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Freshwater mussels are declining throughout their range. Their important ecological functions along with insufficient levels of natural recruitment have prompted captive breeding for population augmentation and questions about the usefulness and applicability of such measures.This article reviews the current state of captive breeding and rearing programmes for freshwater mussels in Europe. It considers the various species, strategies, and techniques of propagation, as well as the different levels of effort required according to rearing method, highlighting the key factors of success.Within the last 30 years, 46 breeding activities in 16 European countries have been reported, mainly of Margaritifera margaritifera and Unio crassus. Some facilities propagate species that are in a very critical situation, such as Pseudunio auricularius, Unio mancus, and Unio ravoisieri, or multiple species concurrently. In some streams, the number of released captive‐bred mussels already exceeds the size of the remaining natural population.Rearing efforts range from highly intensive laboratory incubation to lower intensity methods using in‐river mussel cages or silos. Most breeding efforts are funded by national and EU LIFE(+) grants, are well documented, and consider the genetic integrity of the propagated mussels. Limited long‐term funding perspectives, the availability of experienced staff, water quality, and feeding/survival during early life stages are seen as the most important challenges.Successful captive breeding programmes need to be combined with restoration of the habitats into which the mussels are released. This work will benefit from an evidence‐based approach, knowledge exchange among facilities, and an overall breeding strategy comprising multiple countries and conservation units. [ABSTRACT FROM AUTHOR]

During a detailed faunistic survey of the river Turiec in northern Slovakia in 2013–2022, we found 18 species of aquatic molluscs (8 gastropods and 10 bivalves) at 9 sites. The most species-rich assemblages were in the middle course of this river. The surprisingly low species richness in the lower course could be due to the heavily altered riverbed and polluted water. The most frequently recorded species were the gastropods Ancylus fluviatilis and Radix ampla and the bivalves Pisidium casertanum and P. subtruncatum. The rare and critically endangered bivalve Pisidium tenuilineatum was recorded in 2017 at one site, and two endangered species, Unio crassus and P. amnicum, were found in rich populations in 2022. The presence of these species confirms the relatively good preservation of the middle course of Turiec. The non-native species Physa acuta was found in this river for the first time.

This paper presents results of a malacological survey of the middle and upper section of the Jihlava River, a tributary of the Dyje River (Czech Republic). Twenty-three species of aquatic molluscs (13 gastropods, 10 bivalves) were found at 35 sites during research realised mostly in 2017 and 2020. In comparison with the lower section of the Jihlava River, a significantly lower proportion of bivalves of the family Unionidae was found in the middle and upper sections.

Freshwater mussels (Bivalvia: Unionida) are valuable components of river ecosystems. Among them, the thick-shelled river mussel (Unio crassus) is globally endangered, and its populations are decreasing in many European catchments. Over the last decade, it has become a focal point of environmental conservation programs. A comprehensive understanding of the relationship between the species and its environment is therefore required to optimize those programs. Yet, despite being increasingly studied, freshwater mussel ecological requirements remain complex to establish as they appear to be species- and region-related. We reviewed the potential environmental factors which have been showed to impact freshwater mussels, and therefore possibly U. crassus. We compiled datasets of environmental factors and occurrence of U. crassus across the study area (Wallonia, Belgium). Logistic regression models were built to identify the main driving factors. Results underscore the predominant significance of hydro-geomorphological variables in shaping distribution patterns. Specific stream power had a significantly negative impact on the distribution of U. crassus , while the probability of occurrence increased with catchment area, and with contrasted high- and low flows during the year. The high prediction accuracy of the hydro-geomorphological model supports the idea of a considerable influence of hydro-geomorphological habitat on the species. Other parameters are also relevant in predicting mussel distribution, like the percentage of land covered by meadows, elevated heavy metals concentrations and fish richness. • Regional distribution of an endangered freshwater species is investigated. • Hydro-geomorphological factors are the most predictive indicators. • Specific stream power has a negative effect on the species. • Flow variability and catchment size have a positive effect. [ABSTRACT FROM AUTHOR]

This paper presents results of a malacological survey of the Oslava River, a tributary of the Jihlava River (Czech Republic, Southeastern Moravia). Sixteen species of aquatic molluscs (8 gastropods, 8 bivalves) were found at 14 sites during the research conducted from 2010 to 2018. The research was focused on the endangered bivalve Unio crassus. Its population was confirmed at the lower river section and is connected with the population in the Jihlava River.

Many freshwater mussel species are highly endangered. Knowledge of their occurrence and distribution is a crucial prerequisite for their conservation. The use of environmental DNA (eDNA) is a relatively new approach and an effective tool to detect the presence of target species in freshwater environments. This study aimed to establish a molecular identification tool for eDNA for the endangered thick-shelled river mussel Unio crassus Philipsson in Retzius, 1788 and to analyze the relationships between mussel abundance, stream discharge, sampling distance, and freshwater mollusk DNA signal (detectability and DNA quantity) by using binary logistic and linear regression modeling. We additionally correlated turbidity and mollusk DNA detectability. We developed a species-specific eDNA detection system in silico and tested the efficiency and species specificity on water samples collected downstream of extant populations of different sizes (100–42,000 individuals) in Bavaria, Germany. This system allowed detection of very small U. crassus populations over long distances (>3 km) and excluded amplification of DNA from other potentially co-occurring species. Signal strength was most strongly associated with population size and distance of sampling point from mussel population. Additionally, a negative correlation with stream turbidity was evident. This species-specific tool may help in screening of streams for target species that are otherwise difficult to monitor. [ABSTRACT FROM AUTHOR]

This paper presents results of a malacological survey of the Novohradka River, a small river in the Pardubice Region (Czech Republic, Eastern Bohemia, tributary of the Chrudimka River). Fifteen species of aquatic molluscs (7 gastropods, 8 bivalves) were found at 13 sites during the research realised in 2015 and 2017. The research was focused on the population of the endangered bivalve Unio crassus and its recent occurrence was confirmed at four sites, though its population density is low. Fortunately this population is connected with more numerous population in the Chrudimka River.

Endosymbiotic bacterial species that manipulate host biology, reproduction and mitochondrial genetic diversity have been identified in many metazoans, especially terrestrial arthropods. Until now, the hypothesis that Wolbachia or other bacterial endosymbiont might be absent in mollusks has remained unexplored. We present here preliminary data on bacterial communities in a freshwater mussel Unio crassus—species with doubly uniparental inheritance of mtDNA (DUI). Next generation sequencing of 16S rRNA bacterial gene fragment allowed to identify endosymbiotic Cardinium and sequences that were classified to the order Rickettsiales. Finally, we discovered Wolbachia and confirmed Cardinium infection of Unio crassus using bacterial species-specific primers. Discovering Wolbachia and Cardinium infections in Unio crassus opens new opportunities of further investigations in the second largest animal phylum on Earth, very diversified phylogenetically, widespread geographically and inhabiting many environs, including freshwater, inhabited by the most threatened molluscan species. Considering the problems caused by endosymbionts identified in arthropods, the presence of endosymbiotic factor implies possibility of their influence on taxonomy of threatened unionids, on the results of studies of genetic diversity and proper conservation planning. [ABSTRACT FROM AUTHOR]

The freshwater pearl mussel (Margaritifera margaritifera) and the thick‐shelled river mussel (Unio crassus) are relatively widespread across Europe, but are strongly declining and are now protected by the European Habitats Directive. In the course of this study, 20 pearl mussel and 14 thick‐shelled river mussel streams in Bavaria, Germany, were investigated.The mussel populations were mapped to determine population size and age structure. For the assessment of habitat quality, host fish abundance and physicochemical parameters were investigated, e.g. substratum quality, water chemistry, redox potential, and turbidity. Furthermore, potential risks for the populations such as predation or river maintenance were also recorded and assessed.The average population size and recruitment rates of M. margaritifera populations were lower than in U. crassus populations, with 3517 (2.2% juveniles) compared with 5566 (41.4% juveniles) individuals, respectively. On average, 22.3% of particles were smaller than 0.85 mm in diameter at M. margaritifera sites, whereas the mean proportion of fine particles at U. crassus sites was twice as high, at 41.3%. Other parameters such as redox potential or electric conductivity also indicated more favourable habitat conditions in M. margaritifera streams. Unio crassus seems to be less vulnerable to adverse substratum texture and increased nutrient levels than M. margaritifera.The main threats for U. crassus were physical habitat destruction, predation by muskrat, or a lack of host fish, whereas M. margaritifera mainly suffered from siltation leading to a lack of oxygen supply to the interstitial zone, affecting recruitment. Consequently, conservation strategies need to be species‐specific and address stream‐specific reasons for decline. As a basis, accurate and comparable monitoring data are necessary, which implies the standardization of monitoring protocols. [ABSTRACT FROM AUTHOR]

This paper presents results of a malacological survey of the Chrudimka River between the town of Chrudim and the junction with the Labe (Elbe) River; the territory of the Dolní Chrudimka Site of Community Importance (SCI). Twenty-three species of aquatic molluscs (11 gastropods and 12 bivalves) were found at 21 sites during the research in 2013, 2015 and 2016. The occurrence of a population of rare bivalve Pseudanodonta complanata was recorded in the lower stretch. The research was focused on the population of the endangered bivalve Unio crassus which occurrence was confirmed at nine sites, however its population density was always low. The Chrudimka site is nowadays the only site with the occurrence of this bivalve known in Pardubice Region (Czech Republic, Eastern Bohemia).

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Freshwater mussels are one of the most endangered groups of animals. Translocation is a common conservation measure for protecting them, but the success of this procedure varies widely, and the reasons for failure are frequently unknown.The dispersal and mortality of freshwater mussels was studied using a novel method: the radio tracking of Unio crassus translocated to new sites during a reintroduction project for this threatened species in the River Biała (Poland).Individual mussels were equipped with radio transmitters (10 mussels at each of four sites during low water in autumn, and 10 mussels at each of 11 sites during the spring flood). After being released into the water, the mussels generally moved away from the bank, and then performed erratic movements, dispersing over a larger area on flat parts of the riverbed, finally congregating on steep parts of the river bed in shallow water (0.20 m) near the bank.Mussel mortality was surprisingly low: all survived the autumn trial, and 80% of the radio‐tracked adults survived the spring flood. Some of them were displaced quite far within the channel (>100 m), apparently suffering no harm. The confirmed cases of mortality were a few individuals that had become stranded on gravel bars formed during the flood that were now drying out.Long‐distance displacement was more frequent in lotic or regulated parts of the channel and in the parts where the channel morphology was unstable during the flood.Radiotelemetry should be used to test the suitability of receptor sites before translocation, in order to find places where mussels will choose to stay, and to standardize mussel detectability when monitoring relocation success. [ABSTRACT FROM AUTHOR]

Management decisions in conservation of threatened species require trading off social needs against biodiversity values, including the prioritization of conservation locations, i.e. where conservation efforts should take place. To improve conservation decisions for the thick-shelled river mussel, Unio crassus, a highly threatened temporary parasite on fish, we performed a field study on how mussel and host fish density (European bullhead, Cottus gobio, and common minnow, Phoxinus phoxinus) affect reproduction potential of the mussel at different sites along a river. We assumed that the proportions of gravid mussels would be higher at high mussel density, and result in enhanced glochidia (mussel larvae) encapsulation rates on fish. We also expected the highest 'glochidia density'—a proxy for the potential number of recruits per stream area, assessed by multiplying glochidia encapsulation rates on fish by fish density, to occur at high mussel density sites. Such river sites, producing many offspring and conveying important conservation values, may thus be prioritized. However, contrary to our assumptions, higher glochidia density and higher proportions of gravid mussels occurred at lower density mussel sites. We also found that P. phoxinus had higher glochidia encapsulation rates than C. gobio, possibly related to species-specific behavioural and life-history traits. Even so, glochidia density was similar for both fish species, reflecting comparable ecological functions in hosts. The results of this study suggest that mussel and host fish densities should be considered along with glochidia density in conservation prioritization and management trade-offs. [ABSTRACT FROM AUTHOR]

Recent study was carried out on 6 sampling stations chosen from Dipsiz-Çine Stream between March-October 2010 to determine the morphometric features of 176 specimens collected. In the extent of current study, data related with weight, width, length and height, and distribution of individual numbers of Unio crassus, which is inhabiting Dipsiz-Çine Stream, according to months and sampling stations were presented. 176 individuals of Unio crassus were caught by hand and measured by use of caliper and precision scale, and weight, width, length, height parameters were compared according to months and sampling points, and a monthly image of this distribution was presented. Maximum number of mussel individuals determined as 115 from IV. sampling point. Lowest numbers of individuals were found from II. sampling point as 1. It is aimed to contribute to data pool of Unio crassus by this study which includes data concerning the time before the construction of Çine Adnan Menderes Dam.

Mussels of the family Unionidae are important components of freshwater ecosystems. Alarmingly, the International Union for Conservation of Nature and Natural Resources Red List of Threatened Species identifies almost 200 unionid species as extinct, endangered, or threatened. Their decline is the result of human impact on freshwater habitats, and the decrease of host fish populations. The Thick Shelled River Mussel Unio crassus Philipsson, 1788 is one of the examples that has been reported to show a dramatic decline of populations. Hierarchical organization of riverine systems is supposed to reflect the genetic structure of populations inhabiting them. The main goal of this study was an assessment of the U. crassus genetic diversity in river ecosystems using hierarchical analysis. Different molecular markers, the nuclear ribosomal internal transcribed spacer ITS region, and mitochondrial DNA genes (cox1 and ndh1), were used to examine the distribution of U. crassus among-population genetic variation at multiple spatial scales (within rivers, among rivers within drainages, and between drainages of the Neman and Vistula rivers). We found high genetic structure between both drainages suggesting that in the case of the analyzed U. crassus populations we were dealing with at least two different genetic units. Only about 4% of the mtDNA variation was due to differences among populations within drainages. However, comparison of population differentiation within drainages for mtDNA also showed some genetic structure among populations within the Vistula drainage. Only one haplotype was shared among all Polish populations whereas the remainder were unique for each population despite the hydrological connection. Interestingly, some haplotypes were present in both drainages. In the case of U. crassus populations under study, the Mantel test revealed a relatively strong relationship between genetic and geographical distances. However, in detail, the pattern of genetic diversity seems to be much more complicated. Therefore, we suggest that the observed pattern of U. crassus genetic diversity distribution is shaped by both historical and current factors i.e. different routes of post glacial colonization and history of drainage systems, historical gene flow, and more recent habitat fragmentation due to anthropogenic factors.

A total of 95 species of molluscs were recorded in the Kačák brook valley (Central Bohemia, Czech Republic), in its upper and middle part. Most of the terrestrial survey was conducted in the woody part of the studied area, therefore woodland gastropods represent 31% of the total species number, followed by generalists, hygrophilous and steppe species. Only a few species belong to aquatic molluscs. Twenty four species belong to the Red List of mollusc species in the Czech Republic. Critically endangered Anisus vorticulus, endangered Pisidium amnicum and Unio crassus represent aquatic species. Very attractive are wetlands and marshes in the Kačák brook catchment area, hosting two vulnerable species: Vertigo angustior and V. antivertigo.

Abstract: Freshwater mussels (Unionoida) are globally threatened. The key to the development of effective conservation strategies for this group is to unravel their complicated life cycle, including the obligatory larva, which parasitizes the external tissues of fish. Artificial infestation was conducted for three fish species – Barbus barbus, Chondrostoma nasus, and Squalius cephalus – in order to study the prevalence, site, and intensity of infestation, in relation to the fish species and their individual body size. Fish gut contents were also examined, in order to analyse the possibility of glochidia consumption. Glochidia of Unio crassus are considered parasites of fish gills; however, this study revealed that U. crassus glochidia were attached to the fins of all the host species tested. The number of glochidia attached to C. nasus fins was four times higher than the number of glochidia attached to gills. The prevalence of fin infestation was higher for C. nasus and S. cephalus than for B. barbus. Infestation intensity was highest for C. nasus and lowest for B. barbus, and was also dependent on fin type. The number of glochidia attached to fins demonstrated a nonlinear (log10) relationship with fish body length. Inspection of dissected fish guts showed that all of the fish species tested foraged on glochidia: S. cephalus and B. barbus more frequently than C. nasus. Using predatory fish host species with non‐predatory species in mixed assemblages for artificial rearing of mussels led to increased mortality of fish and decreased infestation intensity. The best conservation practices for freshwater mussels should therefore be based on single fish species that are easy to infest and that do not prey on glochidia. [ABSTRACT FROM AUTHOR]

Freshwater mussels of the order Unionida provide important ecosystem functions and services. Unfortunately, some previously widespread species are now seriously endangered. To restore the historical range of the population of Unio crassus in the Biała River, southern Poland, the species was reintroduced into a series of ‘stepping stones’ joining two remnant populations. During the first phase of the study, the relationships between the abundance of U . crassus , physical habitat, and water quality were studied to assess reintroduction potential. In general, chemical water quality improved upstream from the existing population, favouring the decision for reintroduction, whereas morphological variables worsened. Mussel abundance was correlated negatively with the elevation and slope of channel, organic matter contents, and pH (exceeding 8.0), but positively with silt presence, water conductivity, and concentrations of HCO 3 − , Ca 2 + , and NO 3 − . During the second phase, adult individuals were introduced into one type of functional habitat—marginal channel sectors with still water and fine sediment. Despite the initial very high rate of reproduction in some parts of the upper reach of the river, the juveniles were ultimately recruited only in the lower part of the restored range, resulting in a very rapid change in recruitment at a channel slope of 1.8‰. Recruitment was positively related to silt content, conductivity, and Ca 2 + and HCO 3 – ions, negatively to channel elevation and slope, and water pH. The host fish species showed no correlation with abiotic habitat features within the studied reach. These results imply that most of the habitat traits related to U . crassus occurrence depended on the river's longitudinal profile, not on the chemical water quality, and that final success of introduction should be evaluated after several years. [ABSTRACT FROM AUTHOR]

Adaptation to temperature regimes and host presence may enhance fitness in parasites. In an experimental study, we evaluated the timing of glochidia release by Unio crassus subjected to three spring water temperature regimes in the presence and absence of the host fish Cottus gobio. The timing of glochidia release was delayed at (i) constantly low temperatures (<10°C), in contrast to earlier and pronounced releases at (ii) natural temperature increases that level off at intermediate temperatures (10–15°C), and (iii) higher-than-normal temperatures (10–20°C). Mussels from treatment (i) that had not released glochidia during the experiment did so soon after being moved to the temperature in (ii), indicating a temperature threshold for glochidia release. Neither host fish presence nor the combined effect of temperature and host fish presence significantly affected the timing of glochidia release. The treatment with natural spring water temperatures indicated possible fitness benefits for U. crassus through combined effects of high intensities of glochidia releases and high survival of released glochidia. The furthered understanding of climate change effects on mussel and host phenology in seasonal environments, potentially inducing temporal mismatches of glochidia release to host availability, is key to mussel conservation. [ABSTRACT FROM AUTHOR]

We studied shell variation in the thick-shelled river mussel (Unio crassus Philipson, 1788) sampled from two sites of very different character: (1) a rocky channel (San river) and (2) a fine-sediment channel (Zborowianka river). The analyzed mussels differed significantly between the sampled channels in almost all analyzed phenotypic traits and shape indices. Intersexual variation was so low that it cannot be used effectively for sexing in this species. The growth rate was much higher in the fine-sediment and nutrient-rich channel than in the rocky one, but the asymptotic shell length for the rocky channel was larger than for the fine-sediment channel, suggesting higher survival in the mountainous, unmodified river. Shell size differed significantly within the rocky channel, depending on microhabitat: shell size and dorsal arching were much greater in mussels living in the strong current of the rocky midchannel than in those inhabiting still water at the nearest bank. The results demonstrate that microhabitat conditions significantly determine shell shape. [ABSTRACT FROM AUTHOR]

1. Parasite-host interactions can involve strong reciprocal selection pressure, and may lead to locally adapted specializations. The highly threatened unionoid mussels are temporary parasites on fish, but local adaptation has not yet been investigated for many species. 2. Patterns of local adaptation of one of Europe0s most threatened unionoids, the thick-shelled river mussel (Unio crassus) were investigated. Eurasian minnows (Phoxinus phoxinus) from two rivers (separate drainage areas) were cross-infested in the laboratory with sympatric and allopatric mussel larvae, while bullheads (Cottus gobio), inhabiting only one of the rivers, were infested with sympatric or allopatric mussel larvae. Larval encystment, juvenile mussel excystment and survival were measured. 3. For one river, but not the other, juvenile excystment from P. phoxinus was highest when infested with sympatric mussels. The opposite pattern was found for C. gobio in this river, where juvenile excystment and post-parasitic juvenile survival from allopatric C. gobio were highest. The results thus cannot confirm local adaptation of U. crassus to P. phoxinus in the study rivers, as excystment was not consistently higher in all sympatric mussel-host combinations, whereas there were potential maladaptive signs of U. crassus in relation to C. gobio. There was no loss of encysted larvae 3 days after infestation until juvenile excystment. Most juveniles were excysted between 17 and 29 days after infestation, and the numbers of excysted juveniles increased with fish size. 4. The results have implications for parasite-host ecology and conservation management with regard to unionoid propagation and re-introduction. This includes the need to (1) test suitability and adaptation patterns between U. crassus and multiple host fish species, (2) evaluate the suitability of certain unionoids and host fish strains after more than 3 days, and (3) determine whether large fish produce more juvenile mussels than smaller fish. [ABSTRACT FROM AUTHOR]

Usefulness of Passive Integrated Transponder (PIT tag) technology for observation of behaviour and life history of endangered Unio crassus was tested in two Polish mountain rivers. Dispersion of PIT marked individuals from the place of release did not exceed 3 m. The detection of marked individuals on the rocky bottom was very low (13-39%) and decreasing with time. Ca. 1/3 of implanted PIT tags were rejected, usually within two weeks after implanting; later the rejection did not occur as the PIT tag became fixed in the nacre (very thin on the PIT tag surface adjacent to the flesh, thick with additional fixing structures adjacent to the shell). Nevertheless, 33 tags were detected after three years, some in live individuals which were more numerous on the soft sediment bank (n=12) than on the rocky bottom (n=6). The influence of electromagnetic field on the detection of PIT tags, the possible causes of the tag rejection and mechanisms of tag retention are discussed. It is suggested that PIT tags could be useful as a method of durable individual marking but less suitable for detecting and/or locating the mussels. Controlling of possible tags rejection is indispensable. [ABSTRACT FROM AUTHOR]

Large freshwater mussels (Unionoida) are declining throughout the world. The European bitterling Rhodeus amarus (Bloch, 1782) female spawns its eggs inside the unionids' shells, where fertilisation and further embryonic development take place; thus its reproduction depends fully on the presence of large freshwater mussels. Unio crassus, previously regarded as one of the most numerous unionids in Europe, is now listed in the IUCN Red Data List as being globally endangered. Despite its previous prevalence, it was never reported as a host for the bitterling. A large population of U. crassus was studied in small river at the Świętokrzyskie Mts (Poland), where also electrofishing was conducted. In each bitterling territory located on the study plots, we found individuals of U. crassus, with the bitterling eggs or larvae developing on mussel's gills. That proves that this species can be also used by the bitterling for reproduction. We suggest that this relationship has not been reported to date due to the mussels' rarity and ongoing decline. However, it is also possible that the endangered mussel is a novel host for the bitterling, which is expanding its range throughout Europe. [ABSTRACT FROM AUTHOR]

The thick-shelled river mussel, Unio crassus (Bivalvia: Unionoida), is one of Europe's most-threatened mussels. Finnish populations of U. crassus lie close to the northern limit of its natural distribution. Extirpation of these populations will reduce the range of this endangered species., Growth characteristics of U. crassus were measured in a river running through the Helsinki metropolitan area. Shell dimensions (size-at-age data) and annual shell growth increments were used to reconstruct growth rate and its variation during the lifespan of individual mussels and to investigate the relationship between growth rate and longevity (age-at-death)., Reconstructed growth rates compared well with size-at-age data conventionally used to study individual growth in natural populations, and fitted von Bertalanffy growth functions very well. Based on the same function, reconstruction and size-at-age methods resulted in similar estimates of growth rate., Shell weight explained the body size more reliably than age, suggesting that growth rate varied significantly among individuals. Comparison of individual growth histories revealed a negative correlation between age-at-death and growth rate, i.e. slow-growing mussels lived longer, and vice versa., In comparison with populations from central and southern Europe, U. crassus populations in the north of its range grew more slowly but lived longer, a phenomenon most likely explained by latitudinal changes in ambient temperature., Although northern populations are expected to benefit from a relatively high number of reproductive periods and lower juvenile mortality, the individuals studied here died earlier and suffered higher mortality than expected and an elevated conservative status is identified. Any management programme should take into account the life-history traits essential to the recognition of management units of U. crassus., Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The availability of a rapidly growing number of complete mitochondrial genome sequences provokes high confidence dating approaches. However, even if the congruence between mitochondrial and nuclear markers is reasonable, the resulting topologies are frequently questionable. The unique opportunity to study the evolutionary history of two independent mitochondrial genomes in one phylogenetic context exists in the freshwater mussels family Unionidae. The two lineages function under doubly uniparental inheritance since well before the emergence of the family. Despite the relatively high number of available complete sequences of maternally inherited genomes, comparative analyses are limited by the small number of sequences of counterpart paternally inherited genomes. We have sequenced for the first time the representative set of five sequences (two maternal and three paternal) from the species Unio crassus . Comparative analysis of the phylogenies reconstructed using relevant mitogenomic data available in GenBank (13 species in total) reveal that single – genome inferences are congruent only if the relaxed clock is assumed. [ABSTRACT FROM AUTHOR]

This paper presents results of a malacological survey of four small tributaries of the brook Javorka near Lázně Bělohrad in East Bohemia (Czech Republic). Only 15 species of aquatic molluscs (7 gastropods, 8 bivalves) were found at 31 sites during research realised in 2010, 2011 and 2012. Molluscan assemblages of all tributaries were similar and consisted of common and widespread molluscs as Galba truncatula, Radix peregra, Anodonta anatina, Pisidium subtruncatum or P. casertanum. The occurrence of endangered river pea mussel (Pisidium amnicum) was confirmed at three tributaries but only at lower parts near its inflows to the brook Javorka. The research was focused on the endangered thick-shelled river mussel (Unio crassus), and the occurrence of numerous population was confirmed at the brook Lukavecký potok. In the brook Chotečský potok a small population of this bivalve was found. Only a few specimens were found at remaining two tributaries.

This paper presents results of a malacological survey of the Lužnice River in ca 70 km long stretch between the inflow of its main tributary the Nežárka River in Veselí nad Lužnicí and its confluence with the Vltava River in Southern Bohemia (Czech Republic). Thirty-one species of aquatic molluscs (19 gastropods, 12 bivalves) representing 40% of freshwater molluscan fauna of the Czech Republic were found at 15 sites. Differences of molluscan assemblages upstream and downstream of Tábor town were probably caused by different characters of the watercourse. The occurrence of rare bivalve Pseudanodonta complanata was confirmed. On the other hand, two non-native species Physella acuta and Sinanodonta woodiana were recorded. The research was preferentially focused on the population of the endangered bivalve Unio crassus. Its occurrence was confirmed at 8 from 15 sites under study but altogether only 31 individuals were found. Less than four specimens were found at particular sites but one.

This paper brings results of a malacological survey of the Bystřice River on the territory of the Site of Community Importance (SCI) Bystřice (Eastern Bohemia, Czech Republic). The research was focused on the population of endangered bivalve Unio crassus which is the main object of conservation of this SCI. Twenty one species of aquatic molluscs (11 gastropods, 10 bivalves) were found in 2010. Most of the molluscs belong to common and widespread species. Altogether 26 species have been found during research in the Bystřice River since 2000. Expansion of non-native species Potamopyrgus antipodarum and Physella acuta is the most visible change in comparison with the previous research. The research in 2010 confirmed existence of the occurrence of Unio crassus all over the SCI and also upstream of this site. The population density documented in 2010 was higher than that found during the previous research. In the upper part of the SCI, the density of U. crassus population was at least 1 ind./1 m of the river stream while in the lower part it did not exceed 1 ind. per 10 m of the river. The reason was the canalisation of the river and the absence of suitable sediments. The occurrence of the bivalve was not recorded downstream of the SCI.

This paper brings a review of a malacological survey of the Vsetínská Bečva River, its tributaries, canals and other different aquatic habitats in surroundings (Eastern Moravia, Czech Republic). Vsetínská Bečva is a 58 km long river which rises in 870 m a.s.l. and joins Rožnovská Bečva River in Valašské Meziříčí at 288 m asl. Research was focused on endangered bivalve Unio crassus. Aquatic molluscs at 63 sites were studied in 2007. Altogether 19 species of aquatic molluscs (12 gastropods, 7 bivalves) were found. Three non-native species Potamopyrgus antipodarum, Physella acuta, and Gyraulus parvus, which are rare in Moravia, were found. Occurrence of vulnerable but in this region relatively common prosobranch Bythinella austriaca was confirmed in springs and rivulets. Research based on bivalves provided more interesting results. Existence of two large populations of Unio crassus were documented in two canals (mill races) in Hovězí and Vsetín. Populations of Unio crassus were estimated at 1500–2500 individuals in Hovězí and 1000–4000 individuals in Vsetín. Occurrence of young individuals of Unio crassus were confirmed in both cases. Rare bivalves Pisidium amnicum and P. moitessierianum were found at one site in Canal in Vsetín and also non-native Dreissena polymorpha was found at one site – the gravel pit near Nový Hrozenkov.

This paper brings a review of a malacological survey of Malá Bečva River (Central Moravia, Czech Republic). Malá Bečva is a canal 18 km long and approximately 7 m wide, which runs from the Bečva River near Troubky and flows into the Mostěnka stream (influent to the Bečva River) in altitude 190 m. Aquatic molluscs sites were studied in 2007 at 9 sites. Altogether 23 species of aquatic molluscs (9 gastropods, 14 bivalves) were found. Except non-native species Potamopyrgus antipodarum, which is rare in Moravia, only the occurrence of common and widespread gastropods was documented, while research based on bivalves provided more interesting results. Large populations of three endangered species Unio crassus, Pisidium amnicum, and P. moitessierianum were documented. Population of Unio crassus was estimated at 5000-20000 individuals. This species and also Pisidium amnicum occurred only in the upper part of the Malá Bečva River from Troubky to Chropyně. Aquatic malacofauna of this part showed similarity with Strhanec Canal, which is canal of the Bečva River situated less than 10 km upstream of the Malá Bečva River. Bivalves Unio tumidus, Sinanodonta woodiana, Sphaerium rivicola, S. corneum and also Anodonta cygnea and Pseudanodonta complanata occur only downstream of Chropyně. This change of the malacofauna is probably caused by high nutrient tributary flows into the Malá Bečva River near Chropyně.

This contribution presents new data on the occurrence of the thick-shelled river mussel Unio crassus in the Stradomka river and its tributaries (S. Poland), not surveyed earlier. Live specimens and empty shells were found in 14 localities, mostly in the Stradomka river. [ABSTRACT FROM AUTHOR]

The thick-shelled river mussel Unio crassus is an endangered freshwater bivalve species with a life cycle that includes an obligate glochidial larval stage that attaches to the gills of freshwater fish for several days to months., Fish hosts were identified in nine north-eastern French rivers in order to improve unionid conservation management., Eurasian minnow Phoxinus phoxinus was the most important host at all study sites, with prevalence ranging from 15-100% (122 of 238 individuals infested), and bullhead Cottus gobio usually the second most important (prevalence 0-73%; 37 of 121). Other host species had a relatively low rate of infestation: chub Squalius cephalus (5 of 41), roach Rutilus rutilus (1 of 23), gudgeon Gobio gobio (2 of 50), stone loach Barbatula barbatula (2 of 58) and dace Leuciscus leuciscus (1 of 19). No glochidia were found on spirlin Alburnoides bipunctatus (0 of 24)., Parasitic load usually ranged between one and three per fish (60% of cases), and rarely exceeded 10. First results suggest that U. crassus glochidia influence the weight:length ratio of P. phoxinus but not C. gobio, with infested fish being heavier., To date, the ecological function of P. phoxinus (and C. gobio) in small streams has been largely underestimated or ignored as the species is small, common and of no interest to anglers. There is increasing evidence, however, that P. phoxinus deserves conservation management owing to the essential role it plays in the reproduction, development and distribution of U. crassus., Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Conservation and restoration strategies require knowledge of the autecology of target species. The thick-shelled river mussel Unio crassus is an endangered freshwater bivalve with largely unknown habitat preferences. In this study, hydrological and substrate parameters including shear stress, flow velocity and penetration resistance of substrate were examined in relation to mussel presence/absence data in six streams with recruiting and self-sustaining populations to derive information on the hydrological and substrate habitat requirements of the species., Mean flow velocities between 0.11 and 0.31 m s−1 and absolute maximum values in the study streams ranging from 0.22 to 0.95 m s−1 indicate that the species is tolerant of a wide range of hydrological conditions., Areas with high mussel densities were characterized by low flow velocities (<0.3 m s−1), low mean penetration resistances (0.36 ± 0.52 kg cm−2) as well as by low near-bed shear stress (1.06 ± 0.33 x 10−6 N cm−2) compared with non-colonized sites., Maximum near-bed flow velocity at mussel beds was 0.33 m s−1 and 0.35 m s−1 at 60% depth. Higher flows with maximum velocities of 0.52 m s−1 and 0.95 m s−1 at 60% depth were measured at non-colonized sites. Mussel density was significantly negatively correlated with near-bed shear forces., The results indicate that the hydrological and substrate characteristics of U. crassus habitat are broader than expected. In contrast to the assumption that it depends on moderate to high flows (>0.3 m s−1), streams with low water flow and soft substrate were also identified as suitable habitats., Functional characteristics of the substrate, especially stability and areas with low shear stress (<1.5 N cm−2), seem to be of great importance for the persistence of the species, and monitoring protocols used at present for habitat assessment of U. crassus should be updated., Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Mollusc communities of the Ohře (Eger) River, which drains a large part of northwestern Bohemia, were studied. In total, 43 freshwater mollusc species (22 gastropods, 21 bivalves) were found at 56 sites. The richest malacofauna (31 species) was recorded in the lower river section. One empty shell of Margaritifera margaritifera, a species regarded as critically endangered in the Czech Republic, was found. There are no previous records of this bivalve in the Ohře River and its tributaries, and its recent occurrence there is unlikely. Planorbis carinatus, Unio crassus, Pseudanodonta complanata, Pisidium amnicum and P. moitessierianum, endangered in the Czech Republic, were recorded. The population of U. crassus is the largest in northwestern Bohemia and one of the largest in the Czech Republic. It is divided in two parts by the Nechranice dam lake. Likewise, populations of other species are negatively affected by the dam, as well as by the pollution and watercourse alteration. In some sites the muskrat (Ondatra zibethicus) was observed to predate on unionids and especially on U. crassus. The occurrence of five alien species (Potamopyrgus antipodarum, Physella acuta, Gyraulus parvus, Corbicula fluminea, Dreissena polymorpha) was confirmed. A gradual invasion of C. fluminea was observed; the species is predated by an unidentified animal, probably also muskrat. [ABSTRACT FROM AUTHOR]

Information on the growth and age of the endangered freshwater mussel Unio crassus Philipsson, 1788 is crucial to correctly assess the conservation status of a particular population. This long-term study of individually marked specimens from four populations from central and southwest Germany revealed that zero growth and shrinkage of adult mussels from the upper 50% of their population's length range were common phenomena. Therefore, overall shell length measurements or counting of growth lines on the shells of live adult mussels may grossly underestimate their age and should be regarded as unsuitable for determining the actual age structure of a population. [ABSTRACT FROM AUTHOR]

Freshwater mussels of the order Unionoida have life cycles that include larval attachment to and later metamorphosis on suitable host fishes. Information on the trophic relationship between unionoid larvae and their host fishes is scarce. We investigated the trophic interaction between fish hosts and encysted larvae of two species of freshwater mussels, Margaritifera margaritifera and Unio crassus, using stable isotope analyses of larvae and juvenile mussels as well as of host fish gill and muscle tissues before and after infestation. Due to different life histories and durations of host-encystment, mass and size increase in M. margaritifera during the host-dependent phase were greater than those of U. crassus. δ13C and δ15N signatures of juvenile mussels approached isotopic signatures of fish tissues, indicating a parasitic relationship between mussels and their hosts. Shifts were more pronounced for M. margaritifera, which had a five-fold longer host-dependent phase than U. crassus. The results of this study suggest that stable isotope analyses are a valuable tool for characterizing trophic relationships and life history strategies in host-parasite systems. In the case of unionoid mussels, stable isotopic shifts of the larvae are indicative of the nutritional versus phoretic importance of the host. [ABSTRACT FROM AUTHOR]

The recognition of management units ( MUs) that respect interpopulation distinctions in management needs is central to many biological applications addressing species conservation, biological invasions and ecosystem processes., We present a methodological approach for the evaluation of population-level differences in the host compatibility of natural populations of affiliate (dependent) species. Two experiments were performed to diagnose the sources of variability in the relationships between an endangered freshwater mussel Unio crassus and its host fish species in a fragmented river system in Central Europe., Despite the common approach applied to the conservation of U. crassus throughout the study area, we identified differences in the ability of U. crassus to infest particular host fish species between nearby and recently isolated mussel populations. These differences could strongly influence their reproductive success under a constant conservation regime., The observed differences were accompanied by genetic and morphometric distinctions, as revealed by microsatellite, mt DNA and elliptic Fourier descriptor analyses. The genetic data indicated substantial reproductive isolation, which could have resulted in adaptive or random changes responsible for the differences in host compatibility., We showed that experimental testing of physiological host compatibility can be effectively used for the detection of different MUs of U. crassus, with direct implications for the management of host populations. In combination with traditional genetic and morphometric techniques, this approach may provide a general framework for the recognition and care of management units of affiliate species., Synthesis and applications. Population-level evaluations of host compatibility should be utilized in management unit recognition and the formulation of management targets for affiliate species. We demonstrated that small-scale cross-compatibility testing can be effectively used to diagnose the sources of variability in host relationships with direct management implications. This approach can enhance the targeting of management actions in many biological applications addressing species conservation, biological invasions and ecosystem processes. [ABSTRACT FROM AUTHOR]

Copyright of Knowledge & Management of Aquatic Ecosystems is the property of EDP Sciences and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

ABSTRACT The complex life-cycle of freshwater mussels from the superfamily Unionoidea includes an obligate parasitic phase on a suitable host fish, which makes freshwater mussels particularly susceptible to disruption by environmental stressors. In particular, temperature stress due to rising ambient temperatures resulting from heated effluents, damming of streams and deforestation of banks or climate change may have detrimental effects on threatened mussel species., In this study, the effect of four different temperature regimes (12 °C, 17 °C, 20 °C and 23 °C) on the development and metamorphosis success of Unio crassus encysted on Phoxinus phoxinus was investigated., The highest metamorphosis success as well as the lowest host mortality were found at a temperature of 17 °C. Duration of development was inversely related to the temperature at which the hosts were maintained., Excystment of living juvenile mussels was found between 265 and 357 degree-days for 23 °C and between 433 and 632 degree-days for 12 °C., In conclusion, water temperature during the parasitic phase of U. crassus plays an important role in development and metamorphosis and should be considered in the conservation of the species through catchment management and in the artificial propagation of this species as well as in other endangered mussel species., Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Copyright of Natura Croatica is the property of Natura Croatica and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The malacofauna of the Dyje River, including the lower sections of its five tributaries, is still rich and diversified despite the pollution, regulation and building of several dam reservoirs. In total, 42 freshwater mollusc species (24 gastropods, 18 bivalves) were found at 75 sites. The richest malacofauna (30 species) was recorded in the lower section of the Dyje. Populations of endangered or rare molluscs Theodoxus danubialis, Viviparus acerosus, Lithoglyphus naticoides, Unio crassus, Pseudanodonta complanata, Pisidium amnicum and P. moitessierianum were found. The first three species inhabit only the area under study in the Czech Republic. Especially the occurrence of Unio crassus corresponds with river sections that were only partly affected by human activities (long distance below dam reservoirs, less polluted sections, unregulated watercourses). The occurrence of five invasive species, four originating from other continents (Potamopyrgus antipodarum, Physella acuta, Ferrissia fragilis, Sinanodonta woodiana) and one (Dreissena polymorpha) originally Ponto-Caspian, was confirmed for the Dyje River and its tributaries. The Dyje River formed the main invasion route from the Danube River into Moravia (eastern part of the Czech Republic). [ABSTRACT FROM AUTHOR]

The thick-shelled river mussel Unio crassus is critically endangered throughout its range as a result of increasing human activity and habitat loss. Next generation DNA sequencing was used to develop a set of microsatellite markers that can be used for future ecological and population genetics studies of U. crassus. A total of 11 polymorphic loci were identified and characterized using 57 individuals from two Polish populations. Numbers of alleles ranged from 2 to 15 and expected heterozygosity levels ranged from 0.069 to 0.899. Deficiency of heterozygous genotypes was observed in four loci. Marker independence was confirmed with tests for linkage disequilibrium, however, analyses indicated evidence of null alleles in four loci. The microsatellite markers developed specifically for U. crassus provide a valuable tool for future ecological, population genetic assessments, and conservation management of this species. J. Exp. Zool. 319A:113-116, 2013. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The toxicity of three highly volatile organic solvents - tetrachloroethane (Cl2HC-CHCl2), tetrachloroethylene (Cl2C=CCl2), trichloroethylene (ClHC=CCl2) - was studied on Unio crassus specimens. The tested chlorinated hydrocarbons sink to the bottom of standing water bodies due to their high density; therefore they represent an increased risk for benthic organisms. For a start, all three compounds were applied in 100% saturation. In the case of tetrachloroethylene and trichloroethylene all specimens died within a few days. In the case of tetrachloroethane the intoxication process was much faster, all the specimens died within 4 hours. Tetrachloroethane proved to be the most toxic compound; its diluted solutions were further tested: in the 5% solution all U. crassus specimens died after 24 hours. The 1% solution had a strong disturbing effect on the behaviour of the bivalves, which was gradually moderated and eventually stopped on the 33rd day. These results suggest that the examined chlorinated hydrocarbons are strongly toxic to living organisms even in aquatic medium and cause rapid death even in low concentrations. [ABSTRACT FROM AUTHOR]

The conservation of endangered affiliate species, which are critically dependent on the presence of another species, is often hindered by a poor understanding of the relationships between the interacting partners. The parasitic stage of endangered unionid bivalves constitutes a tight host-affiliate linkage between the mussels and their host fishes. However, the threats resulting from potential shortages of the host species are rarely sufficiently quantified and incorporated into conservation strategies. In this paper, we integrated both host quality and availability analyses to assess the potential threats to the endangered thick-shelled river mussel Unio crassus that result from the impairment of its host resources in Central Europe. The experimental determination of the compatibility of U. crassus parasitic larvae (glochidia) with its potential host fishes revealed an intermediate level of host specificity. At least, some glochidia successfully developed on 14 of the 27 potential hosts that were evaluated. Nevertheless, only three fish species ( Scardinius erythrophthalmus, Phoxinus phoxinus and Cottus gobio) enabled the majority of the attached glochidia to transform successfully. Subsequently, our analysis of host availability at sites inhabited by living or extirpated populations of U. crassus showed that the local extirpations of U. crassus are likely associated with an impaired status of the fish assemblage and with the absence of the primary host fishes. These results indicate that the availability of host fish resources may have played at least an additive role in the present pan- European decline of U. crassus and that the evaluation of host limitation without precise data on the host compatibility or host abundance may be ineffective for identifying the threats to particular species. It demonstrates a strong need for more thorough incorporation of host limitation issues into conservation strategies for U. crassus and probably also for other species of freshwater mussels that were previously considered safe from host limitation. [ABSTRACT FROM AUTHOR]

Abstract: Successful conservation strategies require consideration of species interactions. In their life cycle, freshwater mussels of the family Unionidae depend on a suitable host fish on which their larvae metamorphose into juveniles. This study investigated the host–parasite interaction of different fish species with the endangered European thick-shelled river mussel (Unio crassus) under artificial and natural conditions. Chondrostoma nasus, Cottus gobio, Leuciscus idus, Phoxinus phoxinus, Squalius cephalus, Scardinius erythrophthalmus and three different strains of Gasterosteus aculeatus were identified as suitable hosts. In contrast, U. crassus was not able to metamorphose on Acipenser ruthenus and Rutilus rutilus. In natural U. crassus streams, 16 different fish species were found, with pronounced differences in the occurrence and abundance of suitable hosts. Data from the laboratory infections and from the field investigations strongly suggest that S. cephalus is the most important host for U. crassus in the Danube drainage, despite the fact that some populations exclusively depend on other hosts. The results of this study indicate that (i) an evaluation of host fish importance in unionid mussels should not exclusively rely on standardized infestation experiments without considering natural fish communities, (ii) host suitability can vary within different strains of the same host species, (iii) development time of U. crassus on different hosts varies considerably, and (iv) natural recruitment in functional U. crassus populations can be secured by highly different fish community structures and densities. The wide host range of U. crassus is likely to increase the chances of successful restoration of priority populations for conservation. [Copyright &y& Elsevier]

ABSTRACT [ABSTRACT FROM AUTHOR]

In the framework of the research program "Invertebrate faunistical investigation of the Maramureş", numerous molluscs were collected between 2004 and 2008. This research project also provided a good opportunity to revise the collection of János Wagner (H. Wagner), because a part of the material, collected between 1940 and 1944 by the late staff of the Hungarian Natural History Museum could be preserved there. We present an annotated check-list of the mollusc fauna of Maramureş county, including literature data, some unpublished material from the Wagner collection as well as the results of recent collectings. Compared to other parts of the Carpathians, the mollusc fauna of Maramureş county is relatively poor, considering either the total species richness or the number of endemic and rare taxa. From faunistical and conservation point, Unio crassus, Drobacia banatica, Arianta picea and the Bythinella species are worth mentioning. An important outcome of this study was the discovery of Bythinella molcsanyi type material, which was believed to be fully destroyed to now. [ABSTRACT FROM AUTHOR]

Thick shelled river mussel Unio crassus (included in IUCN Red Data List) is a benthic, filter-feeding animal very susceptible to any changes of water chemistry. In this paper we report on the new sites of this species in the Małopolska region -- an area of strong anthropogenic pressure. Physical and chemical parameters have been compared between 8 rivers inhabited and not inhabited by the mussel. Results indicate that despite economic development of the region, it retains clear rivers; however, subtle differences in water chemistry can influence the occurrence of this endangered species. [ABSTRACT FROM AUTHOR]