Influence of stock origin and environmental conditions on the survival and growth of juvenile freshwater pearl mussels (Margaritifera margaritifera) in a cross-exposure experiment

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European freshwater pearl mussels are among the most critically threatened bivalve molluscs. Margaritifera margaritifera and Margaritifera durrovensis are endangered, and both occur in Ireland and are currently listed separately in Annex II of the European Habitats Directive. This study had the objective of guiding the conservation of freshwater pearl mussels in Ireland based on a genetic characterization of the most important Irish populations of both species using microsatellite and mitochondrial DNA (mtDNA) analyses. Margaritifera durrovensis from the Nore was found to have the highest genetic divergence from all the populations studied; however, there was substantial relatedness between the genetic constitution of the Nore and of central–eastern M. margaritifera populations, placing the Nore pearl mussel within the M. margaritifera taxon (Habitats Directive species code 1029). Two main conservation units of pearl mussel were identified: a mostly salmon‐dependent western cluster and a trout‐dependent central–eastern cluster. The genetic diversity of western Irish freshwater pearl mussels, as expressed by allelic richness and observed heterozygosity, was greater by a factor of two than in central European and central–eastern Irish populations, suggesting a genetic diversity hot spot and low effects of genetic drift and selection. The trout‐dependent central and eastern populations had much lower genetic variability, but had the greatest differentiation and uniqueness. Conservation of freshwater pearl mussels in Ireland should recognize the existence of a minimum of two conservation units (western and central–eastern) that differ in their use of host fish and in geographic isolation. Low levels of genetic drift and inbreeding in western populations should be secured by sustaining optimal habitat conditions favourable for the recruitment of mussels and their migratory salmonid hosts. The small population sizes of central–eastern populations and problems with recruitment require urgent action, e.g. by captive breeding and augmentation, to prevent any further erosion of their genetic variability.

Section: Discussionsupporting

confidence: 74%

Genetic structure of Irish freshwater pearl mussels (Margaritifera margaritifera and Margaritifera durrovensis): Validity of subspecies, roles of host fish, and conservation implications

Geist

Moorkens

Killeen³

et al. 2018

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“…The mussel larvae from this C. gobio strain, however, also had the highest juvenile survival and thus seemed to be more fit than mussels from other fish strains or species. Differences in immune response and nutrient uptake or quality from the allopatric C. gobio may explain this pattern, and may have positively affected the energetic level of juveniles, even if there were no parasitic growth differences among fish strains and species, such as that shown for M. margaritifera (Denic et al, ; Österling & Larsen, ). The quantitative conclusions from these results deserve further evaluation in future studies.…”

Section: Discussionmentioning

confidence: 99%

Local adaptation studies and conservation: Parasite–host interactions between the endangered freshwater mussel Unio crassus and its host fish

Schneider

Nilsson

Höjesjö

et al. 2017

Parasite–host interactions can involve strong reciprocal selection pressure, and may lead to locally adapted specializations. The highly threatened unionoid mussels are temporary parasites on fish, but local adaptation has not yet been investigated for many species. Patterns of local adaptation of one of Europe's most threatened unionoids, the thick‐shelled river mussel (Unio crassus) were investigated. Eurasian minnows (Phoxinus phoxinus) from two rivers (separate drainage areas) were cross‐infested in the laboratory with sympatric and allopatric mussel larvae, while bullheads (Cottus gobio), inhabiting only one of the rivers, were infested with sympatric or allopatric mussel larvae. Larval encystment, juvenile mussel excystment and survival were measured. For one river, but not the other, juvenile excystment from P. phoxinus was highest when infested with sympatric mussels. The opposite pattern was found for C. gobio in this river, where juvenile excystment and post‐parasitic juvenile survival from allopatric C. gobio were highest. The results thus cannot confirm local adaptation of U. crassus to P. phoxinus in the study rivers, as excystment was not consistently higher in all sympatric mussel–host combinations, whereas there were potential maladaptive signs of U. crassus in relation to C. gobio. There was no loss of encysted larvae 3 days after infestation until juvenile excystment. Most juveniles were excysted between 17 and 29 days after infestation, and the numbers of excysted juveniles increased with fish size. The results have implications for parasite–host ecology and conservation management with regard to unionoid propagation and re‐introduction. This includes the need to (1) test suitability and adaptation patterns between U. crassus and multiple host fish species, (2) evaluate the suitability of certain unionoids and host fish strains after more than 3 days, and (3) determine whether large fish produce more juvenile mussels than smaller fish.

“…As adult pearl mussels, they are more typically exposed to open river water. The larval and juvenile periods are the most sensitive life stages (Denic et al, ; Denic, Taeubert, & Geist, ; Geist & Auerswald, ), resulting in many populations that are dominated by adults with little or no successful recruitment (Skinner et al, ).…”

Section: Introductionmentioning

confidence: 99%

Developing a standard approach for monitoring freshwater pearl mussel (Margaritifera margaritifera) populations in European rivers

Boon

Cooksley

Geist

et al. 2019

Self Cite

The freshwater pearl mussel (Margaritifera margaritifera) is an endangered species in Europe, protected nationally and internationally, but with a steadily declining range and abundance owing to pressures such as pollution, river engineering, and illegal exploitation. Despite this, no consistent approaches have been developed around Europe for monitoring pearl mussel populations and their habitats. To address this need, experts on pearl mussel ecology from 11 countries met at a series of workshops in order to develop a protocol for monitoring, published under the auspices of the European Committee for Standardization (CEN). This standard is unique, as it is the first CEN standard dedicated to a single species of conservation concern. The standard is aimed at scientists, conservation bodies, and environmental regulators, and can be used for designing national monitoring programmes as well as reporting on the conservation status of pearl mussel populations under the European Habitats Directive. It contains guidance at the individual site level to determine why populations are failing to recruit, but also addresses the need for a wider‐scale approach to ensure that catchment developments do not have adverse impacts on rivers containing pearl mussels. A pearl mussel monitoring programme needs to investigate the size and viability of populations, as well as the fish hosts (Atlantic salmon, Salmo salar, or brown trout, Salmo trutta) on which pearl mussel larvae depend. Water quality, including variables such as dissolved oxygen, acid–base chemistry, and nutrient levels, is also an essential monitoring component, together with the physical features of the river bed, river flow regimes, and sediment dynamics. It is hoped that this pan‐European approach will improve the ability to compare data across many countries, and will ultimately ensure that the results of monitoring are translated into measures for improving the conservation status of the freshwater pearl mussel throughout its range.