Samples were collected within typical L. saxatilis habitats, far from the contact zones between Crab-Wave and Low-High habitats (see below), in 11 localities: Spain (n = 2 pooled samples), France (n = 1), United Kingdom (n = 2), and Sweden (n = 6) (table S2). For three of the six Swedish localities (SWn1, SWn2, and SWn4, n = 6 sample pools), pools consisted of 24 individuals from both sexes. For the remaining localities (n = 16 pools), pools consisted of 100 female individuals (females were selected to avoid contamination with a sibling species, Littorina arcana, which coexists with L. saxatilis in the United Kingdom and France; males of the two species cannot be distinguished morphologically). Our sampling design includes two axes of environmental differentiation, Crab-Wave and Low-High habitat contrasts. Crab-Wave divergence consists of two ecotypes locally adapted to crab-predation and wave-action, respectively. Low-High shore-level divergence consists of two local microhabitats of differential temperature and desiccation selective pressures along a sharp vertical gradient. See details in the Supplementary Materials. The two axes of environmental differentiation follow different directions across the different sampling localities (see Fig. 1B). Shore-level samples in Sweden were collected for two locations (Ramsö and Arsklovet; SWn3 and SWn5, respectively), where high- and low-shore individuals within each of the Crab and Wave habitats were kept separate. This means that each of these four pools was divided in two pools of 50 females each for analyses of High-Low divergence and combined for analyses of Crab-Wave divergence into pools of 100 females. Thus, the number of pools we used varied according to which axis of divergence (Crab-Wave or Low-High) was considered. Analyses considering Low-High divergence used a total of 18 pools, i.e., excluding Swedish samples that we did not split into high and low shore (table S2). For the Crab-Wave divergence analyses, we used a total of 22 pools (table S2).

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