Pearl Mussel Research Articles

The signal crayfish Pacifastacus leniusculus (Dana, 1852) (Crustacea, Decapoda) is threatening the near future of Margaritifera margaritifera Linnaeus, 1758 (Bivalvia, Unionoida) in the Negro River (NW Zamora, Spain)

We studied the incidence of the signal crayfish on a population of Margaritifera margaritifera in the Negro River (Zamora, Spain) during the summers of 2019 to 2022. The incidence of predation was assessed as a mortality factor in relation to floods and the hydrology of the river. The crayfish were trapped in a small plot of high pearl mussels density monitored since 2002, and collected each summer since 2019, during which time their abundance increased (65 % in three years). Simultaneously, we counted the shells carried by the floods to the gravel riverbanks. The incidence of floods was stable between 2019 and 2022 (10.3 % to 19.4 % of mortality), while at the bottom of the river the mortality of pearl mussels increased due to predation from 2.7 % to 43.3 %. During the 2022 dry season, 29 pearl mussels that had recently died and whose shell edges were widely gnawed by crayfishes were collected from the plot. The shells appeared bitten only in the contour exposed above the gravel, ruling out the possibility that the marks could be the effect of the scavenging of dead specimens by the crayfish. Low intensity trapping barely affected the crayfish population, since the following year their abundance in the controlled section had recovered.

Efficacy of freshwater pearl mussel (Lamellidens marginalis) as a biomonitoring tool for assessing microplastic pollution

The presence of microplastics (MPs) is an emerging concern for aquatic life. The present study investigated the ingestion and absorption of experimental MPs in the internal organ and body tissue of freshwater pearl mussel (Lamellidens marginalis) following exposure for 10 days and using histological procedures. Mussels were exposed to three types of bright color plastics were used such as plastic rope fibers, green PET bottle fragments and brush bristles. A reference group not exposed to MPs was also included. Following exposure, tissues were dissected and embedded in paraffin, and histological sections were examined for the presence of microplastic. Result showed that 31.25% of all the exposed mussels exhibited the ingestion of MPs. Fiber was the highly accumulated MPs followed by filaments. However, none of the mussels showed the ingestion of brush bristles. Accumulation of MPs was the highest for smaller sized categories. Histological observation of mussel’s tissue was also showed the absorption of MPs. The present experiment strongly suggests that the uptake, presence, and potential health impacts of MP should be closely monitored using L. marginalis as bio-monitor organism.

Effects of multiple stressors on recruitment of long‐lived endangered freshwater mussels

Abstract Multiple stressors can affect populations interactively. However, the process by which they affect recruitment remains unclear, especially in long‐lived organisms with complex life cycles. This study explored the bottleneck stages of juvenile recruitment in the endangered freshwater pearl mussel Margaritifera togakushiensis and the interactive effects of nutrients and fine sediments on these stages. The proportion of M. togakushiensis juveniles aged ≤10 years was investigated in 24 rivers from north‐eastern Japan. Margaritifera togakushiensis has a complex life cycle with an obligate parasitic larval stage. Gravid female density, glochidia density (the number of glochidial infections per stream area), and juvenile survival rate, used as indices for adult, parasitic, and post‐parasitic juvenile stages, respectively, were obtained from 11 of these rivers. Twenty‐four population status assessments showed that the proportion of juveniles aged ≤10 years ranged from 0.00 to 0.23, and that they were absent in 12 rivers. Parasitic and post‐parasitic juvenile stages were bottlenecks for recruitment; juvenile survival rates had a stronger positive effect on recruitment in rivers with high glochidia density. Juvenile survival rate was negatively associated with the synergistic effects of nutrients and fine sediments, although factors limiting glochidia density were not identified. These findings suggest that the interaction between nutrients and fine sediments can synergistically reduce the recruitment of M. togakushiensis, indicating an underestimated impact of stressors on mussel recruitment if treated individually. Future mitigation measures should consider both nutrient levels and fine sediments to restore mussel recruitment. These results also emphasize the importance of considering both parasitic and post‐parasitic juvenile stages of mussels to maximize the success of future restoration actions.

Zn, Cu, Ni, Cd, and Pb in the foot tissue of freshwater pearl mussels from Green Belt rivers of Fennoscandia

Margaritifera margaritifera is a critically threatened species in Europe. Metal pollution may be one of the reasons for its decline, but the influence of this factor remains insufficiently studied. Zn, Cu, Ni, Cd, and Pb were determined in the foot tissue of the freshwater pearl mussel Margaritifera margaritifera (bivalvia, margaritiferidae) from Green Belt rivers of Fennoscandia [the Murmansk region (Kola Peninsula) and Karelia (northern part)] and the accumulation of the metals was shown to be variable; however, their levels in the molluscs, being quite long-lived aquatic organisms, do not exceed the established standards of the Russian Federation. The recorded levels of the metals in the foot tissue of the Margaritifera margaritifera can be considered indicators of the background levels of geologic and anthropogenic metal concentrations in the environment influencing the distribution and development of this protected species in the lake-river systems of the Green Belt of Fennoscandia.

Substratum Raking Can Restore Interstitial Habitat Quality in Swedish Freshwater Pearl Mussel Streams

Functional and oxygenated stream beds provide crucial habitat for multiple endangered stream taxa, including endangered freshwater mussels, fishes, and insect larvae. Stream bed restoration measures such as substrate raking are often applied to mitigate excess fine sediment introductions and stream bed colmation, yet such measures are controversial. In this study, we conducted a systematic experiment in which sites with stream bed raking and removal of macrophytes were monitored over two years and compared with before-treatment conditions and untreated reference sites in the Swedish Brånsån stream, which still contains a population of the endangered freshwater pearl mussel Margaritifera margaritifera. The stream bed restoration resulted in improved habitat quality, as evident from decreased substrate compaction, increased redox potential, and oxygen supply into the stream bed. In contrast to previous studies in Central European catchments with more intensive agricultural catchment uses, the effects of the restoration measure were much longer, extending over two years. Consequently, stream bed raking and macrophyte removal can be considered a useful and more long-lasting restoration measure than currently assumed, especially in streams where excess input of fine sediment has already been mitigated, where catchment land use is rather extensive, and where near-natural flow regimes still prevail.

Bioaccumulation of pharmaceuticals and personal care products (PPCPs) in freshwater pearl mussels Hyriopsis cumingii in Poyang Lake

Thirty-five PPCPs were measured in representative freshwater pearl mussels (Hyriopsis cumingii) in Poyang Lake, the largest lake of China, as well as their responses to sedimentary PPCPs. We observed 32 PPCPs in soft tissues of mussels at a total concentration of 2721.5 ± 929.3 ng·g−1 dry weight (dw), much higher than those in sediments (21 PPCPs, 273.2 ± 89.4 ng·g−1 dw). Anti-inflammatories were the primary contaminants detected in both sediments and mussels. PPCP concentrations in mussels exhibited significant organ-specific characteristics, and gonads were identified as a hotspot for these contaminants. Correlation analysis showed that gonads were more likely to assimilate triclosan from sediments. Biochemical analysis revealed a higher physiological sensitivity of glutathione synthesis in gonads to sedimentary PPCPs, suggesting the long-term oxidative damage. Our findings highlight the concern on the potential effects of sedimentary PPCPs to propagation of mussels, and emphasize the necessity to formulate strategies for sedimentary PPCPs control targeting a healthy lake.

NEW DATA ON RELATIONSHIPS BETWEEN FISHES (CYPRINIDAE) AND MOLLUSKS (BIVALVIA, MARGARITIFERIDAE, UNIONIDAE) IN THE RUSSIAN FAR EAST

The fi ndings of bitterling (Rhodeus sericeus) eggs and larvae in the gill chambers of freshwater bivalve mollusks from Razdolnaya River basin are discussed. Bitterling eggs and larvae were registered in Margaritifera dahurica pearl mussels from Komarovka river, a tributary of Razdolnaya River, on the territory of the Ussuriysky Nature Reserve. Bitterling larvae at diff erent stages of development were also found in Nodularia douglasiae mussels from Razdolnaya River.

Genomic selection improves inner shell purpleness in triangle sail mussel Hyriopsis cumingii (Lea, 1852)

Purple pearls, which are exclusively produced by the freshwater pearl mussel, Hyriopsis cumingii (Lea, 1852), are of great economic and ornamental value. Selection mussels with purple inner shell as donor mussel can realize oriented cultivation of purple pearls. This study sought to explore the genomic genetic parameters of inner shell color traits as well as genomic-based methods to improve inner shell purpleness using genomic selection (GS). Genotype by sequencing (GBS) technology was used to generate genome-wide SNPs (n = 97,090) for reference population (n = 496) and candidate population (n = 268). In reference population, the four inner shell color phenotypes, lightness (L*), redness (a*), yellowness (b*), and color difference (dE*) were 59.97 ± 7.27, 2.66 ± 2.57, 1.79 ± 4.77, and 40.65 ± 7.46, respectively, and the genomic heritability of four traits were moderately high (0.39–0.67). GBLUP and four Bayesian models were applied to assess the prediction accuracy, where BayesB performed the best and it was chosen to develop a cost-effective GS method using low-density SNPs. Lastly, one GS method that consisted of BayesB model and 5 K GWAS-informative SNPs was successfully established and the genomic estimated breeding value (GEBV) of each candidate individual was predicted. The mussels with the highest GEBVs (n = 30) were selected as broodstocks for breeding the purple strain (PS), while the rest for control strain (CS). Significant differences in inner shell color a* and dE* were detected between PS and CS, as well as the proportion of deep purple individuals rose from 9.57% and 12.38% to 51.16% and 26.92% in Wuyi (WY) and Chongming (CM) sites, respectively. These results demonstrated that GS was an effective selective breeding approach for enhancing the purpleness of the inner shell in H. cumingii, and opened the door for future commercial GS uses on other aquaculture species.

Impact of Pb Toxicity on the Freshwater Pearl Mussel, Lamellidens marginalis: Growth Metrics, Hemocyto-Immunology, and Histological Alterations in Gill, Kidney, and Muscle Tissue

Pb is one of the most extensively used harmful heavy metals in Bangladesh, and its occurrence in waters affects aquatic organisms significantly. The tropical pearl mussel, Lamellidens marginalis, was exposed to different concentrations (T1 21.93 mgL−1, T2 43.86 mgL−1, and T3 87.72 mgL−1) of Pb(NO3)2 and was evaluated against a control C 0 mgL−1 of Pb(NO3)2, followed by a 96 h acute toxicity test. The LC50 value was recorded as 219.32 mgL−1. The physicochemical parameters were documented regularly for each treatment unit. The values of % SGR, shell weight, soft tissue wet weight, and weight gain remained statistically higher for the control group in comparison with the treatment. No mortality was noted for control units, while a gradually decreased survival rate was recorded for the different treatment groups. Fulton’s condition factor was recorded as highest in the control and lowest in the T3 unit, while the condition indices did not vary between the control and treatment groups. The hemocyte was accounted as maximum in the control and T1, while minimum in T2 and T3. The serum lysosomal parameters also followed a similar pattern, and a significantly low level of lysosomal membrane stability, and serum lysosome activity was noted for T3 and T2 units in comparison to the control group. The histology of the gill, kidney, and muscle was well structured in the control group, while distinct pathologies were observed in the gill, kidney, and muscle tissue of different treatment groups. The quantitative comparison revealed that the intensity of pathological alteration increased as the dosage of Pb increased. The current study, therefore, indicated that intrusion of Pb(NO3)2 in the living medium significantly alters growth performance and hemocyte counts, and chronic toxicity induces histomorphological abnormalities in vital organs.

Aktivitas Antioksidan, Kadar Flavonoid, dan Fenolik Total Cangkang Kerang Mutiara (Pinctada maxima)

Antioxidant compounds are able to dampen or ward off free radicals. Living things including pearl mussels (Pinctada maxima) can produce secondary metabolite compounds (phenolics, alkaloids, and terpenoids). There have been no studies on the antioxidant potential, flavonoid content, and total phenolics of pearl clam shells. This study aimed to determine the antioxidant potential of pearl clam shell ethanol extract (EECKM) in vitro using the DPPH method. In addition, it aims to determine the total flavonoid and phenolic levels of EECKM. The powder of pearl mussel shells is soaked with ethanol and the filtrate is concentrated until a concentrated extract is obtained. Flavonoid and phenolic levels from EECKM were measured using a UV-Vis spectrophotometer. Measurement of antioxidant activity using the DPPH method. EECKM has a yield of 1.73%, flavonoid levels of 0 mg QE/g, and total phenolics of 4.8 mg GAE/g. The extract has the highest antioxidant activity for a concentration of 100 mg / L with a percent value of DPPH radical inhibition of 67.1%.

Locomotion behavior of juveniles of the freshwater pearl mussel Margaritifera margaritifera: A new non-invasive tool for the evaluation of stress effects

The lack of knowledge about the sensitivity of the endangered freshwater pearl mussel (FWPM) Margaritifera margaritifera to environmental pollution and the rapid decline of its populations in Europe, have led to the need of developing non-destructive experimental protocols in order to assess the impact of such pollution. This species has a complex life cycle and the early life stages are considered the most sensitive. This study deals with the development of a methodology for the assessment of juvenile mussels' locomotor behavior using an automated video tracking system. Different parameters were determined such as the duration of the video recording and light exposure as a stimulus during the experiment. Locomotion behavior pattern of juveniles was assessed in control condition and also following exposure to sodium chloride as a positive control in order to validate the experimental protocol developed in this study. Results showed that juveniles locomotion behavior was stimulated under light exposure. Moreover, exposure to sublethal concentrations of sodium chloride (0.8 and 1.2 g/L) for 24 h was found to decrease juveniles’ locomotion by almost three-times, thus validating our experimental methodology. This study allowed to provide a new tool for the assessment of stress condition impacts on the juveniles of the endangered FWPM, highlighting the interest of such non-destructive biomarker of health for protected species. Consequently, this will help in the improvement of our knowledge on M. margaritifera sensitivity to environmental pollution.

Ethanol preservation effects on stable carbon, nitrogen and hydrogen isotopes in the freshwater pearl mussel

Chemical preservatives can alter stable isotope ratios in animal tissues. The effects of preservation on δ13C and δ15N values have been investigated in a variety of species, but not on δ2H values or on the freshwater pearl mussel (FPM, Margaritifera margaritifera) tissues. We evaluated the effect of ethanol preservation (unpreserved vs preserved tissues) over 6 months on the δ13C, δ15N and δ2H values of FPM foot and gonad tissues. Ethanol preservation significantly increased δ13C values (foot 0.4 ‰; gonad 0.3 ‰), whereas it did not significantly affect δ15N values (foot 0.2 ‰; gonad − 0.1 ‰). The positive effect of ethanol preservation on δ2H values (foot 7.1 ‰; gonad 14.5 ‰) and the negative effect on C:N ratios (foot − 0.1; gonad − 0.5) depended on the tissue type, with larger effects found on the lipid-rich gonad. Overall, ethanol preservation affected δ2H values more than the δ13C, δ15N or C:N ratios of FPM tissues. After 1 month of preservation, the isotope values remained rather stable, and significant changes were only observed in δ15N values. The results imply that ethanol-preserved FPM samples can be used if potential shifts in isotopic and elemental ratios are accounted for prior running mixing models for estimating dietary proportions.

Lipid Profile of the Freshwater Pearl Mussel Margaritifera margaritifera Inhabiting Different Biotopes of the Lake-River System of the Kem River, White Sea Basin

The freshwater pearl mussel Margaritifera margaritifera is considered to be the most rapidly declining species of freshwater bivalve, whereas its colonies in rivers of the White Sea Basin remain the most numerous in the world. The lipid profiles of mantle, muscle (foot), and digestive gland tissues of the freshwater pearl mussel from the Kem, Ukhta, and Vozhma Rivers in autumn (end of September, early October) were studied using high performance thin-layer chromatography. The highest total lipid [TL] content was found in the digestive gland. Cholesterol esters, non-esterified fatty acids, phospholipids, and cholesterol were the dominant lipids in all studied tissues. The reduced triacylglycerol content in the mussels was associated with its utilization during the spawning period. The colony of the freshwater pearl mussel inhabiting the Vozhma River was distinguished by higher TL content in the mantle and digestive gland. Data on the size-age characteristics of mollusks from the Kem, Ukhta, and Vozhma Rivers and the relationship between the structural and storage lipid content and size-age parameters are discussed. The results are important for different conservation strategies of endangered species, such as the freshwater pearl mussel, especially in ecological monitoring based on evaluation of the physiological and biochemical state of mollusks and rare natural colonies.

The combined effects of filter-feeding bivalves (Cristaria plicata) and submerged macrophytes (Hydrilla verticillate) on phytoplankton assemblages in nutrient-enriched freshwater mesocosms.

Freshwater ecosystems are threatened by eutrophication, which causes persistent and harmful algal blooms. Filter-feeding bivalve mollusks and submerged macrophytes (SMs) alleviate the eutrophication effects by inhibiting phytoplankton biomass blooms. However, very little is known about whether and how the combined manipulation of filter-feeding bivalves and SMs control eutrophication and influence phytoplankton assemblages. Here, we performed a nutrient-enriched freshwater mesocosm experiment to assess the combined effects of the filter-feeding bivalve Cristaria plicata, a cockscomb pearl mussel, and the macrophyte Hydrilla verticillate on the biomass and composition of phytoplankton assemblages. We found that addition of C. plicata and H. verticillate decreased the water nutrient concentrations and suppressed overall phytoplankton biomass. Further, distinct differences in taxa between restoration and control treatments were observed and noticeably competitive exclusion of cyanobacteria in the restoration treatments occurred. An antagonistic interaction between filter-feeding bivalves and SMs was only detected for total cyanobacteria biomass demonstrating that a larger magnitude of SM restoration may override the effect of filter-feeding bivalves. Our results suggest that manipulation, through the addition of bivalves as grazers, associated with the restoration of SMs, is an efficient approach for reducing cyanobacterial blooms and alleviating eutrophication.

Active movement to coarse grained sediments by globally endangered freshwater pearl mussels (Margaritifera margaritifera)

The freshwater pearl mussel Margaritifera margaritifera is an endangered bivalve which is usually regarded as sedentary, although individual movement has been observed both vertically and horizontally. Little is known about the causes and rates of mussel movement. The objective of this study was to test the effect of microhabitat characteristics on the horizontal movement distance and rates of freshwater pearl mussels. A total of 120 mussels (length range 40–59 mm) were marked individually with passive integrated transponder tags, placed in stream microhabitats differing in their sediment composition and monitored biweekly over a period of 10 weeks. Mussels situated in sand-dominated habitats had a significantly higher mean movement rate (3.2 ± 4.2 cm/day, mean ± SD) than mussels situated in gravel-dominated (1.9 ± 2.7 cm/day) or stone-dominated habitats (1.8 ± 3.2 cm/day). The direction of the movements appeared random; however, an emigration from sandy habitats was observed, probably to avoid dislodgment from these hydraulically unstable habitats. This study demonstrates that freshwater pearl mussels can actively emigrate from unsuitable microhabitats. Once suitable streams with respect to physical, chemical, and biological quality were identified, it is therefore only necessary to identify suitable mesohabitats (area of 10–30 m2) when reintroducing or relocating mussels.

Fish ecology and sediment conditions in freshwater pearl mussel (M. margaritifera) habitats

Stable, healthy populations of locally adapted brown trout (Salmo trutta) are crucial for the reproduction of the freshwater pearl mussel (Margaritifera margaritifera), as they serve as host fish in its life cycle. In the Interreg project Malšemuschel at the river Maltsch/Malše the hydro-morphology and sediment conditions were analysed regarding their influence on the biocoenoses of rhithral waters on granitic bedrock. In this paper, the fish fauna is highlighted in particular. It could be shown that sand on the one hand and finer fractions on the other impair watercourses and their fish fauna in different ways. While an increase in fine sediments (clay, silt) causes shifts in species compositions and leads to a steady decrease in abundances, sand rather influences the hydromorphology by reducing pools, flattening out the river bed and covering structures. Thus, sand seems to more directly influence the biomass and to cause a sharp deterioration of the fish fauna when a certain threshold is exceeded. Furthermore, it could be shown that brown trout populations are strongly related to the presence of natural structures e.g. large boulders or riparian vegetation.

Communication with local community and stakeholders as a key activity to protect oligotrophic streams with freshwater pearl mussel

A complex approach in nature and species conservation is crucial for a real and persisting success of the costly work and effort. A combination of both direct support of the endangered species together with habitat restoration, and cooperation with community members and authorities, present and active within the species’ habitat, is a must. Nevertheless, the latter is often missing in nature conservation work, resulting in a lack of awareness and interest regarding the targeted species among the local population. Here we describe a plan, practice, and results of a complex information campaign aimed at public, local/regional authorities and stakeholders conducted over the period of four years in the catchment of the Malše river (Czech Republic). The campaign focused on the conservation of a rare aquatic mollusc, the freshwater pearl mussel Margaritifera margaritifera, living in streams with high water quality. The species serves as an umbrella species for the conservation of oligotrophic river catchments. The main goal of the campaign was the transfer of knowledge and transition to good management practice in agriculture, forestry, water and fish management. In our experience, the vast majority of stakeholders took part in the educative campaign and showed willingness to adjust their management work in order to protect freshwater pearl mussels. The role of a contact person was crucial, they provided advice to stakeholders and helped channel the communication with local authorities. We compare our experiences with other recent educative campaigns held in Europe, their effects, and their future challenges. We propose key activities in communication with the local community and stakeholders to enhance the impact of nature conservation.

Disturbed by pH? Nacre tablet thickness of freshwater pearl mussels (Margaritifera margaritifera) is a poor temperature proxy

Context The ultrastructure of marine bivalve shells, specifically the thickness of nacre tablets has recently been identified to be related to water temperature. Aims This study investigates whether the nacre tablet thickness (NTT) of freshwater pearl mussel shells can serve as a proxy for stream water temperature. Methods Laboratory-grown juvenile and adult Margaritifera margaritifera from three streams were analysed using scanning electron microscopy to determine the relationship between NTT and water temperature. Key results Neither laboratory-grown juvenile nor adult specimens from the field revealed consistent NTT trends with temperature. Conclusion The NTT of M. margaritifera is largely uncoupled from stream water temperature. Presumably, fluctuations in stream water pH caused physiological stress and forced the animal to allocate more energy to homeostasis than to shell growth. The absence of a coupling between NTT of juvenile mussels and temperature is likely to be due to stronger physiological control on nacre tablet morphology during early stages of ontogeny, favouring thinner tablets with a larger proportion of organics v. CaCO3, which increases the fracture resistance of the shell. Implications Therefore, only if ambient pH remains stable, NTT of freshwater mussels beyond the age of five may serve as a temperature proxy, albeit being associated with a large error.

Riparian detritus vs. stream detritus: food quality determines fitness of juveniles of the highly endangered freshwater pearl mussels (Margaritifera margaritifera)

Detritus is an important energy source of stream food webs. Being a mix of allochthonous and autochthonous sources, it is often unknown, which components contribute to the growth of stream organisms. This study focussed on the comparison of two different detritus types (riparian detritus and stream detritus) with respect to food quality and effects on growth as a fitness parameter of juvenile freshwater pearl mussels (FPM). We performed feeding experiments with juvenile FPM under laboratory conditions using the two detritus types from four different natural sources each. Food quality was determined by analysing the fatty acid composition. Stream detritus (conditioned to stream environment including autochthonous microbes) resulted in significantly higher growth rates of juvenile FPM than predominately terrestrial-based riparian detritus indicating higher food quality. Significantly positive correlations were found between mussel growth and different groups of polyunsaturated fatty acids (PUFA). This suggests that especially trace substances such as long-chained n-3 PUFAs and a high ratio of n-3 to n-6 PUFAs enhance the food quality of stream detritus for juvenile FPM. These results highlight the importance of instream conditioning of detritus for the food mix in headwater streams and the importance of PUFAs for the development of juvenile FPM.

Visitor pressure in protected areas: Interactions between river tourists and freshwater pearl mussels in the Central European national park Šumava

The freshwater pearl mussel Margaritifera margaritifera (FPM) is an endangered bivalve species occurring in oligotrophic rivers. FPMs can be found in the Upper Vltava River in the Šumava National Park (Czech Republic, Central Europe), where suitable conditions exist for both adult and juvenile mussels. Non-consumptive human leisure activities are known to negatively affect wildlife in fragile aquatic ecosystems, and in the Vltava River boating tourism in particular may be a threat to local mussel populations. This study focused on an assessment of interactions between river tourists and the FPM, using “pseudo-mussels” in both field and lab experiments. In the field, fake concrete mussels were exposed at three rest sites for river tourists, and in the lab experiment glued shells were placed at the water-sediment interface in an aquarium tested for mechanical impacts of paddles (dislodgement and crushing). Interactions of river tourists with fake mussels were most frequent at low water levels (10–20 cm), and within 2 m from the nearest river bank. Mussel visibility and the presence/absence of a guide played an important role in people´s reactions, but site-specific effects were also found. Unintentional interactions (60–69%) were mostly observed at less-attractive sites (with a limited area of shallow water where people spent most time outside the river channel), whereas visual and manipulative interactions (76%) dominated at a more-attractive site (the confluence of two rivers where people move across and inspect the river channel). Crash tests revealed that 8.03 ± 1.37 (mean±SD) and 7.88 ± 1.13 (mean±SD) hits by paddles were needed for dislodgment and crushing, respectively. Those findings indicate that the direct effects of recreational boating might be less detrimental than those of accompanying activities (such as wading, bathing, and swimming). The role of trampling (and handling) disturbances should be investigated in more detail to help conserve FPM populations in protected areas.