Published online by Cambridge University Press: 11 May 2009
An account is given of the variations of shape, colour and patterns of surface sculpture of the shells of ovoviviparous, upper-shore periwinkles of the Littorina ‘saxatilis’ speciescomplex from the sea-coasts and estuaries of south-west 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 ‘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/approx. to M.H.W.S.) and aperture measurements made of shells of all lengths from 1–10 mm (age ca. 0·1–2 years). The apertures of pre-emergent shells teased from the brood pouches of mature females, and of ca. 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 - 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