Fine-scale genetic structure overrides macro-scale structure in a marine snail: nonrandom recruitment, demographic events or selection?

The planktotrophic littorinid species Littoraria flava occupies a continuous habitat on rocky shores close to brackish and freshwater sources. Previous studies of this species have shown a moderate genetic structure over a broad geographical scale, with high deviations from Hardy-Weinberg expectations in many allozymic loci. Local-scale subdivision in marine species with a long dispersal phase is unexpected, but occasionally found. Using a horizontal transect at three locations, we examined whether microscale and short-term subdivision also occurred in L. flava populations and, if so, whether this could explain the Hardy-Weinberg deviations. Littoraria flava showed even more structuring on a microgeographical scale (4-300 m) than on a large-scale (> 200 km). The Ewens-Watterson neutrality test showed that 18% of the tests deviated significantly from the neutrality model. A homogeneity test for each locus across samples within transects showed homogeneous and high F-IS values in many loci. These results and the apparent genetic patchiness within transects suggest that asynchronous spawning associated with recurrent colonizations in L. flava can explain the local differentiation without a recognizable pattern. In addition, there could be a balance between these factors and diversifying selection acting on different loci at different times and localities. (c) 2007 The Linnean Society of London.