Does the parasitic freshwater pearl mussel M. margaritifera harm its host?

Many parasites have strong negative impacts on their hosts, but the effects of single-host, non-trophically transmitted parasites can be subtle and are not well understood. We examined the physiological response of juvenile brown trout (Salmo trutta) to encystment by the parasitic larvae (glochidia) of the freshwater pearl mussel, Margaritifera margaritifera. Glochidia abundance was positively correlated to host body size and was accompanied by significant spleen enlargement at 31 days postexposure, but not before (15 days) or after (160 days). Compared to controls, encysted gill lamellae were significantly thicker and longer, and tended to have fewer mucous cells which may have facilitated encystment. There were no significant difference in mean blood haematocrit between encysted and uninfected trout, but encysted trout took c. 6 h longer to reach basal ventilation rate than controls suggesting that glochidiosis may impose a respiratory burden to brown trout. These findings may have implications for the artificial propagation of the freshwater pearl mussel because the effects of glochidia on host respiratory performance appear to be additive. Therefore, aiming for high glochidia loads may not be the best option for mussel propagation programmes, if this compromises host fitness and hence the probability of successful glochidia excystment.     

Comparative characteristics of the lipid status of gills of juvenile Atlantic salmon infected with glochidia of the freshwater pearl mussel living in rivers of the European North

A comparative study of the lipid status of gills of juvenile Atlantic salmon infested with glochidia of the freshwater pearl mussel inhabiting the Vuokinyoki (White Sea basin) and Syuskyuyanyoki (basin of Lake Ladoga) rivers in the fall is carried out. The most infested gills of juvenile salmon and the lower water temperature (1.3°C) in the Vuokinyoki River reveal higher cholesterol levels and cholesterol/phospholipids ratios, which are due to the influence of infection factors; thereby they slow down biochemical processes. It is noted that variations in the lipid composition in juvenile salmon the gills of which are infected with glochidia reflect the development of adaptive responses that maintain homeostasis of juveniles under invasion.     

Survival of glochidial larvae of the freshwater pearl mussel, Margaritifera laevis (Bivalvia: Unionoida), at different temperatures: a comparison between two populations with and without recruitment

The viability of free-living glochidia of the freshwater pearl mussel (Margaritifera laevis) was studied in the laboratory at water temperatures of 10 degrees C, 15 degrees C and 20 degrees C. To obtain glochidia, gravid female mussels were collected from the Chitose River, inhabited by adult and juvenile mussels, and from the Abira River, where only adult mussels were found. Daily survival rates of glochidia from each population at various water temperatures were significantly different, and survival time was longest at the lowest temperature in each population. Maintenance of some field mussel populations might become difficult at higher water temperatures due to the short survival time of glochidia and expected low density of host fish. Daily survival rates of glochidia were compared between the Abira population at 15 degrees C and the Chitose population at 20 degrees C, since these temperatures were close to the mean water temperature during the period of glochidial release in the respective rivers. Daily mean survival rates were significantly different between the Abira population at 15 degrees C and the Chitose population at 20 degrees C. Mean glochidial survival rate for the Chitose population changed from 85.3% to 66.2% from 9 to 13 h, whereas that for the Abira population dropped suddenly from 80.4% to 34.2% from 10 to 14 h after the initiation of experiment. Absence of juveniles in the Abira River might have been caused by the low glochidial viability. Survival times of free-living glochidia in Margaritiferidae tend to be shorter than in other families in Unionoida. A trade-off is suggested between high fertility and low glochidial survival rate in Margaritiferidae.     

The freshwater pearl mussel <Emphasis Type="Italic">Margaritifera margaritifera</Emphasis> L.: Metamorphosis,growth, and development dynamics of encysted glochidia

The growth and morphogenesis of glochidia of the freshwater pearl mussel Margaritifera margaritifera on the gills of the Atlantic salmon Salmo salar in the Syuskyuyanyoki River (Karelia) are studied. Comparative analysis of histological features of glochidia depending on the age of the cyst is made, and the results of studies of the relationship and the influence of seasonal changes in water temperature on growth and morphogenesis of glochidia, which is essential for adaptation of living organisms and ensuring the sustainability of the participants of the parasite–host relationship, are presented.     

Habitat traits, population structure and host specificity of the freshwater pearl mussel Margaritifera margaritifera in the Waldaist River (Upper Austria)

In European streams and rivers, the freshwater pearl mussel (Margaritifera margaritifera L., 1758) faces extinction. This is also true for the Waldaist River, with 20,000 specimens recorded in the early 1990s then Austria’s most important pearl mussel river. Nowadays, there is only a single 320 m stretch with noteworthy mussel densities. During an in-depth survey of this river stretch in 2010, we detected a total of 2,774 specimens. Mussel microhabitats were confined to patches of sand and fine gravel (psammal and akal) at run sections of the river, stabilized by large boulders. Pearl mussels avoided large accumulations of fine sediments. Typically situated at undercut slopes, preferred microhabitats were 0.25–0.50 m deep at baseflow with current velocities (at 40% depth) of 0.2–0.6 m s^?1. A comparison of the present stock with data from 1997 revealed a rapid decline in mussel density down to 27%. We also noticed strongly reduced growth and a high mortality of medium age classes. Juvenile mussels were completely lacking. With respect to host specificity in terms of glochidia survival, the brook char Salvelinus fontinalis (Mitchill, 1814), a suitable host in North America, shed glochidia within eight days. In the brown trout Salmo trutta L., 1758, two strains were investigated. Glochidia survival, growth and prevalence were significantly higher in the Danish than in the Austrian hatchery strain.     

Regional monitoring of freshwater pearl mussel <Emphasis Type="Italic">Margaritifera margaritifera</Emphasis> in the County of Norrbotten,Sweden

A total of 16 freshwater pearl mussel Margaritifera margaritifera rivers out of 64 known freshwater pearl mussel rivers are included in the Administrative County of Norrbotten regional monitoring program. First surveys were done in 1994 and the results show that only three rivers have viable populations according to national criteria despite little visible human impact and that juvenile mussels (<50 mm in length) have been found in all rivers at every occasion. The results also indicate that there might be “missing years” for juvenile recruitment suggesting that freshwater pearl mussel populations at the extreme end of the species range might be dependent on “golden moments” in order to keep up viable populations. Deeper knowledge and better tools for determining viable populations might be necessary in order to make the right decisions on how to manage the rivers and the freshwater pearl mussel populations.     

Do individual Activity Patterns of Brown Trout (Salmo trutta) alter the Exposure to Parasitic Freshwater Pearl Mussel (Margaritifera margaritifera) Larvae?

The hypothesis that interindividual differences in the activity of brown trout alter the exposure to parasitic freshwater pearl mussel glochidia was tested in a Swedish stream. Wild yearling brown trout (N = 103) were caught, individually tagged for identification and scored for open‐field activity during standardized laboratory tests in June. Fifty gravid freshwater pearl mussels were relocated to the stream, where after the trout were released back into the stream. The fish were recaptured in October (N = 35), checked for glochidia encystment (infested individuals: n = 6) and re‐scored for open‐field activity traits. Swimming velocity during the test was higher in fish infected with glochidia, suggesting that high activity could increase their exposure to glochidia. Potentially, as metabolism rate and ventilation rate typically increase with activity, elevated activity may lead to an increased likelihood of glochidia passing over the gills. This novel finding suggests that glochidia infestation is non‐random and that the behaviour of the host fish can influence the likelihood of glochidia infestation.     

Historical geography of pearl harvesting and current status of populations of freshwater pearl mussel Margaritifera margaritifera (L.) in the western part of Northern European Russia

This paper summarises data on the occurrence of freshwater pearl mussel Margaritifera margaritifera in the western part of Northern European Russia adjacent to White Sea, Barents Sea, Onega Lake and Ladoga Lake basins. Also, this article provides an overview of the literature-based data and archive materials on the history of pearl harvesting. We include a list of rivers of Northwestern Russia, where pearls were harvested during the period of the sixteenth to twentieth centuries. The pearl mussel populations at the present time exist in at least 24 water streams in Northwestern Russia. Many of these populations are in high abundance and are able to reproduce normally. Data on the status of populations are given in this paper. Data indicate that the pearl mussel population in Russia contains >143.5 million individuals, but this estimation is certainly undervalued. Timber floating (timber floating is the transport of timber on waterways), hydro-engineering construction, pollution of rivers by industrial wastes and introduction of alien species, as well as a reduction in host fish species numbers can all be cited as the main factors that transformed the ecosystems inhabited by pearl mussel of Northern Russia. In addition, methods used in the Russian Federation for decreasing the anthropogenic load on pearl mussel populations were considered.     

三角帆蚌钩介幼虫在两种寄主鱼上的寄生效果及其对寄主鱼血浆生化指标的影响

利用黄颡鱼(Pelteobagrus fulvldraco)、尼罗罗非鱼(Tilapia nilotica)对三角帆蚌(Hyriopsis cumingii)钩介幼虫进行了寄生实验,分析了幼虫在寄主鱼上的寄生周期和脱落数量与寄主种类、寄主鱼单尾体重的相关关系.以未寄生幼虫的罗非鱼为空白对照,测定分析了罗非鱼在幼虫寄生前、脱落后主要血浆生化指标的变化.实验结果显示:黄颡鱼、罗非鱼平均每千克体重脱落稚贝的数量分别为(26 435±6 430)只、(26 327±3 091)只,两种寄主鱼间不存在显著差异;脱落的稚贝数量随着寄主鱼体重的增加而增加,并呈显著线性相关.在水温(24±1)℃条件下,黄颡鱼组、罗非鱼组的稚贝脱落高峰分别出现在寄生后的第8 d、第7 d,罗非鱼组表现为明显的"早脱落"现象.罗非鱼在寄生前后血浆总蛋白(TP)、白蛋白(Alb)、总糖(Glu)、总胆固醇(Tc)和高密度脂蛋白(HDL)含量均未出现显著变化,而血浆甘油三酯(TG)、低密度脂蛋白(LDL)含量显著下降(P=0.001,0.020).未寄生组罗非鱼的7项指标均未出现明显变化.实验结果将为三角帆蚌钩介幼虫变态发育相关营养因子的探讨提供理论参考.     

Influence of cortisol on the attachment and metamorphosis of larval Utterbackia imbecillis on bluegill sunfish (Lepomis macrochirus)

The larvae of unionid freshwater mussels (i.e., glochidia) undergo a parasitic stage requiring their attachment to the external epithelia of fish hosts, where they metamorphose into free-living juveniles. We describe the physiological effects in bluegill sunfish (Lepomis macrochirus) of infection with glochidia from the paper pondshell (Utterbackia imbecillis). Glochidia accumulation on bluegill increased dramatically at concentrations of 2000 glochidia liter(-1) and above, reaching a maximum attachment density of about 30 glochidia g(-1) fish at 4000 glochidia liter(-1). Plasma cortisol was the most sensitive indicator of biological effect to glochidial exposure, increasing significantly in hosts exposed to 2000 glochidia liter(-1) or greater. Glochidia were 31% more likely to undergo successful juvenile metamorphosis when attached to bluegill with elevated plasma cortisol, largely due to the enhanced survivorship of these larvae during the first 48 h after infection. We tested the hypothesis that glochidial attachment and juvenile metamorphosis were stimulated directly by plasma cortisol in fish hosts. Bluegill were given an intraperitoneal injection of cortisol, then infected with 1000 glochidia liter(-1) at 48 h after hormone supplementation. Cortisol-injected fish had a 42% increase in the number of attached glochidia g(-1) fish and a 28% increase in larval metamorphosis compared to sham-injected and control fish. We provide evidence that cortisol enhances glochidial metamorphosis on hosts by improving the retention of attached glochidia. This study gives insights into the influence of host physiology on glochidial attachment and juvenile mussel transformation.     

The impact of land use on the mussel Margaritifera margaritifera and its host fish Salmo trutta

Today, land use impacts a major proportion of all streams. Here, landscape features in corridors along streams and water chemical factors were analyzed in relation to recruitment of the threatened freshwater pearl mussel (Margaritifera margaritifera) and its host fish the brown trout (Salmo trutta). Mussel recruitment and trout density were negatively related to forest clear-cuts. Mussel recruitment was negatively related to water color and turbidity. Therefore, the threats to the mussel may be severe, as low mussel recruitment may be caused by direct effects on the juvenile mussels and indirect effects on the host fish. High proportions of lakes and ponds were found to be positive for recruitment and for trout, and deciduous forest was positively related to trout. The combination of investigations at different scales at the landscape level and at in-stream levels may be applicable to find threats to other threatened species. The results indicate that forestry activities may negatively affect recruitment of freshwater pearl mussels and its host fish. Reductions of forestry activities and the retaining of intact quantity and quality of the riparian zones next to streams, both for the mussel and its host fish may be important conservation measures to restore freshwater pearl mussel populations.     

High levels of multiple paternity in a spermcast mating freshwater mussel

Multiple paternity is an important characteristic of the genetic mating system and common across a wide range of taxa. Multiple paternity can increase within‐population genotypic diversity, allowing selection to act on a wider spectre of genotypes, and potentially increasing effective population size. While the genetic mating system has been studied in many species with active mating behavior, little is known about multiple paternity in sessile species releasing gametes into the water. In freshwater mussels, males release sperm into the water, while eggs are retained and fertilized inside the female (spermcast mating). Mature parasitic glochidia are released into the water and attach to the gills of fish where they are encapsulated until settling in the bottom substrate. We used 15 microsatellite markers to detect multiple paternity in a wild population of the freshwater pearl mussel (Margaritifera margaritifera). We found multiple paternity in all clutches for which more than two offspring were genotyped, and numbers of sires were extremely high. Thirty‐two sires had contributed to the largest clutch (43 offspring sampled). This study provides the first evidence of multiple paternity in the freshwater pearl mussel, a species that has experienced dramatic declines across Europe. Previous studies on other species of freshwater mussels have detected much lower numbers of sires. Multiple paternity in freshwater pearl mussels may be central for maintaining genetic variability in small and fragmented populations and for their potential to recover after habitat restoration and may also be important in the evolutionary arms race with their fish host with a much shorter generation time.     

Variation in the <Emphasis Type="Italic">COI</Emphasis> gene of the freshwater pearl mussel <Emphasis Type="Italic">Margaritifera margaritifera</Emphasis> from River Vuokkijoki

The freshwater pearl mussel Margaritifera margaritifera L. is one of the most endangered freshwater mussels in the world. Effective conservation of threatened species requires not only ecological, but also genetic information from the target species and populations. Since low genetic diversity can reduce the ability of a species to adapt to environmental changes, maintaining genetic diversity has been identified as one of the key elements in successful conservation programs. We examined genetic variation of the freshwater pearl mussel from the River Vuokkijoki, Karelia, Russia. We sequenced a fragment of the cytochrome c oxidase subunit I gene (COI) from 22 individuals and compared the data to 32 previously published COI sequences available in GenBank. We identified 10 different COI haplotypes in the sequenced samples, three of which had not been previously reported. Our results show that the River Vuokkijoki has high genetic diversity and suggest that the colonization of this northern freshwater pearl mussel population might have occurred from multiple and even distant refugia. Therefore, the freshwater pearl mussel population of the River Vuokkijoki is valuable for the conservation of the whole species.     

Practical experience in the rearing of freshwater pearl mussels (Margaritifera margaritifera): advantages of a work-saving infection approach,survival, and growth of early life stages

The critically endangered freshwater pearl mussel (Margaritifera margaritifera Linnaeus 1758) is the target species of an Austrian conservation project that involves captive breeding. In order to optimize the operational procedure, controls were conducted at several decisive stages, including infection of host fish (for which a time- and work-saving enclosure approach was tested), larval growth during the parasitic stage, growth of juvenile mussels in climate chambers at different temperatures, and growth and survival of re-introduced juveniles in field cages. High infection rates could be attained under near natural conditions. Distinctive patterns in the way the gill arches of the host fish were infected could be detected. Encysted glochidia showed significantly different successive growth stages, related to water temperature. In all, five distinctive growth stages could be detected in the course of the first 562 days of observation. The stages are described and the respective daily increments given. Very high survival rates were achieved during hibernation in the field as well as at the laboratory. The study suggests a way for saving time in the infection procedure that can more effectively be invested in an intensive maintenance of juveniles at the laboratory and during hibernation in the field.     

Survival and extinction of the southern populations of freshwater pearl mussel Margaritifera margaritifera in Russia (Leningradskaya and Novgorodskaya oblast)

Information on the southern populations of the freshwater pearl mussel in Russia (Novgorodskaya and Leningradskaya oblast) is very scarce and has never been mentioned in the international scientific literature. Pearl fishing used to be common in this area in the fifteenth to eighteenth centuries but collapsed afterwards. Long-term field survey undertaken in 2006–2012 revealed that eight populations survived and at least 22 became extinct. The total number of mussels found was about 50,000, with the largest population in one small river consisting of approximately 40,000 individuals. Juveniles were found in six populations. The data obtained are of interest for assessing the resilience and stability of pearl mussel populations in response to potentially negative environmental and particularly anthropogenic impacts. The results are expected to inform future management strategies for conservation of suitable habitats for successful pearl mussel reproduction. The decline in host fish populations in the rivers appears to be the most significant threat to the survival of the existing pearl mussel populations.     

The freshwater pearl mussel <Emphasis Type="Italic">Margaritifera margaritifera</Emphasis> in Bavaria,Germany—Population status,conservation efforts and challenges

The freshwater pearl mussel Margaritifera margaritifera is a highly specialized, sensitive and critically endangered freshwater bivalve with a complex life cycle. An overview on the population status in Bavaria, Germany, one of the species’ main areas of distribution in Central Europe, is presented. Using the example of the freshwater pearl mussel and a Bavarian conservation project for this species, a scheme for the development of site-specific conservation concepts is discussed.     

Cross-resistance of largemouth bass to glochidia of unionid mussels

We tested whether host fish that acquired resistance to glochidia of one mussel species were cross-resistant to glochidia of other species. Largemouth bass (Micropterus salmoides) were primed with 4-5 successive infections of glochidia of Lampsilis reeveiana. The percentage of attached glochidia that survived and transformed to the juvenile stage (transformation success) was compared between primed fish and na?ve controls. Transformation success of L. reeveiana, Lampsilis abrupta, Villosa iris, and Utterbackia imbecillis was significantly lower on primed fish (37.8%, 43.5%, 67.0%, and 13.2%, respectively) than on control fish (89.0%, 89.7%, 90.0%, and 22.2% respectively). Immunoblotting was used to analyze the binding of serum antibodies from primed fish with glochidia proteins. Antibodies bound to glochidia proteins of similar molecular weight from L. reeveiana and L. abrupta. Bound proteins of V. iris differed in molecular weight from those of the Lampsilis species. There was no binding to specific glochidia proteins of U. imbecillis or Strophitus undulatus. Our results indicate that host-acquired resistance can extend across mussel genera and subfamilies and might involve both specific and nonspecific mechanisms. Understanding the specificity of acquired resistance of hosts to glochidia could enhance understanding of the evolutionary and ecological relationships between mussels and their host fishes.     

Asymmetric dispersal structures a riverine metapopulation of the freshwater pearl mussel Margaritifera laevis

Unidirectional water flow results in the downstream‐biased, asymmetric dispersal of many riverine organisms. However, little is known of how asymmetric dispersal influences riverine population structure and dynamics, limiting our ability to properly manage riverine organisms. A metapopulation of the freshwater pearl mussel Margaritifera laevis may be sensitive to river currents because mussels are repeatedly exposed to downstream drift during floods—a parasitic life stage is the only, limited period (~40 days) during which larvae (glochidia) can move upstream with the aid of host fish. We hypothesized that water‐mediated dispersal would overwhelm upstream dispersal via host fish, and therefore, that upstream subpopulations play a critical role as immigrant sources. To test this hypothesis, we examined the effects of both up‐ and downstream immigrant sources on the size of target subpopulations in the Shubuto River system, Hokkaido, Japan. We found that target subpopulation size was dependent on the upstream distribution range of reproductive subpopulations and the number of upstream tributaries, which are proxies for the number of potential immigrants moving downstream. In contrast, little influence was observed of downstream immigrant sources (proximity to downstream reproductive subpopulations). These results were consistent even after accounting for local environments and stream size. Our finding suggests that upstream subpopulations can be disproportionately important as immigrant sources when dispersal is strongly asymmetric.     

Physiological and ecological hosts of Popenaias popeii (Bivalvia: Unionidae): laboratory studies identify more hosts than field studies

1. Freshwater mussels are critically endangered in North America, making it important to understand their environmental requirements at all life stages. As glochidia (larvae), they attach to fish hosts where they undergo substantial mortality, making this transition important in their life cycle. Larval host fish requirements have typically been described using data from laboratory infestations to determine suitable hosts. 2. Laboratory infestations circumvent many natural barriers that prevent infestation of physiologically compatible fishes by mussel larvae. While such methods are invaluable for identifying ‘physiological hosts,’ they cannot fully describe realised ‘ecological hosts’ in the field. 3. We studied Popenaias popeii in the Black River in New Mexico, because it is of conservation concern and it is the only mussel species present, facilitating identification of glochidial infestation. To explore the difference between physiological hosts and ecological hosts, we conducted a 3‐year field study of fishes infested by P. popeii glochidia. 4. Substantially fewer fish species were infested by P. popeii in the wild (10 of 20 observed) than had been identified as physiological hosts in laboratory trials (24 of 31). We combined data on fish abundance, proportion of fish hosts infested (prevalence) and the number of glochidia per fish (intensity) and identified three fish species that probably contributed substantially more to mussel recruitment by carrying more glochidia than other host species. 5. Similarities in behaviour among these fishes allowed us to hypothesise routes of infestation, such as benthos‐feeding by catostomids, that allow glochidia to infest these hosts at higher rates than other suitable hosts. Overall, this approach provides a method of quantifying the relative importance of different species of host fish in the mussel lifecycle.     

Acquired resistance of bluegill sunfish Lepomis macrochirus to glochidia larvae of the freshwater mussel Utterbackia imbecillis (Bivalvia: Unionidae) after multiple infections

The effects of multiple infections on the host-parasite relationship between bluegill sunfish (Lepomis macrochirus) and parasitic glochidial larvae of the freshwater mussel Utterbackia imbecillis were examined. Na?ve, young-of-the-year bluegills were infected with glochidia and placed in individual observation chambers. Each day, water was drained from each chamber and the numbers of dead glochidia, live glochidia, partially metamorphosed glochidia, and fully metamorphosed juvenile mussels were counted. The same fishes were infected a total of 4 times. After 2 infections, the fish began to exhibit evidence of acquired resistance to glochidia. During the third and fourth infections, this resistance was clearly evidenced by the marked increase in the percentage of dead and live glochidia shed during the first 5 days of the infection and by the significant decrease in the success of metamorphosis. The total number of glochidia that successfully attached to the fish decreased significantly during the fourth infection relative to the first. The number of larvae attached to the host fish was positively correlated with the size of the fish during the first infection but was negatively correlated during all subsequent infections. Variance to mean ratios indicated that larvae were aggregated among host fishes during the infections. This study has important implications in propagation and conservation efforts of this endangered group of organisms.