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.
As adults Littorina littorea and L. saxatilis have overlapping distributions, both on the shore and on a geographical scale, this implies that they are affected by similar selective forces to a large extent. The two species have, however, different reproductive strategies. Littorina littorea has a planktonic larval stage lasting 4-6 weeks, whereas L. saxatilis has direct development. This influences dispersal rates of the two species, and gene flow and effective population size are both assumed to be much smaller in L. saxatilis than in L. littorea. Intraspecific variation in morphological and allozyme characters differs between the two species. Littorinu littorea is homogeneous for both types of characters, both within and between populations, while populations of L. saxatilis may show pronounced differences in shell characters with micro-environment changes, and allozyme variation is markedly higher in this species too. The polymorphism in shell characters of L. saxatilis and the monomorphism of L. littorea, may be expected from theory as a consequence of the different dispersal strategies. Also the lack of divergence between geographically separate populations of L. littorea, but the high degree of between population differentiation in L. saxatilis, could be attributed to the high and low dispersal rates, respectively. However, the larger allozyme variation within subpopulations of L. snratilis compared to that of L. littorea, is surprising as the former species has a smaller effective population size than the latter. Furthermore, there is no correlation between dispersal ability and overall allozyme heterozygosity, when 10 additional species of Littorinn are included in the comparison. It is suggested that different evolutionary backgrounds of different species will possibly explain this, as neither selectionist nor neutralist models alone account for the observation. That is, an earlier passage through a population bottleneck may, for example, influence the genetic variation for a long period of time.
Twenty-three enzymes and five shell parameters were screened in 11 subpopulations of Littorina saxatilis Olivi ( = L. rudis Maton) occupying different habitats over a 1 km stretch of coastline. Shell morphology varied considerably and consistently with respect to degree of exposure, and since there is evidence that such morphology is at least partly under genetic control, it is likely that natural selection selects particular genotypes at particular locations. There was significant allozyme heterogeneity between neighbouring subpopulations, sometimes only metres apart, but little of the allozyme variability could be related directly to environmental pressures. Thus, with the exception of the Odh locus, the considerable morphological differentiation between snails from exposed and sheltered sites was not reflected in differentiation of those genes coding for electrophoretically assayed enzymes. At the Odh locus, virtually all the genetic differentiation between subpopulations was-attributable to differentiation between habitat types. Two loci, Sod-l and Ant-1, showed highly significant genetic disequilibrium, and possible reasons for this are explored. The population structure as assessed electrophoretically accords well with the stepping-stone model which permits greater differentiation of neighbouring populations than the island model, and which seems realistic in the ovoviviparously reproducing L. saxatilis, where the greater part of gene flow is likely to occur through the occasional migration of adults between contiguous populations.
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