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Conservation Genetics and Ecology of European Pearl Mussels

The work of Annual Award Winner, Dr Juergen Geist, of the Technical University of Munchen

Despite the fact that mollusc species play important roles in many aquatic ecosystems, often little is known about their ecology, biodiversity and population genetics. Freshwater pearl mussels (Margaritifera margaritifera L.) are among the most critically endangered freshwater invertebrates, facing serious population declines and local extinctions. Freshwater pearl mussels are flagship, indicator, keystone and umbrella species. They have a complex life cycle with specific habitat requirements during different life stages. Thus, they can be seen as a model species for designing conservation strategies in stream ecosystems.
The goal of this study was to contribute knowledge for designing conservation strategies for the species by combining conservation genetics and ecological investigations.
Altogether 14 polymorphic microsatellite markers were developed for M. margaritifera, representing the first published microsatellite markers for a European freshwater bivalve mollusc (order Unionoida). The markers revealed wide ranges of allelic richness and heterozygosity levels and proved to be suitable for monitoring of neutral genetic divergence and diversity for describing the current genetic structure of pearl mussel populations.
The genetic diversity and differentiation of the last and most important central European pearl mussel populations from the drainages of Elbe, Danube, Rhine, Maas and Weser were assessed in order to determine conservation units (CUs), to select priority populations for conservation, and to deduce conservation strategies on a genetic basis for free-living populations and for supportive breeding. A highly fragmented population structure and different levels of genetic diversity within populations were detected. This observation can most likely be explained by historic, demographic and anthropogenic effects.
The methodology of non-destructive sampling with no impact on living populations was established for pearl mussel DNA-analyses (dead individuals and haemolymph sampling). In addition, the successful use of shell-DNA was demonstrated. The potential for using mollusc shells in DNA-based analyses and the required precautions and limitations to avoid erroneous results were discussed.
This study also explored the potential of separating annual growth increments of pearl mussel shell carbonate layers, and their suitability as long-term archives for up to 100 years. Stable isotope investigations of inner nacreous and outer prismatic shell carbonate increments demonstrate that pearl mussel d13C shell carbonate signatures record individual metabolic signals extending over several years and that a high percentage of respiratory CO2 (community and mussel respiration) contributed to shell aragonite formation. In combination with d15N signatures of mussel tissues and potential food sources, these analyses allowed an assessment of the trophic level and of the origin of the mussel diet.

The study on the status of host fish populations and on fish species richness in European pearl mussel populations characterised typical fish communities in pearl mussel streams and revealed that a lack of host fish only seems to be limiting pearl mussel reproduction in specific areas. Intact and functional pearl mussel populations were found to occur under extremely oligotrophic conditions with lower host fish densities and biomasses than in disturbed central European populations without juvenile recruitment. The effects of stocking measures with glochidia infected host fish as a conservation strategy were discussed.
Due to the pearl mussels´ wide geographical distribution, their complex life cycle and extraordinary reproductive strategy, the global phenomenon of serious decline can have different and multiple reasons in different regions. Conservation strategies in general and those for freshwater pearl mussels in particular can greatly benefit from a holistic and combined approach of integrating conservation genetics and ecological studies. Such strategies retain a maximum of the species´ biodiversity and evolutionary potential on the one hand, and try to identify specific habitat requirements during all stages of the life cycle and restore the habitat on the other hand. Conservation strategies for pearl mussels can be key examples for the development of conservation strategies for other aquatic organisms and the ecosystem functioning upon which they depend. Generally, conservation efforts should move away from single species and pattern protection towards process and persistence conservation for ecosystems.

Present occupation:

Juergen Geist´s general research focus lies in the fields of Aquatic Ecology, Molecular Ecology, Ecotoxicology, Conservation Genetics and Evolutionary Functional Genomics with a specialisation on mussel and fish species. At present, Juergen is a Postdoctoral Research Scholar in the field of ecotoxicogenomics at the Aquatic Toxicology Laboratory, Department of Anatomy, Physiology and Cell Biology at the University of California in Davis, USA. His current project on “Evolutionary Functional Genomics in Aquatic Environments” includes field and laboratory research on the sublethal effects of stress response in aquatic species, with a main focus on changes in gene expression patterns at the mRNA and protein level. He also teaches a graduate course in “Aquatic Ecology and Conservation” at the Technische Universitaet Muenchen, Germany.