid mussels. Dense beds of non-native macrophytes (Ceratophyllum demersum and Egeria densa) were found to produce adverse anoxic and hypoxic conditions potentially fatal to mussels in a field survey of Karāpiro, the most downstream in the Waikato River hydrogeneration reservoir chain (Chapter 4). Here, adverse conditions at the sediment-water interface in littoral zones were mediated by reservoir management of water-level and water-flow, and by macrophyte control via herbicide application in the lower-lacustrine section. A subsequent field survey extended the Chapter 4 results to show that effects of non-native macrophytes at the sediment-water interface depended on macrophyte species and overarching hydrology, whereby adult-skewed mussel population size-structures were present in the lower-lacustrine of Karāpiro but not in the upper-riverine section where recruitment was occurring (Chapter 5). The final chapter combined previous findings to show how various hypothetical scenarios of fish and macrophyte invasions could operate separately (non-native fish only) or in combination to disrupt E. menziesii recruitment (Chapter 6). This hypothetical analysis highlighted the importance of considering the threats of both non-native fish and macrophytes, which operate primarily on different stages of the unionid life-cycle, in freshwater mussel conservation and management. Due to the long life-span of unionids, recognition of non-native species impacts contributing to adult-skewed mussel population size-structures may provide an opportunity to restore disrupted mechanisms supporting their recruitment before local extinction occurs. Globalisation and energy demand facilitate continued biotic homogenisation and loss of associated ecosystem services. In this context, the role of management in preventing and mitigating the impacts of biological invasions on sensitive species with affiliate relationships will become increasingly important in freshwater ecosystems in the future

Mussel parasite richness and risk of extinction.

Parasite conservation is important for the maintenance of ecosystem diversity and function. Conserving parasites relies first on understanding parasite biodiversity and second on estimating the extinction risk to that biodiversity. Although steps have been taken independently in both these areas, previous studies have overwhelmingly focused on helminths in vertebrate hosts over broad scales, providing low resolution and excluding a large proportion of possible host and parasite diversity. We estimated both total obligate parasite richness and parasite extinction risk in freshwater mussels (Unionidae and Margaritiferidae) from Europe and the United States to provide a case study for considering parasite conservation in a severely understudied system. We used currently reported host-parasite relationships to extrapolate parasite diversity to all possible mussel hosts and then used the threat levels of those hosts to estimate the extinction risk for both described and undescribed parasites. An estimated 67% of parasite richness in freshwater mussels is undescribed and over 80% of the most host-specific groups (digenean trematodes and ciliates) are undescribed. We estimated that 21% of this total parasite fauna is at immediate risk of extinction, corresponding to 60 unique species, many of which will likely go extinct before being described. Given the important roles parasites play in community structure and function and the strong ecosystem engineering capacities of freshwater mussels, such extinctions are likely to severely affect freshwater ecosystems. Our detailed study of mussel parasites provides compelling evidence for the hidden conservation threat to parasites through extinction cascades and shows parasites are deserving of immediate attention

Conservation of the Giant Freshwater Pearl Mussel (Pseudunio auricularius = Margaritifera auricularia Spengler, 1793) in Spain

Las almejas de agua dulce o náyades son uno de los grupos faunísticos más amenazados del planeta. Entre ellos, la almeja gigante del Ebro o náyade auriculada (Pseudunio auricularius, Spengler 1793) es el bivalvo unionido más amenazado de Europa, y la población de la cuenca del río Ebro en España se encuentra en una situación crítica. El objetivo de esta tesis es aumentar el conocimiento científico del estado de conservación actual de la especie en España con el fin de aplicar acciones de gestión significativas para reducir su riesgo de extinción. El descenso de las poblaciones de náyades (Orden Unionida) en la cuenca del Ebro es generalizado, pero la situación en los canales asociados es crítica, especialmente desde 2013 cuando se registró un episodio de mortalidad masiva de P. auricularius en el Canal Imperial de Aragón (CIA). En el marco de un estudio a largo plazo con el objetivo de evaluar las densidades de náyades en los canales, los resultados obtenidos mostraron una disminución continua y generalizada de todas las especies nativas, que terminó con la extinción local de Anodonta anatina (Linnaeus, 1758), Potomida littoralis ( Cuvier, 1798) y Unio mancus Lamarck, 1819, junto con miles de ejemplares muertos de P. auricularius. Una posible explicación de estas disminuciones es el envejecimiento y la posible muerte debido a la senescencia. Para conocer la relación entre la longitud de la concha y la edad en P. auricularius en la cuenca del Ebro, se analizó su patrón de crecimiento mediante el conteo de anillos de concha de ejemplares adultos y jóvenes junto con datos de crecimiento de juveniles criados en cautividad. Los modelos sigmoidales generalizados de von Bertalanffy y Richards presentaron el mejor ajuste, mostrando que la tasa de crecimiento de P. auricularius es relativamente lenta después de los 30 años, cuando se alcanza el máximo crecimiento. La edad máxima estimada para un ejemplar en la cuenca del Ebro fue de 68 años. Otra posible explicación de la disminución de la población fue la contaminación ambiental derivada de las actividades antropogénicas. Considerando la falta de conocimiento previo sobre la sensibilidad de la especie, se realizaron por primera vez pruebas de toxicidad aguda contra metales pesados y amonio utilizando juveniles de la cría en cautividad. De esta forma, se determinaron los valores de concentración letal (CL50 y CL10) para cadmio, cobre, níquel, zinc, cromo, plomo y amonio. Además, se evaluaron los valores de la concentración sin efecto no observado (NOEC), los valores de concentración de efecto mínimo observado (LOEC) y las concentraciones tóxicas máximas aceptables (MATC) para cada contaminante probado. Al comparar los resultados de P. auricularius con los datos disponibles de otras especies de náyades, parece ser más sensible al cadmio y al cobre, y menos sensible al níquel, el plomo y el amoníaco total. Como una de las principales acciones previstas para la recuperación de la especie, se establecieron cultivos de laboratorio para desarrollar un protocolo de cría en cautividad de juveniles de P. auricularius. Se probaron los efectos de varias condiciones sobre la supervivencia y el crecimiento. Los mejores resultados correspondieron a un tratamiento en recipientes de vidrio, a una densidad de 0,2 ind/L, utilizando agua del mismo río Ebro, sustrato, detritus y fitoplancton, y sin aireación extra. Las tasas más altas de supervivencia y crecimiento alcanzadas fueron del 60 % a los 100 días y una longitud de concha de 2,56 mm a las 30-32 semanas de vida. La cría en cautividad se considera una de las herramientas de gestión más importantes para mejorar el estado de conservación de P. auricularius, y estos valiosos resultados permiten establecer, por primera vez, las mejores condiciones para maximizar la supervivencia de los juveniles en condiciones de laboratorio. Finalmente, y con el objetivo de aumentar las probabilidades de supervivencia de los ejemplares en el CIA, en 2017 se inició un plan de translocación. Los ejemplares adultos fueron traslocados desde el CIA al río Ebro utilizando parcelas previamente caracterizadas como aptas para la supervivencia de P. .auricularius. Se traslocaron un total de 638 especímenes (291 especímenes en 2017, 291 en 2018 y 56 en 2019). La monitorización, un año después, permitió estimar una tasa de supervivencia entre el 40 y el 70%. Por otro lado, el grupo de control que quedó en la CIA tuvo una tasa de supervivencia más baja de solo el 19,7% después de un año. En base en estos primeros resultados, se confirmó que los ejemplares de P. auricularius trasladados al cauce principal del río mostraron una mayor supervivencia que los que permanecieron en el canal. En general, la información generada servirá de apoyo para la aplicación de las mejores medidas de restauración con el fin de frenar la curva de declive de esta icónica especie ribereña en España.Freshwater mussels are one of the most threatened faunal groups in the planet. Among them, the Giant Freshwater Pearl mussel (Pseudunio auricularius, Spengler 1793) is the most endangered unionid bivalve in Europe, and the population of the Ebro River basin in Spain is in a critical situation. The aim of this thesis is to increase the scientific knowledge of the current conservation status of the species in Spain in order to apply meaningful management actions to reduce its risk of extinction. The decline of freshwater mussel (Order Unionida) populations in the Ebro basin is widespread, but the situation in the associated canals is critical, especially since 2013 when an episode of mass mortality of P. auricularius in the Canal Imperial de Aragón (CIA) was recorded. In the framework of a long-term study aiming to assess mussel densities in the canals, the obtained results showed a continued and generalized decrease of all native mussel species, ending with the local extinction of Anodonta anatina (Linnaeus, 1758), Potomida littoralis (Cuvier, 1798) and Unio mancus Lamarck, 1819, together with thousands of P. auricularius' dead specimens. One possible explanation for these declines is aging and possible death due to senescence. To find out the relationship between shell length and age in P. auricularius in the Ebro basin, its growth pattern was analysed by counting the shell rings of adult and young specimens together with growth data of captive-bred juveniles. The generalized von Bertalanffy and Richards sigmoidal models presented the best fit, showing that the growth rate of P. auricularius is relatively slow after 30 years, when maximum growth is reached. The maximum age estimated for a specimen in the Ebro basin was 68 years. Another possible explanation of the population decline was environmental pollution derived from anthropogenic activities. Considering the lack of previous knowledge on the species' sensitivity, acute toxicity tests against heavy metals and ammonium were carried out for the first time using juveniles from a captive breeding program. In this way, the lethal concentration values (LC50 and LC10) were determined for cadmium, copper, nickel, zinc, chromium, lead and ammonium. In addition, non-observed-effect concentration (NOEC) values, lowest-observed-effect concentration (LOEC) values and maximum acceptable toxicant concentrations (MATC) were assessed for each contaminant tested. When comparing the results of P. auricularius with available data from other mussel species, it seems to be more sensitive to cadmium and copper, and less sensitive to nickel, lead, and total ammonia. As one of the main actions planned for the recovery of the species, laboratory cultures were established to develop a captive breeding protocol for juveniles of P. auricularius, by testing the effects of several conditions on survival and growth. The best results corresponded to a treatment in glass containers at a density of 0.2 ind/L, using river water, substrate, detritus and phytoplankton, and without extra aeration. The highest survival and growth rates achieved were 60% at 100 days and 2.56 mm shell length at 30-32 weeks of life. Captive breeding is considered one of the most important management tools to improve the conservation status of P. auricularius, and these valuable results allow establishing, for the first time, the best conditions for maximizing juvenile survival under laboratory conditions. Finally, and with the objective of increasing the survival probabilities of the living specimens left in the CIA, a translocation plan was initiated in 2017. Adult specimens were translocated from the CIA to the Ebro River using plots previously characterized as suitable for the survival of P. auricularius. A total of 638 specimens (291 specimens in 2017, 291 in 2018, and 56 in 2019) were translocated. A monitoring survey, one year after, allowed estimating a survival rate between 40 and 70%. On the other hand, the control group left in the CIA had a lower survival rate of just 19.7% after one year. Based on these first results, it was confirmed that the specimens of P. auricularius translocated to the main river channel showed higher survival than those that remained in the canal. Overall, the autecological information collected will provide support for the application of the best restoration measures in order to bend the curve of decline of this iconic riverine species in Spain