1- Climate influences population genetic variation of marine species. Capturing those impacts remains challenging for marine fishes dispersing over a large geographic scale spanning steep environmental gradients. This requires an extensive spatial sampling of individuals or populations, representative of seascape heterogeneity, combined with a set of highly informative molecular markers able to reveal climatic-associated genomic variations.

2- We explored how space, dispersal and environment shape the genomic patterns of two marine fish species. We hypothesized that population structure and climate-associated genomic signatures of selection will be stronger in the less mobile species, as restricted gene flow tends to facilitate the fixation of locally adapted alleles.

3- To investigate our hypothesis, we genotyped two species that share a common environment but have contrasting dispersal abilities, the white seabream (Diplodus sargus) and striped red mullet (Mullus surmuletus). We collected 823 samples across the Mediterranean Sea, which ranks among the oceanic basins the most affected by climate change and human pressures. We used genotyping by sequencing (GBS) to detect 8 206 Single Nucleotides Polymorphisms (SNPs) for seabream and 2 794 for mullet. For each species, we identified highly differentiated genomic regions and disentangled the relative contribution of space, dispersal and environmental (climate, productivity) variables on genetic structure to test the prevalence of gene flow and local adaptation.

4- We observed contrasted patterns of gene flow and adaptive genetic variation between the two species. The seabream showed a separate Alboran sea population but further panmictia across the Mediterranean Sea, while the mullet revealed additional differentiation. The within-Mediterranean differentiation of mullet was significantly correlated to summer – and winter temperatures as well as productivity. Functional annotation of climate-associated outlier SNPs then identified candidate genes involved in heat tolerance that could be examined to further predict species’ responses to climate change.

5- Our results illustrate the key steps of a comparative seascape genomics study that aims to unravel the evolutionary processes at play in marine species, in order to better anticipate their response to climate change. Defining population adaptation capacities and climatic niches can then serve to include complementary evolutionary processes in species conservation planning. 

SNP data for each species considering all filtered SNP, putatively neutral SNP and outlier SNP as detected by PCAdapt.