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.