Neighbourhood size and the importance of barriers to gene flow in an intertidal snail

Abstract

The littorine gastropod Bembicium vittatum has direct development from benthic egg masses, and shows high levels of genetic subdivision in the Houtman Abrolhos Islands, Western Australia. Parallel studies of dispersal and the scale of variation of allozyme frequencies were used to estimate the extent of the neighbourhood, the area of complete genetic mixing. Estimates based on the variance of dispersal distances were 58 to 69 m, very similar to the estimate of 62 m from the median dispersal distance. The allozymes indicated mixing over greater distances, up to 150 to 300 m. Combined with estimates of population density at two sites, these values suggest neighbourhood sizes of 990 to 37 200 adults. However, estimation of effective population size based on a model of isolation by distance gave very low values of 22 to 38, raising doubts about the applicability of this method. Nevertheless, there was an overall pattern of isolation by distance along about 11 km of nearly continuous suitable habitat, with smooth clinal variation in allelic frequencies over distances of 2 to 6 km at individual loci. In contrast, there was no pattern of isolation by distance, and allelic frequencies showed less smooth patterns, along a discontinuous series of islands over the same distance. Within this discontinuous series, genetic subdivision, as measured by G_ST, was twice as large at distances of 1 km as for populations on continuous shores, demonstrating the importance of even relatively short water gaps as barriers to gene flow in this directly developing intertidal species.