Freshwater Pearl Mussel Research Articles

Abstract The freshwater pearl mussel ( Margaritifera margaritifera , FPM) was 200 years ago widespread in the upper Weiße Elster catchment (Vogtland, Germany). Due to population collapse, only a few hundred individuals remain today, primarily from captive breeding activities. This decline significantly reduced the species’ filtering capacity and its contribution to the river’s natural-filtration capacity. To assess this contribution, we measured FPM clearance rates and estimated the historical population sizes based on historical records. We also examined how shell length, water temperature, and food quality affect the clearance rates. At 15–19 °C (typical summer conditions), FPM showed an average clearance rate of 1.95 L h −1 mussel −1 . Rates varied depending on shell length and food quality. Based on historical data, the population size around 1800 likely ranged from minimum 250,000 to 3,125,000 individuals. Relating these estimates to the summer discharge of the river catchment, FPM may have filtered 2.7–33.6% of the flow around 1800. In contrast, today’s remaining population filters just 0.027%. It can therefore be concluded that FPM once played a substantial role in maintaining water quality in the Weiße Elster catchment, a role that has become negligible today.

Inhibition of opioid growth factor receptor (OGFR) promotes wound healing in the freshwater pearl mussel (Hyriopsis schlegelii).

A scanning electron microscopy based survey of the impact of infestation by parasitic freshwater mussel glochidia upon the gill morphology of two species of salmonid fish.

Molluscan growth histories can be retrieved from sclerochronological data. The extreme longevity of the endangered freshwater pearl mussel (Margaritifera margaritifera) (Mollusca: Bivalvia) makes this species ideal for sclerochronological studies where long timeseries are needed. However, the species is included in The IUCN Red List of Threatened Species, for which reason the number of potentially reproducing individuals collected from the extant populations should be kept down. In this study, museum specimens of M. margaritifera, collected originally from northern Finland in 2002, were explored. Timeseries of annual shell growth increments were produced from both valves of each individual mussel. The approach of paired valves benefitted cross-dating between the growth records and improved chronology statistics, helping to keep down the number of explored individual mussels. On the other hand, it was also found that the growth of left and right valves may exhibit differentiated patterns. All specimens provided increment data over the 1976–2002 period, that is, the mussels were around 30–40 years old when sacrificed. Midsummer (July) temperature was the only climate variable correlating statistically significantly with the mean shell growth record over the common period (1976–2002). However, this climatic factor did not fully explain a shell growth curtailment around the year 1991. Likely, the negative excursion in shell growth was caused by a high sediment influx due to anthropogenic changes in the catchment. Our findings are based on one site chronology which means the results should not be straightforwardly extended to other streams and habitats. This limitation notwithstanding, the results suggest that sample collection strategies can be used to build sclerochronological datasets for M. margaritifera and other endangered bivalve species.

ABSTRACTThe freshwater pearl mussel Margaritifera margaritifera (FPM) has substantially declined throughout Europe. It is a target species in conservation of freshwater biodiversity, with several restoration and captive breeding efforts in place. Understanding the spatiotemporal patterns of genetic diversity in relation to environmental factors is crucial in increasing the success of such projects. Based on nonlethal haemolymph sampling from 42 (sub)populations throughout Ireland, a comprehensive assessment of the genetic diversity and differentiation of Irish FPM was realized. Given the previous identification of a genetic FPM diversity hotspot in Ireland, the debate about (sub)species ranks related to the co‐occurrence of M. margaritifera and M. durrovensis and the establishment of captive breeding efforts, the island setting of Ireland provided an ideal study case. Genetic differentiation of Irish freshwater pearl mussel populations suggests the existence of four distinct genetic clusters, which should be managed as separate conservation units. In a few highly inbred populations with occurrence of multiple individuals with the exact same genotypes, genetic drift likely explains the strong differentiation from other populations. Genetic diversity was particularly high in the most intact rivers at the west coast with occurrence of salmon as a host fish. In contrast, genetic variability was much lower in highly degraded rivers in the south‐eastern part of Ireland where current census population sizes are low. The identified clusters and their different levels of diversity require conservation‐unit specific prioritization: Habitat and host fish conservation are key for the genetically most diverse Western Irish FPM populations to avoid genetic drift effects and population bottlenecks, whereas the unique but less diverse FPM requires special attention to avoid further erosion of their variability. Captive breeding should maintain the high level of genetic variation and may also include representation of additionally identified conservation units in the future.

Parasites often have a large impact on their hosts and can alter host phenotype to increase their own fitness, a phenomenon known as extended phenotype. Studies demonstrating extended phenotype for non-trophically transmitted parasites are scarce. Unionid mussels have a parasitic life stage adapted to parasitize fish which can affect host behavior, habitat use and growth rates, raising the question if parasitic freshwater mussels can also manipulate their host fish to compensate for downstream dispersal and to reach habitats favorable for newly excysted juvenile mussels. Wild-caught, parasite-naïve juvenile brown trout (Salmo trutta) were PIT-tagged, and half of the individuals were infested with parasitic larvae from the freshwater pearl mussel (Margaritifera margaritifera), all individuals were then returned to their home stream. During the following year, trout were tracked to investigate movement and habitat use, and also periodically recaptured to measure growth and body condition factor. The infested trout showed significantly higher upstream movement than non-infested trout and were more often recaptured in stream sections with slow-moving shallow water, particularly during the parasite excystment period (270 d post infestation). These data suggest that the juvenile mussels were successfully transported an average of 170 m upstream from the host trout release points to stream sections favorable for adult mussels. Infested trout survived as well as the non-infested, but had a significantly lower specific growth rate than non-infested trout. These results indicate a first example of extended phenotype in unionid mussels and highlight the importance of understanding glochidia-induced changes to host fish behavioral ecology.

Application of K-means clustering and spectroscopic analysis for rapid sorting of inner Shell colors in freshwater pearl mussels Hyriopsis schlegelii

Human impacts have led to dramatic declines of the endangered freshwater pearl mussel Margaritifera margaritifera (FPM) and to a shift of its main distribution areas to more undisturbed but colder headwaters. In this study, we investigated whether feeding on high-quality food (i.e. food rich in n-3 polyunsaturated fatty acids) can compensate for growth limitations due to low water temperatures and whether food quality influences the survival rate of juvenile FPM. Therefore, we conducted feeding experiments with juvenile FPM at three different temperatures (11, 15 and 19°C) and with three different food sources. Afterwards, mussels were reared in cages in a stream to analyse survival during the first winter. Our results showed that food quality can compensate to some extent for growth limitations due to low water temperatures, but sufficiently high growth rates of juvenile FPM can only be achieved by the availability of high-quality food in warmer (> 15°C) environments. The availability of high-quality food during the main growing season also improves the survival of FPM over the first winter. Consequently, restoration efforts must aim to restore habitats in the historical main distribution areas of FPM and to create habitat conditions that allow the development of high-quality food.

Captive breeding of freshwater mussels is essential for conservation strategies to maintain populations of endangered bivalve mollusks. In the Czech Republic, breeders use detritus from natural sources, preferably from nearby streams and spring areas, to prepare pearl mussels for release into the target localities to reestablish sustainable populations in their native habitats. This strategy is subject to sudden changes, and breeders must consider different detritus qualities between seasons and the unpredictability of weather conditions. Therefore, harvesting natural detritus on a regular basis is challenging and time-consuming, and more samples require more testing, which increases the cost of captive breeding. We proved that juvenile freshwater pearl mussels grew faster when fed detritus stored frozen in a freezer than when fed the same chilled detritus. Therefore, detritus can be harvested under optimal conditions and stored in a freezer for the remaining season, thereby simplifying and reducing mussel conservation costs. Moreover, this approach offers a sustainable, environmentally friendly method that does not require the industrial production of artificial feeds.

Freshwater pearl mussels (Margaritifera margaritifera) are vital bioindicators in river ecosystems, with their population health reflecting broader environmental conditions. Mussel population declines are well documented across Europe, and in Estonia, Pudisoo River supports the last remaining population. Long-term monitoring also indicates a significant decline in Pudisoo River, but the causes remain underexplored. This study aimed to assess how geology, geomorphology, and land use changes could impact pearl mussel population trends and distribution. We conducted field surveys, bioindication tests, and hydrological and morphometric analyses to evaluate physicochemical and environmental parameters in the Pudisoo River. The study revealed a declining but stabilizing population, concentrated in river sections with higher gradients and coarser riverbed substrates. Sediment transport and accumulation, influenced by both natural and human-induced factors, significantly affect habitat suitability, especially for juvenile mussels. Positive correlations were found between mussel populations and river sinuosity, suggesting that the varied microhabitats created by sinuosity, such as gradients in flow velocity and sediment deposition, play an important role in supporting the species. This finding highlights the importance of directing in-channel restoration efforts towards enhancing channel morphology to create more diverse habitats. Research on sediment dynamics and hydrological connectivity will be crucial for developing effective habitat management strategies for the species, especially considering the impacts of land use and climate change.

Abstract The thick-shelled river mussel Unio crassus is categorized as Endangered on the IUCN Red List, and searching for surviving populations is urgent. We surveyed for this species in Kaliningradskaya Oblast, a Russian territory lying between Poland and Lithuania, where empty shells of the species had been reported from two rivers. There are at least 125 rivers and numerous small streams in the region, and as a comprehensive survey of all of these watercourses is infeasible, we used a method developed for surveys of the freshwater pearl mussels Margaritifera margaritifera and Margaritifera laevis. This involved a remote assessment of the forests and lakes in the river basins to identify sites potentially suitable for U. crassus based on criteria used for pearl mussels, followed by site surveys. We surveyed six sites and discovered U. crassus in five of those, only two of which support healthy populations. The existence of other U. crassus populations in Kaliningradskaya Oblast is unlikely. This study underscores the critical role of riparian arboreal vegetation for freshwater mussels. The conservation of U. crassus in rivers surrounded by farmlands is challenging because of siltation, eutrophication and other processes that negatively impact the riverine environment. Even the abandonment of these farmlands does not necessarily lead to improvements in mussel survival. Any plans for the restoration of U. crassus will require concurrent restoration of riparian arboreal vegetation.

Shell and pearl formation in bivalves is a sophisticated biomineralization process that encompasses immunological and mineralization aspects, particularly during shell repair and the initial stages of pearl cultivation when a nucleus is inserted. Here, we describe a novel C-type lectin, HcLec1, isolated and characterized from the freshwater pearl mussel Hyriopsis cumingii Lea. Immune challenge, RNA interference (RNAi) experiments, ELISA, and antibacterial assays were employed to investigate the role of HcLec1 in innate immunity. We also established shell damage repair and pearl nucleus insertion models to examine the impact of HcLec1 on the biomineralization process in Hyriopsis cumingii Lea. In vitro calcium carbonate crystallization assays were conducted to explore the direct role of HcLec1 in calcium carbonate crystal formation. The HcLec1 gene sequence is a full-length cDNA of 1552 bp, encoding 240 amino acids. HcLec1 comprises an N-terminal signal peptide and a carbohydrate-recognition domain (CRD), with QPD (Gln-Pro-Asp) and MND (Met-Asn-Asp) motifs for polysaccharide binding. Tissue expression analysis showed that HcLec1 is predominantly expressed in the gill tissue of Hyriopsis cumingii Lea under normal conditions, and its expression is significantly elevated in both gill and pearl sac tissues following nucleus insertion for pearl cultivation (P < 0.05). After immune stimulation with Aeromonas hydrophila and lipopolysaccharides (LPS), HcLec1 expression levels significantly increased in both cases (P < 0.01), indicating a role in bivalve innate immunity. RNA interference (RNAi)-mediated knockdown of HcLec1 led to a significant decrease in the expression levels of immune-related genes (WAP, α2m, and Lyso) and mineralization-related genes (CA, CHS, Nacrein, and Pif) (P < 0.05). In animal models for shell damage and nucleus insertion in pearl cultivation, HcLec1 showed a consistent expression pattern, with an initial significant decrease followed by a marked increase, peaking at day 14 (P < 0.05). This suggests a role for HcLec1 in pearl formation and shell repair. The recombinant HcLec1 protein demonstrated binding affinity to LPS and PGN, a robust ability to agglutinate Escherichia coli, Staphylococcus aureus, Aeromonas veronii, and Aeromonas hydrophila, and significantly inhibited bacterial growth (P < 0.05). Moreover, rHcLec1 promoted calcite crystal formation in saturated calcium carbonate solutions and altered crystal morphology. The HcLec1 gene plays a pivotal role in both innate immunity and biomineralization in the triangle sail mussel. This study enhances our understanding of the functional diversity of C-type lectins and provides a foundation for future studies on shell repair and pearl growth.

ABSTRACT The freshwater pearl mussel, Lamellidens marginalis is a promising candidate species for pearl farming in India and Southeast Asia. In India, this species is being intensively cultured in ponds, tanks, cisterns and freshwater integrated multi-trophic aquaculture systems, where the chance of accumulating ammonia is very high. However, no information is currently available on the effects of ammonia on the physiology of this mussel. Here, we conducted a study of the responses of Lamellidens marginalis to ammonia exposure. The 96 h LC50 of un-ionised ammonia was 4.28 mg L−1. Mussels were challenged by exposure to un-ionised ammonia at half the 96 h LC50 for 96 h. Exposure induced significant alterations in the serum biochemical profile (P < 0.05) of mussels, encompassing changes in total protein levels, alkaline phosphatase, aspartate aminotransferase and alanine aminotransferase. The activities of antioxidative enzymes, including superoxide dismutase, catalase and glutathione peroxidase, and the biomarker malondialdehyde, were significantly impacted (P < 0.05) by ammonia challenge. The antioxidant defence system may be able to protect mussels from oxidative stress during early exposure to ammonia. However, prolonged exposure to ammonia impaired antioxidant activities, emphasising the importance of maintaining water quality for a healthy environment for sustainable freshwater pearl mussel farming.

Since the 1700s, global wetlands have declined by 3.4 million km2. Wetland quality loss is a key driver of freshwater pearl mussel (FPM) population decline in peaty catchments. GIS techniques were used to determine wetland cover in 1834 and 2023, in eight peaty FPM catchments in Ireland, based on historical Ordnance Survey mapping (1834) and Irish National Land Cover (NLC) mapping (2023). Historical catchment wetland change (1834–2023) ranged between a net loss of 7.03 to 29.96%. Wetland coverage in 2023 varied between 53.85 and 83.53%. In 1834, that coverage ranged from 59.72 to 89.85%. The Hydromusindex (HDi) was developed to evaluate catchment-scaled historical wetland change, which for each of the catchments studied was a net loss. This damage index was based on the ratio of the 2023 catchment wetland coverage in proportion to the 1834 baseline scenario, and the catchment proportional coverage of each wetland cover type weighted by their potential contribution towards water storage; higher HDi values indicate increased damage. The HDi values of eight studied catchments ranged from 21 to 46. The HDi can be used to assess catchment restoration design and to rank FPM catchments for restoration, under the Nature Restoration Law. Measurable targets for catchment-scaled restoration have been produced. The sum of weighted wetland scores, in contrast to predefined reference values, can be used for rapid damage evaluation. An estimated 41.06% of Ireland’s plantation forestry is located on peatlands and peat soils, thereby highlighting considerable opportunities in terms of meeting the EU Nature Restoration Law’s targets.

In this paper we present the results of our field survey in the Kushereka River and preliminary data concerning the size structure, age composition, and population density of the freshwater pearl mussel M. margaritifera in the studied sites.

ABSTRACTFreshwater mussels are traditionally monitored by visual observations which is time‐consuming and can be difficult depending on water conditions. Environmental DNA (eDNA) is an attractive alternative since it can detect a species in the water without visual observations. Furthermore, since the DNA can potentially travel downstream in the river systems, presence of a species can be detected even away from the population of mussels. In this paper, we evaluate and describe how dPCR (digital PCR) technique can be used to efficiently monitor four freshwater mussel species: the freshwater pearl mussel Margaritifera margaritifera from Margaritiferidae family and three species from Unionidae family: the depressed river mussel Pseudanodonta complanata, the painter's mussel Unio pictorum, and the thick shelled river mussel Unio crassus, which are all regarded endangered in many regions worldwide. dPCR assays for the four mussel species were developed and tested in silico before conducting field surveys. The verification step in the field was carried out in two Swedish rivers with confirmed populations of the freshwater pearl mussel (M. margaritifera). Furthermore, two other rivers with unknown presence of the endangered freshwater mussels were surveyed for occurrence of the four mussel species, using the capacity to multiplex several species simultaneously in a dPCR reaction. We furthermore showed that the target DNA concentrations probably depend on the season and water level, which may largely affect the detection probability and interpretation of the results in terms of population size.

Climate change poses a significant threat to freshwater ecosystems by causing increases of average water temperatures, and more frequent and extreme heating events. Freshwater mussels are declining globally, and the distribution of the freshwater pearl mussel (Margeritifera margeritifera) has decreased dramatically over the past century. Even though it is likely that climate change is contributing to the decline of the species, little is known about the specific mechanisms involved. Here, we test how short episodes of water temperatures above the known thermotolerance range affect the survival and growth of the early post parasitic juvenile phase of freshwater pearl mussels. We also test if previous experience with elevated water temperatures can modify survival and growth responses to subsequent high-temperatures exposure. Mortality was very low in all treatments (< 5%) and not affected by the temperature treatments, while growth rate was positively affected by temperature. Our results suggest that juvenile mussels can survive short periods of heat stress when other environmental conditions are favourable. Future studies should therefore address how heat stress affects survival in combination with other stressors, such as reduced availability of dissolved oxygen.

In Southern Norway, there are strong indications that acidic precipitation is the main reason for the extinction of 94% of all known populations (n = 47) of the vulnerable pearl mussel Margaritifera margaritifera (L). An example is the famous pearl mussel river, the River Audna, where the death of both young and old mussels took place in the period 1930 - 50. When the pH is in the range 5.0 - 5.5 or lower for prolonged periods of time, the mussel can no longer tolerate the acidity. The only three known localities remaining in this part of the country, with ageing populations of the pearl mussel, are all lowland streams that are only very little acidified. The main threat there is eutrophication.

Genome-wide association analysis reveals genetic architecture of growth and inner shell color traits in freshwater pearl mussel Hyriopsis cumingii (Lea 1852)

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.