Habitat related allozyme variation on a microgeographic scale in the marine snail Littorina mariae (Prosobranchia: Littorinacea)

The marine snail Littorina mariae Sacchi & Rastelli occurs in high numbers in the littoral zone on fucoid macro‐algae. The eggs are laid on the seaweed and d e velopment to miniature snails takes place without any pelagic larval stage. We have mapped the genetic variation of 30 enzyme loci in populations from eight small islands within 15 km of each other on th swedish west coast. The original intention was to investigate the magnitude of gene flow within and between islands. However, we soon realized that our basic assumption of neutral genetic variation was forcefully violated in at least one locus, arginine kinase (Ask). Allele frequencies of Ark were strongly associated with type of habitat. Therefore the main part of this study focused on allele frequency distribution in different habitats. Eight of the 30 loci screened were polymmorphic but we mainly considered the four most polymorphic ones (total heterozygosity between 0.32 and 0.57). All four showed significant heterogeneity between subpopulations, Ask especially so; 42% of the total variation of Ark was explained by differentiation between samples, and 90% of this variation was attributed to differences between different types of habitats (more or less exposed to wave action). In contrast, peptidase (Pep‐1) and phosphoglucomutase (Pgm‐2) varied in a way predicted by neutral theory; between sample variation being mainly attributed to differentiation bvetween islands. The variation in phosphoglucose isomerase (Pgi) was less consistent. In some islands there w as an obvious difference between different habitats, but on other islands we found no significant difference.Not taking into account the extreme, presumably sclected, variation in Ark, we concluded that the metapopulation of Littoriana mariae that we studied was divided into semi‐isolated populations, between which the average rate of migration was in the range of a fe w individuals per generation.