The marine gastropod Littorina saxatilis has different ecotypes in shores only a few meters apart. This has both taxonomic and evolutionary implications. Here we report on an extreme type of within-shore dimorphism in shell characters. In the wave-exposed rocky shores in northwestern Spain, we found one form of L. saxatilis in the upper-level barnacle zone. It had a white, ridged shell, with black bands in the grooves. Another form confined to the lower-shore mussel belt had a smooth shell that was either white and tessellated or darkly colored. These two forms cooccured in a narrow midshore zone together with individuals that had combined characters, but were present in low frequencies (II %-29%). We used principal-component analysis of metric shell characters to study variation in shell size and shape. We found that the upper-shore form was larger than the lower-shore form. We also found small but significant differences in shell shape. Experiments in a common laboratory environment suggested the differences in shell ornamentation and color are inherited, but the individuals did not develop the morph-specific characters until a shell height of about 3 mm. The occurrence of mainly two distinct forms may suggest the presence of two species that hybridize. An analysis of five polymorphic enzyme loci in populations of snails from three geographically separated sites indicated, however, that there was no positive correlation between morphological distances and genetic distances among populations on a geographic scale (tens ofkilometers). Thus, we rejected the hypothesis oftwo species. However, on a microgeographic scale (meters), genetic differentiation between groups with the same form was less than differentiation between forms. This indicated a partial barrier to gene flow between the two forms, and preliminary mate choice data suggested this was caused by nonrandom mating in the midshore zone of overlap.
Natural selection is one of the most fundamental processes in biology. However, there is still a controversy over the importance of selection in microevolution of molecular traits. Despite the general lack ofdata most authors hold the view that selection on molecular characters may be important, but at lower rates than selection on most phenotypic traits. Here we present evidence that natural selection may contribute substantially to molecular variation on a scale of meters only. In populations of the marine snail Littorina saxatilis living on exposed rocky shores, steep microclines in allele frequencies between splash and surf zone groups are present in the enzyme aspartate aminotransferase (allozyme locus Aat; EC. 2.6.1.1). We followed one population over 7 years, including a period of strong natural perturbation. The surf zone part of the population dominated by the allele atwl' was suddenly eliminated by a bloom of a toxin-producing microflagellate. Downshore migration of splash zone snails with predominantlyAat120 alleles resulted in a drastic increase in surf zone frequency of Aat 20, from 0.4 to 0.8 over 2 years. Over the next four to six generations, however, the frequency of Aat120 returned to the original value. We estimated the coefficient of selection ofAat120 in the surfzone to be about 0.4. Earlier studies show similar or even sharperAat clines in other countries. Thus, we conclude that microclinal selection is an important evolutionary force in this system.
With 5 figures in the text)The ovoviviparous Littorina saxatilis (Olivi) can be found in many intertidal habitats. Winkles from different habitats have different morphologies and behaviours, which are usually argued to reflect adaptations. Whether the different forms are caused by genetic differentiation or due to phenotypic plasticity is, however, less discussed. Our aim was to document morphological and behavioural differences among one exposed rock population, one sheltered boulder population, and one mud flat population, and we reared offspring in a common laboratory environment to see if the differences persisted. We used principal component analysis (PCA), based on 19 measurements, to compare size and shape differences. We also used PCA and linear regression to study allometry and compare growth trajectories. Behavioural differences were studied in three laboratory trials.Littorina saxatilis from the rock were relatively small, had small aperture lips, thin shells, blunt spires, and wide columellae. They preferred to stay in cracks and to be above the water surface, and they quickly emerged out of their shells after disturbance. L. saxatilis from the boulder shore were relatively large, had large lips, thick shells, pointed spires, and narrow columellae. They preferred open surfaces, submergence, and after disturbance they slowly emerged out of the shell. L. suxatilis from the mud flat were relatively small, had small lips, thin shells, pointed spires, and narrow columellae and most behaved similarly to the boulder shore snails. Although less pronounced, most of these morphological and behavioural differences persisted in the snails that we reared in the common environment. This indicates some genetic differentiation. All groups grew allometrically, and most groups differed in growth trajectories. Despite similar ages, the reared females were generally larger than the reared males, indicating higher growth rate in females. Sexual maturation seemed to be reached at a certain size rather than at a certain age. Theories predict genetic differentiation in intertidal snail species with low dispersal, but some investigations show results that contradict the theories, while others agree with them. Our study confirms the prediction that L. saxatilis should be genetically differentiated.
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