Thermal selection shifts genetic diversity and performance in blue mussel juveniles

Mussels from the genus Mytilus, key inhabitants of the benthos, are important for the aquaculture industry and one of the most sustainable sources of animal protein available. Species within the Mytilus edulis complex ( M. edulis, M. galloprovincialis and M. trossulus ) are commonly found in temperate regions globally and can easily hybridise whenever their geographic distributions overlap. In the Baltic Sea, populations are formed by M. edulis and M. trossulus hybrids with low levels of M. galloprovincialis introgression. Given the economic and ecological relevance of mussels, this study aimed to investigate mechanisms through which their resilience towards global warming may be fast-tracked. For this, we developed two cohorts of juvenile mussels (i.e. recently settled animals) from the Baltic Sea (Kiel, Germany), one exposed to an extreme heat event early in life and one naïve to this stressor. Both cohorts were then exposed to experimental temperatures at the proposed upper thermal limit for this population, 21 ^° C to 26 ^° C, with animal performance measured after 25 days. We then assessed the impacts of thermal stress on the genetic composition of each cohort by genotyping 50 individuals using the blue mussel 60K SNP-array. We observed a significant increase in M. edulis genotypes together with a decrease in M. trossulus in the S cohort in comparison with NS juveniles. We also found that exposure to high temperature has an effect on the performance of mussel cohorts, reducing dry tissue weight of the selected individuals. Results from this study provide relevant insights on how selection through thermal stress impacts performance and genetic composition of blue mussel juveniles, with key implications for understanding and managing mussel populations under future warming scenarios.