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Two morphological varieties of Littorina saxatilis, widespread around the United Kingdom, are a thin-shelled, high-shore morph (L. saxatilis H) and a thick-shelled, mid-shore animal (L. saxatilis M). Mitochondrial DNA (mtDNA) analysis by PCR-RFLP was used to test whether gene flow between these morphs is restricted. At Galloway, Scotland, replicated sampling (different years and different transects over a distance of 800 m) has been undertaken. One mtDNA haplotype is predominant in H and a different haplotype common in the M animals. Repeatability in space and time suggests a real H/M differentiation. A similar pattern of mtDNA haplotype variation is seen in a single sample of the Swedish E and S morphs of Littorina saxatilis. However, this pattern is not evident everywhere. Variation in the mtDNA and four nuclear DNA loci was examined within and between L. saxatilis H and M morphs from the south coast of England. Because shape variation in this region additionally separates into three shape groupings (regions identified from multivariate morphometric analysis where shape is more homogeneous within, than between groups), generic variability was examined within and between these groupings as well as between H and M. On the south coast, an apparent association of shape and mtDNA haplotype is identified, but AMOVA analysis shows no support for the association being with shape grouping or H and M morphs. Although the nature of this shape-genotype association is unknown, a mtDNA haplotype and an allele at the nuclear CAL-2 locus are confined mainly to one shape group. Analysis of association of mtDNA haplotype with H and M morphology suggests a strong correlation can be found in some areas (Galloway, Mumbles (south Wales), and between similar morphs in Sweden) yet no association is seen at others (Ravenscar, UK, Ballynahown, Ireland, and the south coast of England). Thus, unravelling the basis of the H and M forms will require more detailed studies, with replication, as at Galloway, and also with additional molecular markers.

Variables related to shell shape have been measured in littorinids from brackish lagoons and coastal sites. After removal of size related effects, the data were analyzed using multivariate techniques. On Canonical Variate 1 there was good separation of the shells of the lagoonal animals from those of animals from the coast and a tidal lagoon. The former, for example, had lighter, and therefore thinner, shells for any given shell size and a smaller jugosity of the aperture lip. The lagoonal shells from Golam Head and the coastal animals from Robin Hood's Bay could each be separated clearly from the other samples. Although there are clear morphometric differences in the shells, it is not possible without appropriate breeding experiments to raise the lagoonal animals from L. saxatilis var. lagunae (L. tenebrosa) to species status. The importance of conserving lagoonal habitats is considered in terms of the preservation of biodiversity