SPATIOTEMPORAL GENETIC-STRUCTURE AND GENE FLOW BETWEEN 2 DISTINCT SHELL MORPHS OF THE PLANKTONIC DEVELOPING PERIWINKLE LITTORINA-STRIATA (MOLLUSCA, PROSOBRANCHIA)

The planktonic developing periwinkle Littorina striata produces both n odulose and smooth shells, which were originally regarded as 2 separat e species. Although both morphs occur microsympatrically, their distri bution is not random. Nodulose shells predominate at wave-sheltered si tes, whereas smooth shells are more common at wave-exposed sites. The degree of genetic similarity between the 2 shell types and their micro geographic 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 c omparable magnitude to gene flow between both morphs, (3) the populati on genetic structure of L. striata remains stable over a sampling peri od of 3 yr, and (4) genetic and morphological distances between popula tions are not correlated. These results thus confirm the conspecific s tatus of the 2 shell types and suggest that shell variability and spat ial patterning in L. striata persist in the presence of intense gene f low.