Study Sites & Sampling
We selected sites to maximize the range of climatic variation within our study area (Irish Sea), and included sites separated by potential barriers to dispersal and gene flow (e.g., divergent residual currents and habitat gaps). We surveyed 120 sites consisting of either natural or artificial rocky substrates along the eastern coast of the Republic of Ireland and Northern Ireland, and the western coast of Wales, England and Scotland between June 2018 and June 2019 for the abundance of our two focal species (Fig. 1A). We focused on areas lacking suitable habitat that may represent large-scale barriers to dispersal. Two large stretches of coastline devoid of populations of one or both species were identified, representing putative dispersal barriers. The first of these was the eastern coastline of Ireland between Coliemore and Rosslare in which, of 25 sites surveyed, none contained S. umbilicalis and 17 (68%) contained N. lapillus . The northern coastline of Wales, east of Llanddulas to the southwestern coastline of Scotland at Brighouse Bay formed the second coastal gap in which, of 16 sites surveyed, two (12.5%) contained S. umbilicalis (with only one individual found at one of these sites) and five (31.3%) containedN. lapillus . The main channel of the Irish Sea was considered the third “habitat gap” separating Welsh, English, and Scottish sites from those in Ireland and Northern Ireland. Twelve sites were selected for genetic sampling, including sites at either end of each habitat gap where thriving populations of both species were found (Fig. 1). From each site, 20 adult individuals of each species were sampled resulting in 240 individuals per species. Tissues were frozen following collection, and subsequently preserved in 99% ethanol prior to DNA extraction.