Freshwater Mussels Research Articles

Controls on the Barium and Strontium Isotopic Records of Water Chemistry Preserved in Freshwater Bivalve Shells.

Biogenic carbonates, including bivalve shells, record past environmental conditions, but their interpretation requires understanding environmental and biological factors that affect trace metal uptake. We examined stable barium (δ138Ba) and radiogenic strontium (87Sr/86Sr) isotope ratios in the aragonite shells of four native freshwater mussel species and two invasive species in five streams and assessed the effects of species identity, growth rate, and river water chemistry on shell isotopic composition. Shells were robust proxies for Sr, accurately reflecting 87Sr/86Sr ratios of river water, regardless of species or growth rate. In contrast, shell δ138Ba values, apart from invasive Corbicula fluminea, departed widely from those of river water and varied according to species and growth rate. Apparent fractionation between river water and the shell (Δ138Bashell-water) reached -0.86‰, the greatest offset observed for carbonate minerals. The shell deposited during slow growth periods was more enriched in lighter Ba isotopes than the rapidly deposited shell; thus, this phenomenon cannot be explained by aragonite precipitation kinetics. Instead, biological ion transport processes linked to growth rate may be largely responsible for Ba isotope variation. Our results provide information necessary to interpret water chemistry records preserved in shells and provide insights into biomineralization processes and bivalve biochemistry.

Description of a new freshwater mussel species of Pletholophus, Simpson, 1900 (Bivalvia, Unionidae) from Guangdong, China

The Pearl River Basin, China’s second-largest freshwater basin, hosts a significant diversity of species and a highly endemic freshwater mussel fauna. In this study, a new species from the Liuxi River in Guangzhou, Guangdong, China, Pletholophus guangzhouensissp. nov., is described based on morphological diagnostic features and molecular phylogenetics. The glochidia shells of the new species are subtriangular, medium-sized, and have a styliform hook on the ventral angle of each valve. Phylogenetic analyses based on the COI and 28S rRNA gene fragments indicated that Pletholophus guangzhouensissp. nov. is the sister to Pletholophus tenuis + Pletholophus reinianus. The pairwise uncorrected COI p-distance analysis demonstrated genetic distances ranging from 5.27% (between P. guangzhouensissp. nov. and P. tenuis) to 11.06% (between P. guangzhouensissp. nov. and P. honglinhensis). Our findings suggest a significant underestimation of the diversity of freshwater mussel species in Guangdong. Further field collections and systematic studies are necessary to fully explore the biodiversity of this region. Furthermore, integrative classification methods and genetic research are essential for informing the development of effective conservation strategies.

Multisystemic inflammatory disease in Pheasantshell (Unionidae, Actinonaias pectorosa) associated with Yokenella regensburgei infection at sites experiencing seasonal mass mortality events.

Freshwater mussels are integral components of riverine ecosystems, influencing water quality, nutrient cycling, and habitat characteristics. Enigmatic freshwater mussel declines, often characterized by sudden mass mortality events, pose significant challenges to conservation efforts. The Clinch River, a freshwater biodiversity hotspot in Virginia and Tennessee, USA, has experienced several enigmatic mass mortality events since 2016. Studies have reported bacteria associated with moribund Pheasantshell (Actinonaias pectorosa) during mortality events in the Clinch River, specifically Yokenella regensburgei. Despite reports of bacterial infection, little is known about their role as pathogens. Through a multiyear case-control study, combining in-situ experiments, field surveys, histology, bacterial isolation, and high-throughput sequencing, we assessed the role of bacteria in Pheasantshell (Actinonais pectorosa) mortality at three sites in the Clinch River. Between May 2021 and December 2023, we collected 21 wild moribund free-living A. pectorosa and 68 hatchery-reared A. pectorosa maintained in silos at the same sites and investigated differences in pathology and microbiologye between groups. No silo mussels presented clinical signs of disease, or gross or microscopic lesions associated with pathological conditions leading to mortality. Our findings reveal a significant association between Yokenella regensburgei and severe multisystemic and multifocal infiltrative hemocytosis with necrosis, consistent with sepsis. Lesions associated with yokenellosis were of sufficient severity and physiological significance to explain mortality in infected hosts. Although our study does not explain the cause of these infections, it confirms that mussels at our study sites are ultimately dying with an infectious disease and that Y. regensburgei can be pathogenic in free-living mussels. Our results underscore the importance of considering bacterial diseases in wild mussel populations and emphasize the need for further research to elucidate the epidemiology and pathogenicity of Y. regensburgei. Overall, our study highlights the importance of integrated approaches combining pathology, microbiology, and epidemiology in freshwater mussel conservation efforts.

The potential compensatory mechanism between pmbA and tldD governing the virulence of Aeromonas veronii and its implications on the immune response in freshwater bivalve (Hyriopsis cumingii)

Aeromonas veronii is an opportunistic pathogen that causes diseases in aquatic animals, but its key virulence factors remain unclear. In order to evaluate the feasibility of potential compensatory mechanism to be a novel target for developing live attenuated vaccine, virulence-related phenotypes and the immune response of H. cumingii were measured. Then two single-deletion mutants ΔPmbA, ΔTldD and one double-deletion mutant ΔP&T were constructed by homologous recombination. The hemolytic activity of double-deletion mutant ΔP&T showed dramatically lower than wide-type GL2 (P < 0.05), and the two genes could significantly regulate the proteolytic activity on skimmed milk (P < 0.05), except for casein (P > 0.05). All mutants exhibited attenuated capacities to form biofilms (P < 0.05), and pmbA mRNA was upregulated dramatically (P < 0.05) under exogenous threats modeled by osmotic stress, in which tldD mRNA was overlapped. Besides, the absence of one gene resulted in significantly higher (P < 0.05) mRNA level of another gene. ftsH mRNA was restrained dramatically (P < 0.05) in all mutants, and aerA mRNA was suppressed remarkably (P < 0.05) in ΔP&T. Challenge tests showed that the pathogenicity of ΔP&T on Hyriopsis cumingii was further reduced compared with the two single-deletion mutants, and its LD50 was 6.31-fold than that of GL2. In addition, enzymatic activities in the hemolymph and hepatopancreas of ΔP&T group were significantly lower (P < 0.05) than those in GL2 group at different time points, and their peaks in hepatopancreas were found almost at 12 h post challenge, which were supported by the qPCR results of HcIL-17 and TR. Besides, histopathological examination displayed that the degree of tissue damage in ΔP&T group was alleviated remarkably compared with GL2 group. Finally, bacterial loads in the hemolymph and hepatopancreas of ΔP&T group were lower significantly (P < 0.05) than other groups. The above results demonstrate that the compensatory mechanism between the genes pmbA and tldD is a novel target to attenuate A. veronii, and the double-deletion mutant ΔP&T is helpful for the development of live attenuated vaccine.

Reproductive Strategies and Embryonic Development of Autumn-Spawning Bitterling (Acheilognathus rhombeus) within the Mussel Host.

We investigated the reproductive strategies and embryonic development of Acheilognathus rhombeus (a bitterling species that spawns in autumn) within its freshwater mussel host in the Bongseo Stream, South Korea. By focusing on survival mechanisms during critical stages of embryonic development, the selective use of mussel gill demibranchs by the bitterlings and associated adaptive traits were observed over 1 year. A significant diapause phase occurs at developmental stage D, which lasts for approximately 7 months, allowing embryos to survive winter. Development resumes when the temperature exceeds 10 °C. Minute tubercles on the embryos (crucial for anchoring within the host gill demibranchs and preventing premature ejection) exhibit the largest height during diapause, and the height decreases when developmental stage E is reached, when growth resumes. Acheilognathus rhombeus embryos were observed in 30.5% of the mussels, mostly within the inner gills, thereby maximizing spatial use and oxygen access to enhance survival. These results highlight the intricate relationship between A. rhombeus and its mussel hosts, demonstrating the evolutionary adaptations that enhance reproductive success and survival. This study provides valuable insights into the ecological dynamics and conservation requirements of such symbiotic relationships.

The reproductive costs of bitterling fish and zebra mussel parasitism to a unionid mussel

Unionid mussels are one of the most vulnerable groups of animals, with multitude of factors responsible for the decline of their populations in the freshwater ecosystems. Many of those causes stem from interspecific relationships. Direct data on the reproductive costs incurred to unionids through parasitism by fish and other mussel species are incomplete. We used field experiments to test whether reproduction of the swollen river mussel (Unio tumidus) is affected by parasitism by the European bitterling fish (Rhodeus amarus) and zebra mussel (Dreissena polymorpha). We documented that bitterling brood can develop in the gills of female, male and hermaphrodite mussels. Our research showed that male mussels carried more fish offspring than female U. tumidus and bitterling embryo survival appeared higher in the outer gill demibranchs. Parasitised female U. tumidus suffered reduced fecundity in terms of a reduced number of mature ovulated oocytes and increased oocyte degeneration. Our data lend clear support to the hypothesis that bitterling fish embryos parasitizing mussel gills and epibiont zebra mussel fouling unionid shells represent a major cost to the reproductive success of their host mussels. Extended gonadal activity (multiple spawning) of U. tumidus spreads their spawning time over a longer period, which may mitigate the costs of parasitism.

Assessment of the viral contamination of fecal origin over a wide geographical area using an active approach with Dreissena polymorpha

Biomonitoring appears to be a key approach to assess chemical or microbiological contaminations. The freshwater mussel, Dreissena polymorpha (D. polymorpha), is a suitable tool already used to monitor chemical and, more recently, microbiological pollution. In the present study, we used this sentinel species to monitor viral contamination of fecal origin over a wide geographical distribution. An active approach was implemented based on caging of calibrated and pathogen-free organisms with the same exposure conditions, allowing spatio-temporal comparisons between different water bodies. In addition, different types of sites were selected to investigate the range of environmental concentrations that D. polymorpha are able to translate. Different viral genome targets were measured: norovirus genogroup I and II (NoV GI and GII) and F-specific RNA bacteriophages belonging to the genogroup –I and –II (FRNAPH-I and -II). Total infectious FRNAPH were also monitored.D. polymorpha was able to translate a wide range of concentrations for all the viral targets studied, meaning that this sentinel species can be used for both low and highly anthropised sites. Moreover, D. polymorpha caging proved effective in achieving gradients of viral contamination of fecal origin pressure and to highlight the contribution of tributaries to the main rivers. D. polymorpha provided spatial and temporal variations of the viral contamination. It allowed to highlight the prevalence of the FRNAPH-I and -II genogroups according to the caging site. FRNAPH-II was found to be dominant in urban areas and FRNAPH-I in rural areas. This strategy uses the caging of the sentinel species D. polymorpha on selected sites with standardised analysis methods has proven to be a promising tool for characterizing viral contamination at both large and very fine scales.

Off the conservation radar: the hidden story of Europe's tiny pea clams (Bivalvia: Sphaeriidae)

AbstractThis study highlights the conservation problems faced by the tiny freshwater bivalves of the family Sphaeriidae, also known as pea, pill, or fingernail clams (or mussels) in Europe. Despite their global distribution, assumed ecological importance, and potential uses, basic knowledge about their taxonomy, biology, and ecology is very limited and much lower than for the larger freshwater bivalve taxa. Immediate scientific priorities are required to fill knowledge gaps regarding their taxonomy, genetic diversity, distribution, reproductive cycle, ecosystem functions, and population trends. Such fundamental knowledge is necessary to identify specific threats and develop appropriate conservation actions. Deploying environmental DNA analysis at a large scale could be a valuable way to fill gaps in distribution and strengthen monitoring in areas where local taxonomic knowledge is lacking. Until taxon-specific management plans can be developed, we recommend that efforts concentrate on the general protection and restoration of wetland habitats, implementing pollution control measures, and managing invasive species. These actions should be complemented by community engagement through citizen science initiatives. Additionally, prioritising data collection to fill existing knowledge gaps and updating conservation statuses (Red Lists) based on comprehensive assessments will be crucial. Implementing these actions will provide a starting point for the broader protection of freshwater ecosystems, thus benefiting pea clams and other interconnected species within these habitats.

Still waters and deep: Habitat characteristics, density and distribution of a lake-dwelling population of the freshwater pearl mussel Margaritifera margaritifera

The freshwater pearl mussel, Margaritifera margaritifera is widely considered to be confined to fast-flowing, near-pristine rivers, and is highly endangered throughout its Holartic range. Here we describe a large population of some 150–200,000 individuals of the freshwater pearl mussel living in an oligotrophic Irish lake.Employing underwater photographic techniques and snorkel diving, mussels were found in two separate locations in the lake. In the first location, 200 m from the shore, mussels were found at a maximum density of 43 m−2, in a largely random distribution in the sandy bed substrate of the lake, up to 9.5 m deep. In the second shoreline location, mussels were observed in dense aggregations up to 118 individuals m−2, in sandy pockets in between boulders and bedrock, in waters 1–2 m deep. In both locations, mussels were found associated with macrophytes – the charophyte Nitella sp. in the deeper location and the quillwort Isoetes lacustris in the shallower shoreline location. Juvenile mussels (>1.5 cm length) were observed lying exposed on the sediment surface in the shallow littoral location, indicating that recruitment occurs in this population, although no estimates of juvenile density were obtained.The discovery of a large population of lentic Margaritifera margaritifera has significant implications for the species’ conservation and indicates that there may be substantial lentic populations of the species elsewhere. The environmental envelope of the species is clearly much wider than thought and our discovery challenges assumptions about the fundamental feeding and reproductive biology of this endangered species.

Riverscape genomics of the endangered freshwater mussel Lampsilis rafinesqueana

Abstract Historical periods of rapid climate change and associated direct and indirect impacts such as glaciation and restructuring of drainage systems are important in shaping biodiversity, lineage divergence, and contemporary genetic variation amongst many aquatic taxa. Through an understanding of the impacts of these ancient changes in land‐ and waterscapes on current diversity, practitioners can account for a species' unique evolutionary history when developing conservation strategies. We investigated intraspecific genetic diversity of the critically endangered freshwater mussel, Lampsilis rafinesqueana, endemic to the interior region of the U.S.A. Our study (1) characterises genetic variation within and amongst populations to infer biogeographic history and (2) applies genetic information to potential conservation strategies. We applied restriction‐site associated DNA sequencing and mitochondrial DNA sequencing to investigate genetic diversity within and amongst populations of L. rafinesqueana from across the species' distribution. We then considered this information in the context of historical riverscape processes and applicability to conservation. Variation within rivers was similar, but we found different genetic populations associated with two major ecoregions, the Central Interior Highlands and Osage Plains. The plains population displayed an Ne value approximately 10‐fold lower than the highlands population. Historical demographic modelling suggests the two populations diverged approximately 1.2 million years ago and then both populations underwent a substantial decrease in population size around 19,700 years ago. These two major events are each correlated with known periods of historic rapid climate change. We identified two distinct populations of a federally endangered mussel, L. rafinesqueana. Contemporary genetic diversity of this species was strongly influenced by direct and indirect impacts of rapid historic shifts in climate. The highlands population has undergone a recent expansion, typical of post‐Pleistocene climate change, while also having a much larger Ne value than the plains population. Our results suggest low levels of gene flow amongst the two distinct populations across generations. We recommend that conservation managers for this species take into account relative difference in diversity between populations when developing management strategies. In many species, assisted gene flow amongst genetically unique populations should only be considered in extreme circumstances; however, in L. rafinesqueana, low levels of assisted migration amongst populations may be a valuable tactic to maintain natural levels of genetic variation. Our study shows that biogeographical frameworks can be useful for developing conservation strategies that serve to retain important genomic variation that represents the evolutionary potential of target freshwater species.

Mussels with luggage: the influence of artificially attached “backpack” devices on mussel movement behavior

BackgroundFreshwater mussels are important keystone and indicator species of aquatic ecosystems. Recent advances in sensor technology facilitate applications to individually track mussels and to record and monitor their behavior and physiology. These approaches require the attachment of sensor devices as “backpacks” to the outer shell surface. The interpretation of such data makes it necessary to understand the influence of these attachments on the horizontal and vertical movement behaviors of freshwater mussels. Over a series of mesocosm experiments, this study systematically investigated the effects of three size- and wiring-specific variants of artificially attached backpacks on the horizontal and vertical movement behavior of Anodonta anatina.ResultsAcross all experiments, equipping mussels with backpacks did not result in a significant influence on horizontal movement for any of the backpack variants. In contrast to this finding, the big backpacks with a high ratio between backpack volume and mussel length resulted in a significantly negative effect on vertical movement, indicating a potential for adverse effects of such devices on mussels, especially in natural settings.ConclusionsThe findings of this study show that assessing the effects of attached devices on mussels requires a species-specific evaluation of potential impacts on the endpoints of interest. Especially for vertical movement patterns, selection of the smallest available devices appears mandatory.

Paving the Way for Sustainability: A Study on Porous Asphalt Mixtures Reinforced with LDPE Plastic Waste and Freshwater Mussel (Pilsbryoconcha Exilis) Shell Filler

This study aimed to explore the impact of different fillers, namely conventional (stone ash) and alternative (mussel shell powder), along with varying percentages of Low-Density Polyethylene (LDPE) additives, on the performance of porous asphalt mixtures. Asphalt mixtures' Marshall Stability and Flow were examined with LDPE percentages of 0%, 2%, 4%, and 6%. Results highlighted that the choice of filler and LDPE content significantly influences the mixture's stability and flow. A shift from 100% stone ash to 50% mussel shell powder resulted in a slight decrease in stability for 0% and 2% LDPE. In contrast, the stability was notably higher for 4% and 6% LDPE, including the alternative filler. Mixtures with 100% stone ash exhibited increased flow values with rising LDPE concentrations, while mixtures with 100% mussel shell powder showed an opposite trend. Furthermore, mixtures containing mussel powder generally had higher flow values than those with stone ash at similar LDPE percentages. The study underscores the intricate relationship between filler types and LDPE additives in influencing the properties of asphalt mixtures. These findings pave the way for the potential use of alternative fillers in asphalt mixtures, emphasizing the importance of understanding material interactions for optimal pavement design.

Temporal dynamics of condition index and glochidia larval occurrence in freshwater mussel, Lamellidens corrianus (Lea, 1834) collected from a wetland of Mymensingh, Bangladesh

We report the temporal pattern of condition index (CI), and glochidia larval presence and discharge in freshwater mussel, Lamellidens corrianus, obtained from the Darikathal Beel at Trishal upazila of Mymensingh district in Bangladesh. Samples were drawn monthly over a year from July 2021 to June 2022. The mean shell length, shell height, shell breadth, tissue wet weight, and shell dry weight were 86.29±8.36 mm, 19.31±1.42 mm, 40.31±4.90 mm, 12.52±4.77 g, and 12.05±2.85 g, respectively. The male-female sex ratio was 1.00:2.00 with no sexually undifferentiated animals in the sample.

Vital effects and the fractionation of rare earth elements and yttrium during uptake by and transfer within freshwater bivalves and their shells

Mussels are commonly used as bioarchives in environmental monitoring, yet the impact of vital effects on the trace element or isotope ratios used as biogeochemical proxies is often only ill constrained. A prime example of such trace elements are the Rare Earth elements and Yttrium (REY) which have become (micro)contaminants in freshwater systems worldwide. We here report on the distribution of REY in different soft tissues and in the shells of freshwater bivalve A. anatina, commonly known as “duck mussel”, from the Danube River in Hungary and the Vistula River in Poland. Both rivers are contaminated with anthropogenic Gd from contrast agents used in magnetic resonance imaging (MRI). Regardless of the mussels' origin, all of their compartments show very similar shale-normalised REY patterns. None of the samples show any anthropogenic Gd anomaly, implying that in freshwater anthropogenic Gd from MRI contrast agents is either not bioavailable or that REY from ambient river water are insignificant for the REY budget of freshwater mussels. Compared to ambient water, the bivalves bioaccumulate the REY with preferential uptake of Ce and of light REY over heavy REY. However, REY concentrations in mussels are similar to or lower than those in their potential food source, with minor fractionation along the REY series besides slight preferential uptake of La and Y. Comparison of shells and tissues reveals the systematic oxidative decoupling of Ce from its REY neighbours, probably due to the presence of Ce(IV) solution-complexes in the mussels' extrapallial fluid. Despite possible REY fractionation during their initial uptake, vital effects do not impose any major control on REY fractionation during REY transfer within the mussels or during formation of their shells. Mussel shells may, therefore, conveniently be used for environmental monitoring of REY without major disturbance from vital effects.

Sulfate sensitivity of early life stages of freshwater mussels Unio crassus and Margaritifera margaritifera

Sulfate is increasingly found in elevated concentrations in freshwater ecosystems due to anthropogenic activities. Chronic exposure to sulfate has been reported to cause sublethal effects on freshwater invertebrates. Previous sulfate toxicity tests have mostly been conducted in hard or moderately hard waters, and research on species inhabiting soft water is needed, given that freshwater organisms face heightened sensitivity to toxicants in water of lower hardness. In the present study, we examined sulfate sensitivity of two endangered freshwater mussel species, Unio crassus, and Margaritifera margaritifera. Glochidia and juveniles of both species were subjected to acute and/or chronic sulfate exposures in soft water to compare sulfate sensitivity across age groups, and effective concentrations (EC)/lethal concentrations (LC) values were estimated. Mussels were individually exposed to allow relatively larger numbers of replicates per treatment. Chronic sulfate exposure significantly reduced growth, foot movement, and relative water content (RWC) in juvenile mussels of M. margaritifera. Mussels at younger stages were not necessarily more sensitive to sulfate. In the acute tests, LC50 of glochidia of M. margaritifera and U. crassus was 1301 and 857 mg/L, respectively. Chronic LC10 was 843 mg/L for 3-week-old U. crassus juveniles, 1051 mg/L for 7-week-old M. margaritifera juveniles, and 683 mg/L for 2-year-old M. margaritifera juveniles. True chronic Lowest Effective Concentration for 7-week-old M. margaritifera may be within the 95% interval of EC10 based on RWC (EC10 = 446 mg/L, 95%CI = 265–626 mg/L). Our study contributed to the understanding of sulfate toxicity to endangered freshwater mussel species in soft water.

Dead or alive: the effect of shells and living individuals of Sinanodonta woodiana (Lea, 1834) on habitat selection and behaviour of European unionid bivalves

1. Ecosystem engineering freshwater bivalves, burrowing in the substratum and accumulating shell deposits, transform bottom habitats. Especially the invasive Asian bivalve Sinanodonta woodiana (SW), due to its rapid growth, large size, and high fecundity, can affect benthic communities. Here, we determined its effect on habitat selection and behaviour of endangered native bivalves, Anodonta cygnea and Unio tumidus. 2. We conducted laboratory preference assays (Experiment 1: choice between two substrata) exposing the native bivalves to pure sand (control), shells (several densities on the sand surface or burrowed), or living SW. Then, we tested their locomotion and burrowing (Experiment 2) on pure sand and substrata contaminated with shells or living SW. 3. In Experiment 1, native bivalves avoided shells, but not living SW. Burrowed and larger shells were avoided compared with those on the surface and smaller ones, respectively. 4. In Experiment 2, U. tumidus exposed to SW delayed activity initiation (in response to living bivalves), increased locomotion (living bivalves, surface shells), and reduced burrowing depth (living bivalves, all shells). Anodonta cygnea exposed to SW reduced locomotion speed (living bivalves, shells), and reduced burrowing duration (burrowed shells) and depth (living bivalves, burrowed shells). 5. SW (especially shell beds) constitutes another emerging threat to native bivalves, impairing their burrowing and inducting active avoidance. As SW expands its distribution with climate warming, the range and strength of its impact is likely to increase, reducing the area available to native bivalves, exposing them to environmental dangers (due to burrowing limitation) and deteriorating physical condition (energetic resources used for excessive locomotion).

Biologically driven isotope fractionation in ultrastructurally different shell portions of freshwater pearl mussels (Margaritifera margaritifera): Implications for stream water δ18O reconstructions

AbstractOxygen isotopes in stream water can serve as natural tracers of watershed dynamics. Freshwater pearl mussels provide δ18Owater estimates that overcome temporal and spatial limitations of instrumental records. The reliability of shell‐based δ18Owater reconstructions depends on understanding which shell layer biomineralizes closer to oxygen isotopic equilibrium with ambient water. To determine this, both the (outer) prismatic and (inner) nacreous sublayers of the outer shell layer were sampled. Over 2500 isotope values were obtained from shells collected from the Our River (Luxembourg) and from mussels cultured in tanks at constant temperature and monitored δ18Owater. Calculated δ18Owater from the prismatic portion was in excellent agreement with monitored δ18Owater, while δ18Oshell of the nacreous portion was systematically offset by +0.43‰, overestimating δ18Owater by +0.53‰. Although shell portions were formed simultaneously from the same extrapallial fluid, they underwent different fractionation mechanisms, presumably due to differences in carbonic anhydrase activity catalyzing mineralization processes.

Fully Developed Open Channel Flow Over Clusters of Freshwater Mussels Partially Buried in a Gravel Bed

AbstractThe present study uses results of eddy‐resolving numerical simulations to investigate the open channel flow over large clusters of freshwater mussels (Unio elongatulus) partially buried in a rough, gravel bed. The density of the mussels forming the array varies from 26 to 500 mussels/m2. The flow structure is analyzed at large distances from the leading edge of the mussel bed, where the flow can be considered fully developed. The effects of changing the mussel bed density, the filtering discharge, the burial level and the roughness of the bed surface in which mussels are burrowed, are investigated in terms of flow field, turbulent structures, drag forces, and bed shear stresses. It is found that strong interactions occur between energetic eddies generated by the larger gravels on the exposed bed surface and by the mussel shells. Simulations results show that for a burial depth close to 50% and a ratio between the average gravel size and the mussel protruding height of 0.13, the shell induced turbulence becomes dominant for mussel bed densities around 50 mussels/m2. The influence of the bed roughness becomes less relevant with increasing mussel density, as the generation of energetic eddies is mostly controlled by mussel‐to‐mussel interactions. For fixed bed roughness, burial level and filtering velocity, the mean streamwise drag force and the associated drag coefficient for the exposed part of each mussel decrease with increasing mussel density, even if strong variations are observed for individual mussels. For constant mussel bed density and burial level, the mean streamwise drag force and the mean drag coefficient decrease slightly with increasing bed roughness. Increasing the burial level decreases not only the drag forces but also the drag coefficients because of the more streamlined shape of the top of the mussels. Strong active filtering acts toward decreasing the mean streamwise force and the mean drag coefficient. The spanwise drag forces contribute significantly to the total drag force, especially for high mussel bed densities. Based on smooth bed calculations, bed‐averaged shear stresses are reduced in highly dense clusters.

Growth and longevity of two imperilled mussel species from the Edwards Plateau of Central Texas and its implications for freshwater mussel conservation and management

Abstract Life history information such as growth and longevity have been useful for understanding evolutionary relationships and predicting species responses to management and habitat alteration for aquatic species. For unionid mussels, which are globally imperilled, life history information remains unknown for a majority of unionid mussels and because of this has not been broadly used to guide mussel conservation efforts. To address this knowledge gap, growth and longevity were estimated for Cyclonaias petrina, Texas pimpleback, and Lampsilis bracteata, Texas fatmucket, using thin‐sectioning and validated using cross‐dating. Both species are proposed for listing under the US Endangered Species Act. Growth and longevity estimates differed between C. petrina (K = 0.065, 0.086, 0.101; L∞ = 55.03, 76.44, 94.43) and L. bracteata (K = 0.187, 0.208; L∞ = 61.40, 61.52), and growth for L. bracteata correlated to annual flow indices. Cross‐dating revealed high interseries correlations (R = 0.400–0.573), indicating estimates can be viewed with some certainty. Growth and longevity estimates indicate C. petrina is positioned near the K endpoint and L. bracteata near the r endpoint along the r/K life history continuum. This suggests C. petrina should be favoured in stable habitats where disturbance is minimal, whereas L. bracteata is expected to tolerate habitats with frequent and likely stochastic patterns of disturbance. Knowledge of growth and longevity along with life history position provides a qualitative basis to help scientists and practitioners better anticipate how species will respond to environmental change and management actions. Given the conservation status of C. petrina and L. bracteata the life history findings from this study are timely and should be useful in their conservation.

The first record of Aspidogaster conchicola (Trematoda: Aspidogastridae) in the freshwater mussel Unio pictorum in Croatia

Although Aspidogaster conchicola is a common and widespread parasite of freshwater molluscs, knowledge about its distribution and hosts in Croatia is still insufficient. The aim of our research was to investigate the presence of A. conchicola in the freshwater mussel Unio pictorum, which is widely distributed in the rivers of Europe. The study was conducted in the Mrežnica River, where U. pictorum is a common species. Five specimens of U. pictorum were collected by scuba diving in October 2021. The dissection of mussels was carried out in the laboratory, and all tissues were inspected under a stereo microscope. Determination of the parasite was performed based on the number and arrangement of alveoli. The results showed that the prevalence of infection was 40% (two out of five mussels were infected), and the intensity of infection was 2 to 3 specimens per mussel. All parasites were detected in the mussels’ pericardial cavity. In conclusion, this is the first record of A. conchicola in U. pictorum in Croatia.