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Blue mussels from the genus Mytilus are an abundant component of the benthic community, found in the high latitude habitats. These foundation species are relevant to the aquaculture industry, with over 2 million tonnes produced globally each year. Mussels withstand a wide range of environmental conditions and species from the Mytilus edulis complex readily hybridize in regions where their distributions overlap. Significant effort has been made to investigate the consequences of environmental stress on mussel physiology, reproductive isolation, and local adaptation. Yet our understanding on the genomic mechanisms underlying such processes remains limited. In this study, we developed a multi species medium-density 60 K SNP-array including four species of the Mytilus genus. SNPs included in the platform were called from 138 mussels from 23 globally distributed mussel populations, sequenced using a whole-genome low coverage approach. The array contains polymorphic SNPs which capture the genetic diversity present in mussel populations thriving across a gradient of environmental conditions (~59 K SNPs) and a set of published and validated SNPs informative for species identification and for diagnosis of transmissible cancer (610 SNPs). The array will allow the consistent genotyping of individuals, facilitating the investigation of ecological and evolutionary processes in these taxa. The applications of this array extend to shellfish aquaculture, contributing to the optimization of this industry via genomic selection of blue mussels, parentage assignment, inbreeding assessment and traceability. Further applications such as genome wide association studies (GWAS) for key production traits and those related to environmental resilience are especially relevant to safeguard aquaculture production under climate change.

Knowledge of population structure and genetic diversity within and between wild and farmed populations of gilthead sea bream (Sparus aurata) and European seabass (Dicentrarchus labrax) is important to achieve sustainable aquaculture production of these species and to assess the risk of genetic impacts of fish escaped from farms. Previous population genetic studies on these species have been based on a limited number of genetic markers and samples. In this study, these features were assessed using samples from 24 seabream and 25 seabass populations distributed throughout the Mediterranean Sea, and 3 wild seabream Atlantic populations. Samples were genotyped with a newly developed combined species SNP array that includes ~60K SNPs. Data from sequencing pools of individual DNA from the same populations were also used. Different approaches were employed for identifying the extent of population stratification within species. The effective population size (a parameter inversely related to the rate at which genetic variability is lost) was estimated for each population based on linkage disequilibrium. Population structure results revealed a clear differentiation between wild and farmed populations in both species. Wild populations showed a low degree of differentiation, particularly in seabream. Despite this, a slight differentiation was observed between Atlantic and Mediterranean seabream populations and between western and eastern Mediterranean seabass populations. However, farmed populations were quite heterogeneous and showed a high degree of differentiation. Some farmed populations of both species showed a genetic makeup similar to that found in wild populations. In general, the effective population size was large (> 1000) for wild and small (< 100) for farmed populations of both species. About 40% of the seabream and 80% of the seabass farmed populations had estimates of effective population size smaller than 50 highlighting the need of applying measures to control t