Spatio-temporal genetic structure and gene flow between two distinct shell morphs of the planktonic developing periwinkle Littorina striata (Mollusca: Prosobranchia)

The planktonic developing periwinkle Littorina striata produces both nodulose and smooth shells, which were originally regarded as 2 separate species. Although both morphs occur microsympatrically, their distribution is not random. Nodulose shells predominate at wave-sheltered sites, whereas smooth shells are more common at wave-exposed sites. The degree of genetic similarity between the 2 shell types and their microgeographic spatio-temporal genetic structuring were investigated using allozyme electrophoresis. This indicated that: (1) both morphs share a common gene pool, (2) gene flow between populations is high and of comparable magnitude to gene flow between both morphs, (3) the population genetic structure of L. striata remains stable over a sampling period of 3 yr, and (4) genetic and morphological distances between populations are not correlated. These results thus confirm the conspecific status of the 2 shell types and suggest that shell variability and spatial patterning in L. striata persist in the presence of intense gene flow.