Marine invertebrates with planktonic-dispersing larvae are assumed to be good dispersers over long distances. This high dispersal capacity implies a high gene flow between populations and a homogeneous population genetic structuring over wide geographic scales. The marine gastropod Melarhaphe neritoides has a long-lived planktonic larval dispersal stage and allozyme data suggest that it is genetically homogeneous over its whole European distribution area. In contrast, preliminary mtDNA sequence data uncovered a remarkable degree of genetic diversity and genetic structuring on smaller geographic scales. In order to explore this mtDNA diversity and structuring in M. neritoides we started to survey sequence variation at COI and 16S rDNA all over the Azores archipelago. These data reveal that the Azorean populations share very few haplotypes, both between and within islands. Hence, it seems that M. neritoides with its long-lived planktonic larval stage nevertheless shows a strong local population genetic structuring and thus challenges the current paradigm that correlates modes of larval development with levels of genetic structuring. It also stresses the importance of the sampling intensity (both in terms of numbers of specimens and genetic markers) to avoid experimental biases when assessing genetic diversity.

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