Freshwater pearl mussels (Margaritifera margaritifera) are among the most critically threatened bivalve molluscs worldwide. An understanding of spatial patterns of genetic diversity is crucial for the development of integrative conservation strategies. We used microsatellites to study the genetic diversity and differentiation of 14 populations of M. margaritifera in central Sweden, an area which was described as a major secondary contact zone in postglacial colonisation for other species. Genetic diversity of Swedish pearl mussel populations was much greater than in central and southern Europe but similar to the genetic diversity observed in the northeastern portion of their European range. Genetic differentiation among populations was pronounced but to a large extent independent from present-day drainage systems. The complex patterns of genetic diversity and differentiation in pearl mussel seem to be strongly influenced by the species' high degree of specialisation and extraordinary life history strategy which involves facultative hermaphrodism and an obligatory encystment stage on a host fish. Genetic drift effects and anthropogenic disturbances resulting in reduction of population size and loss of connectivity are less pronounced in northern pearl mussel populations compared to those in central and southern Europe.
1. The freshwater pearl mussel Margaritifera margaritifera (L.) has become scarce and threatened throughout most of its distribution range, and is consequently listed as 'endangered' by the IUCN. It has been suggested and used as an indicator species and even as an umbrella species. Despite this, few studies have examined factors affecting its catchment-level population status.2. This work tested the hypothesis that it is possible to predict freshwater pearl mussel population viability (reproducing/not reproducing) and population status in central Sweden from a combination of geospatial data describing riparian land-cover and use, water chemistry data, and electrofishing data describing the abundance of the host fish species (brown trout, Salmo trutta) in 56 streams.3. Mussel population viability was best predicted by total phosphorus. Using higher-resolution data (six classes) on mussel population status, host fish abundance (0+ trout) was the best predictor. At abundances below 5 0+ trout per 100 m 2 and above a concentration of 8 mg L -1 total phosphorus, only three viable populations existed.4. Geospatial data could be used to predict levels of total phosphorus and thereby indirectly to predict mussel viability. Geospatial data could not be used to predict abundance of 0+ trout, which is thought to depend more on instream factors. This study demonstrates that geospatial data on riparian land-use and cover can be a useful tool for screening waters potentially harbouring non-reproducing mussel populations.
The freshwater pearl mussel Margaritifera margaritifera is an endangered species in Sweden with more than 600 known populations distributed in 16 out of 21 counties. Only approximately one-third of these populations are considered viable and healthy with signs of recent juvenile recruitment. From 2011 to 2017, research documented an increased mortality in this species, of up to 100% in some populations, but no etiological cause of these mortalities has been identified. With this paper, we provide current knowledge of locations where mass mortality of freshwater pearl mussel has been found in Sweden and discuss possible causes. Postmortem sampling and histopathological findings from two counties in 2016-17 detected lesions in digestive glands indicating a reduced capacity for nutrient uptake. Results from these macroscopic and microscopic investigations also indicate a reduction in, or a lack of, reproductive output compared with reference populations.
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