Selection and migration in two distinct phenotypes of Littorina saxatilis in Sweden

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Abstract
Two distinct phenotypes of the marine snail L. saxatilis, the ‘E morph’ from exposed rocky habitats, and the ‘S morph’ from nearby sheltered boulder shores, were released and survivors recaptured at 3-monthly intervals. Survival rate of E morphs was significantly higher than that of S morphs in the exposed habitat, whilst the revers was true for the sheltered site. When released in an environment intermediate between the exposed and sheltered habitats, both morphs survived equally well. However, snails native of this area, ‘I morphs’, survived better than both E and S morphs here. Recapture rates were highest in spring. Large snails of both phenotypes were selected against in the exposed habitat. In the sheltered environment, small E morph snails survived better than large ones, and both sizes of S morphs survived equally well. E morph individuals transplanted to a different exposed locality than their native one, had lower survival rates compared to native individuals. This effect was not found among transplanted S morph snails. Distances migrated since time of release were on average greater in the sheltered than in the exposed habitat, and within the range of 1–4 m per 3 months, for both habitats. In a laboratory experiment, both morphs showed a shelter-seeking behaviour, most pronounced in the E morph. It is proposed that the strong selection differences between the morphs within the two different habitats, the absence of a planktonic larval stage, and a restricted migration among juvenile and adult snails, maintain a genetic polymorphism within the species, responsibel for considerable parts of the large phenotypic differences observed between the two morphs E and S.