Host-dependent genetic variation in freshwater pearl mussel (Margaritifera margaritifera L.)

Freshwater fishes are vulnerable to changes in water quality, physical habitat and connectivity resulting from drought, particularly in regulated rivers. When adequate river flows return, the recovery of populations might depend on the duration and consequences of drought. Rivers of the highly regulated Murray–Darling Basin in south-eastern Australia terminate at two large, shallow lakes that are separated from the estuary by tidal barrages. Over-abstraction of water and widespread prolonged drought (1997–2010) placed the lakes under severe environmental stress, culminating in critical water level recession from 2007 to 2010. Concurrently, most freshwater fish populations collapsed. We investigate shifts in fish assemblages resulting from habitat inundation in the lakes following the drought. The inundation and re-connection of the lakes and fringing habitats led to a substantial reduction of salinity throughout the region, and aquatic vegetation shifted from salt-tolerant to freshwater species. Fish assemblages became increasingly characterized by common freshwater taxa (ecological generalists), including high proportions of alien species. There were no indications of population recovery for three threatened species. The findings emphasize that short-lived fishes with specialized habitat requirements are vulnerable to severe population declines during prolonged drought in regulated rivers, which might restrict their recovery when adequate flows return.     

Physicochemical stream bed characteristics and recruitment of the freshwater pearl mussel (Margaritifera margaritifera)

1. The freshwater pearl mussel (Margaritifera margaritifera) is endangered and of conservation importance. We used its survival/mortality during the critical post‐parasitic phase as a biological indicator for the habitat quality of the stream substratum. 2. We established and tested biological, physical and chemical methods of assessing the stream bed in 26 streams from seven European countries. We analysed penetration resistance, texture, the concentrations and ratios of C, N, S, P, Fe, Mn in fine material <100 μm, and redox, pH and electric conductivity at the surface and at 5 and 10 cm into the substratum. 3. Sites with high stream bed quality (promoting pearl mussel populations with good juvenile recruitment) had coarser and better sorted substrata with significantly lower quantities of fines, and a higher Mn concentration in the fines, than poor quality sites. Redox potential (Eh) at sites without recruitment differed markedly between the free‐flowing water at the surface and at 5 and 10 cm in the bed, whereas no differences were detectable at good quality sites. This was also true of electric conductivity and, to a lesser extent, pH. The stream bed at sites lacking pearl mussel recruitment had a more variable and higher penetration resistance, indicating clogging of the interstitial macropore system by the deposition of mud and compaction of the stream bed. 4. Our results show that habitat quality for pearl mussels depends strongly on the exchange between the surface and the interstices, which is governed by physicochemical characteristics of the stream substratum. Combined measurements of penetration resistance, depth gradients of Eh and texture were most suitable for assessing stream bed quality, while water chemistry was insufficient because of the decoupling of interstitial and free‐flowing water at poor quality sites.     

Population density and age structure of the freshwater pearl mussel, Margaritifera margaritifera, in two Iberian rivers

1. The European freshwater pearl mussel, Margaritifera margaritifera (Bivalvia: Unionoida), is one of the most threatened mussels. The Iberian populations of this species are considered peripheral because their distinct characters such as growth rate and longevity, and require development of effective conservation strategies. 2. We assessed population density and age structure of pearl mussels in two Galician rivers (Eo and Masma in north‐west Spain). Four sampling sites were selected in each river to cover stretches of 100 m. The mean density of mussels in each of these sampling transects was estimated using the adaptive sampling technique, given that mussels occur at low densities and are highly aggregated in these rivers. 3. Age structure was inferred for each population using length–age keys. The empty shells encountered during sampling were used to determine the length of the specimens at different ages (years), together with length‐at‐age data from shells previously analysed for computing growth rates from the same rivers. Water samples from both rivers were analysed for typical physicochemical parameters. 4. Mean densities were very variable, even within the same river (from 0.27 to 6.55 m^?2 in the River Eo and from 0.98 to 2.38 m^?2 in the River Masma). Individuals in the 0‐ to 5‐year age class were scarce in both rivers. 5. Margaritifera margaritifera showed a preference for the strip of river bed within 1.5 m from the river bank and avoided sites at greater distances. The species also showed a preference for sites with more than 80% tree cover and avoided sites with <50% cover. 6. Iberian populations exhibit the highest growth rate, together with the lowest maximum age and maximum length known for M. margaritifera. Detailed knowledge about these peripheral Iberian populations will contribute to developing strategies for conservation and management of this endangered species.     

Timing of spawning and glochidial release in Scottish freshwater pearl mussel (Margaritifera margaritifera) populations

1. The timing of reproduction was investigated in six Scottish freshwater pearl mussel populations from 1993 to 2002. Gravid females were examined and the release of mussel larvae (glochidia) was monitored. 2. Annual spawning (oviposition) and spat (glochidial release) events occurred during June to July and June to September, respectively. 3. Between‐river differences in timing seem to be related to water temperature. Mussels in the warmest rivers tend to spawn and spat first, and vice‐versa. 4. Thermal variations also seem to influence the timing of reproduction within rivers, which can be delayed by several weeks during cold years. At least 3000°‐days occur between annual episodes of glochidial release. 5. The timing of spawning is determined gradually, probably by a thermal summation effect. 6. The release stage occurs as a sudden, synchronised event, with most of the glochidia spat over 1–2 days, indicating that it is triggered by an environmental cue. Sudden changes in water temperature and/or river level often result in spats, and the underlying mechanism may be respiratory.     

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.     

Physical microhabitat requirements of freshwater pearl mussels,Margaritifera margaritifera (L.)

Surface sediment diatoms from the east coast of Lake Tanganyika were analysed using ordination and classification techniques, and compared with assemblages previously described from the northern part of the lake. Grain-size analyses were performed on subsamples. Four groups of diatom assemblages were recognised. The first group clusters samples taken in the north, far from the Rusizi river mouth. The second group comprises samples taken on silty sediment along the Tanzanian coast, including one sample taken near the mouth of the Malagarazi river and those from the northernmost part of the lake. The third group comprises surface sediments along the Burundian coast (near Ramba and Magara), and the fourth is characterised by epipsammic taxa. A sample taken near the central arm of the Malagarazi river is included in the latter group. The impact of small rivers on the diatom assemblages in the surface sediments is restricted to the mouth area.     

Freshwater pearl mussel (Margaritifera margaritifera) glochidiosis in wild and farmed salmonid stocks in Scotland

The Corbicula of the Saone at Lyon were sampled every month from September 1996 to December 1999. These populations are characterised by a single annual reproduction period starting in May or June and ending in September or October in which two peaks of intensity can be distinguished; the presence of one or more cohorts according to year; low growth in spite of a growth period that generally lasts from March to October; and a lifespan of about 5 years. Although the effect of toxic substances can be considered, it appears that the reason for the low recruitment levels observed from 1997 to 1999 are above all due to the small quantities of food available, especially chlorophyll a.     

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.     

Parasitic freshwater pearl mussel larvae (Margaritifera margaritifera L.) reduce the drift-feeding rate of juvenile brown trout (Salmo trutta L.)

In this paper we describe, for the first time, the effects of freshwater pearl mussel (Margaritifera margaritifera L.) encystment on the drift-feeding behavior of juvenile brown trout (Salmo trutta L.). Because both mussel and salmonid populations are often threatened, this study not only adds knowledge to the understanding of host-parasite systems, but it is also of conservation value. Individual trout, mussel-encysted (25.1?±?5.7 larvae?·?g^?1 body weight, n?=?5) or non-encysted (n?=?5), were fed with chironomid larvae in a flow-through stream aquarium. Feeding trials were filmed and analyzed by counting the numbers of chironomid larvae each individual ate, and by estimating the prey-capture distance. Non-encysted trout had a significantly higher drift-foraging rate than did encysted trout, and they captured significantly more prey further away from their focal point. The reduced foraging success of encysted trout was mainly due to their failure to catch prey relatively further from their focal point. This suggests that reduced foraging success of encysted trout may be due to poorer energetic status, but the physical effects of mussel larvae on prey handling time cannot be ruled out. Encysted trout caught approximately 20 % fewer prey, which would result in a reduction in growth potential during the period of mussel encystment. Reduced energetic status might also result in reduced competitive ability or in increased exposure to predation risk.     

Population dynamics of glochidia of the freshwater pearl mussel Margaritifera margaritifera L., parasitizing on juvenile Salmonidae fishes in northern water reservoirs

This paper presents data on the population dynamics of glochidia of the freshwater pearl mussel Margaritifera margaritifera L. (1758) (a declining species of European fauna), parasitizing on juvenile Salmonidae fishes in rivers of northern Europe. It was found that the number of glochidia parasitizing on gills of juvenile salmon and trout in explored water reservoirs are stimulated by negative binomial distribution. Estimation of the distribution parameter allows us to obtain statistically valid data on the population number of pearl mussel and to judge the stable character of interactions in the host-parasite system (balance of host specimens that are resistant and nonresistant to infection).     

Sensitivity of newly transformed juveniles of the freshwater pearl mussel: Margaritifera margaritifera to acute toxicity of a wide range of contaminants

Hydrobiologia - The freshwater pearl mussel (FWPM) Margaritifera margaritifera is a bivalve mollusk critically endangered in Europe. There is a special concern about the conservation of this...     

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.     

Characterization of cyclophilin D in freshwater pearl mussel (Hyriopsis schlegelii)

Cyclophilin D (referred to as HsCypD) was obtained from the freshwater pearl mussel (Hyriopsis schlegelii).The full-length cDNA was 2 671 bp,encoding a protein consisting of 367 amino acids.HsCypD was determined to be a hydrophilic intracellular protein with 10 phosphorylation sites and four tetratricopeptide repeat (TPR) domains,but no signal peptide.The core sequence region YKGCIFHRIIKDFMVQGG is highly conserved in vertebrates and invertebrates.Phylogenetic tree analysis indicated that CypD from all species had a common origin,and HsCypD had the closest phylogenetic relationship with CypD from Lottia gigantea.The constitutive mRNA expression levels of HsCypD exhibited tissue-specific patterns,with the highest level detected in the intestines,followed by the gonads,and the lowest expression found in the hemocytes.     

Growth Biorhythms of the Freshwater Pearl Mussel Margaritifera margaritifera (Bivalvia,Margaritiferidae), Nemina River Population (Karelia)

Biology Bulletin - Individual linear growth was studied in 23 specimens of the freshwater pearl mussel Margaritifera margaritifera by measuring successive annual growth rings on the shell surface....     

The relationship between the freshwater pearl mussel (<Emphasis Type="Italic">Margaritifera margaritifera</Emphasis>) and its hosts

Endangered freshwater pearl mussels Margaritifera margaritifera reveal a complex life cycle with an obligate host-dependent phase. Only two species, Atlantic salmon Salmo salar and brown trout S. trutta, are important hosts in Europe, indicating a high degree of specialization. Whilst freshwater pearl mussels with their filtering activity can provide important ecosystem services and indirectly improve the habitat quality for their salmonid hosts, their direct effects on physiological stress, reduced swimming performance, and increased mortality at high rates of infestation all support a parasitic character of the mussel during its hostdependent phase. From an evolutionary perspective, both the much greater generation time of the parasite compared to the host, as well as the great distribution ranges of M. margaritifera and its hosts should favour local adaptation patterns. The variable suitability of different salmonid strains and species as hosts for M. margaritifera and the resulting differences in the performance of larvae during the host-dependent phase indicate that host-management strategies should focus on maintaining high quality hosts at a regional scale to avoid selection or genetic drift effects which could erode the genetic and evolutionary potential for adaptation to changing environmental conditions.     

Extensive allozyme monomorphism in a threatened species of freshwater mussel, Margaritifera hembeli Conrad (Bivalvia: Margaritiferidae)

A threatened but under-studied component offreshwater biodiversity in North America is thenative freshwater mussels (Bivalvia:Unionoidea: Margaritiferidae and Unionidae). Genetic data suggest that these mussel speciesgenerally exhibit levels of variability similarto other invertebrates. We surveyed allozymevariation in the Louisiana Pearlshell, Margaritifera hembeli (Margaritiferidae), athreatened freshwater mussel. Five examinedpopulations are monomorphic for 25 allozymeloci, the first report of a native freshwatermussel species with extensive allozymemonomorphism. Low genetic diversity appears tobe characteristic of margaritiferids, as anANOVA indicated that mussels of the familyMargaritiferidae have significantly lowerlevels of heterozygosity than the mussels ofthe family Unionidae. Margaritiferids havestrong habitat preferences and modification ofhabitat leads to rapid loss of populations.Although bottlenecks are known to cause lowgenetic variability, margaritiferids mayexhibit meta-population structure withextinction/re-colonization dynamics leading tolow genetic variability. Margaritiferidsgenerally exhibit a patchy distribution with acolonization rate that is approximately twicethat of extinction. Tests of themetapopulation hypothesis will requireadditional allozyme population genetic data aswell as hypervariable microsatellite loci.     

Evaluation of genetic structure across freshwater mussel community (genus Elliptio) in the Altamaha River basin

Using mitochondrial DNA sequence data and genotypic data from four microsatellite loci, we evaluate the population genetic structure for three common species of the freshwater mussel genus Elliptio in the Altamaha River basin in Georgia, USA. These data are intended to evaluate whether distinct management strategies should be applied to upper and lower reaches of the Altamaha, which are separated by some discontinuities in appropriate habitat and may have been differentially influenced by heavier urbanization in the upstream reaches. Our results show no significant structure of mitochondrial data for E. dariensis, E. hopetonensis, or mussels in the E. icterina–complanata group. However, microsatellite data do suggest moderate genotypic structure of mussels in the E. icterina–complanata group, consistent with (but not necessarily caused by) the noted habitat disjunction. Application of these data may prove useful in management of rare endemic congeners such as the Altamaha spinymussel, E. spinosa, in showing that management resources can be applied uniformly throughout the Altamaha basin.