Hello,
I know this is a very old string, but I'm new to CONCH-L and am catching up. The paper
Genetics and taxonomy of ribbed mussels (Geukensia spp.), By Sarver et al. 1992 addresses this question fully. As for my experience I believe that G.demissa is the East coast species that spans from Rhode Isaland to South Florida and that G.granosissma is the Gulf of Mexico species. I personally have not found any G.demissa North of the Keys.
Derrick
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