The natural history and taxonomy of shell variation in the periwinkles Littorina saxatilis and Littorina rudis

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Abstract
An account is given of the variations of shape, color and patterns of surface sculpture of the shells of ovoviviparous, upper-shore periwinkles of the Littorina saxatilis species-complex from the sea-coasts and estuaries of southwest England. Variations of shell shape derive from the differing geometries of the helically coiled shell cone. The rate of expansion of the cone is constant in each individual shell but varies in the component shells of a population and (more widely) between one population and another. The area of the shell aperture is a measure of the expansion rate. Samples of L. saxatilis were taken from some 65 localities covering a wide range of shore environments and, whenever possible, at closely comparable tidal levels (the Pelvetia (P) belt/.apprx. to MHWS [mean high water springs]) and aperture measurements made of shells of all lengths from 1-10 mm (age .apprx. 0.1-2 yr). The apertures of pre-emergent shells teased from the brood pouches of mature females, and of .apprx. 0.4-0.6 mm length, were also measured. Population aperture-size comparisons were founded primarily on subsamples of shells 10 .+-. 0.5 mm long.sbd.the P-10 aperture-size statistic; and the possible adaptive value of aperture (and foot) size variations were sought through matching relationships with varying conditions of wave attack, shore gradient, platform structure and other features, of which assessments were made in the field. Logarithmic plots of the sequences of aperture-size/shell-length measurements of pre-emergent and post-emergent shells show that the cone geometries are genetically determined and there is some evidence of the adaptive selection of some geometries and ranges of aperture size in preference to others, mostly in the early post-emergent stages of growth. The distributions of the P-10 mean aperture size values (range .apprx. 8-24 mm2) in different localities, on different types of shores and at different levels were strongly indicative of the presence within the L. saxatilis complex of 2 spp. Populations from the type-localities of the canal walls of Venice and the neighborhood of the Bere Alston peninsula in the Tamar estuaries were measured and analyzed. The aperture specifications of L. saxatilis (Olivi) from Venice and L. rudis (Maton) from the Tamar correspond, respectively, to rocky shore and boulder- and stone-beach populations sampled during the present survey. Distinctive features of shell color, sculptural patterns and degree of development of the columella lip of the shell within the 2 spp. confirm these conclusions. The species are re-described to include the additional features as aids to their identification. Some tentative views are added on subspeciation.