Figure 1. (A) Sites surveyed (circles) and sampled (stars) in the UK and Ireland for the intertidal marine invertebrates Nucella lapillusand Steromphala umbilicalis . Sites are coloured according to the presence or absence of one or both species (black = both species present; grey = only N. lapillus present; white = both species absent). Identified habitat “gaps” are indicated with black lines and labelled in capitals. Water bodies are labelled in italics. (B) Release locations of the gastropod Steromphala umbilicalis , in the larval dispersal model, showing sample sites (labelled, red-yellow points) and interim stepping-stones sites (blue points). The size and colour of sample site markers indicate the degree of convergence (i.e., net sink sites are large and yellow) or divergence (i.e., net source sites are small and dark red) for one season of dispersal (June-September). The “100%” legend in the bottom left corner indicates the size of a site marker that had no net gain or loss of particles at the end of a spawning season. The black arrows indicate the sample sites which are connected (no year-on-year spread shown, background connectivity only). Online version in colour.