Fig. 1

Map of flat periwinkle sampling sites characterized in this study. Littorina obtusata samples are represented by orange circles and L. fabalis samples by green squares. Symbols filled with both colors (i.e. half orange and half green) indicate populations where mitochondrial introgression was observed: orange circles half filled with green correspond to L. obtusata samples carrying typical mtDNA haplotypes from L. fabalis; green squares half filled with orange correspond to L. fabalis samples carrying typical mtDNA haplotypes from L. obtusata; the filling is not proportional to the percentage of introgression. A particular locality where hybrid individuals were previously identified (Cabo do Mundo, Portugal) is represented by a black star. a Overview of the sampling sites across the distribution range of the two species, highlighted by dotted shorelines: in white the region where only L. obtusata is found and in black the regions where both L. obtusata and L. fabalis occur along the North Atlantic coast (adapted from [26]). In the middle, L. obtusata and L. fabalis specimens (from Iberian Peninsula), in the standard position for morphometric analysis of the shell. b - e Zoom-in of the sampling sites in Iberian Peninsula, Wales, Norway and Sweden. Locality codes follow those in Table 1 with the letters referring to the country. Maps were obtained from DEMIS World Map Server (http://www2.demis.nl/worldmap/mapper.asp) and Google Maps (https://www.google.com) in 2018