| Abstract: | Translocation experiments were used to test the effect of habitat on growth and shape of three contrasting phenotypes of Bembicium vittatum: dwarf, highly domed snails from an usually dry tidal pond; large, moderately domed snails from a sheltered, regularly inundated pond; and relatively flat snails from a vertical, exposed shore. Snails from both ponds grew nearly twice as fast in the wet pond as in the dry pond, indicating a high degree of plasticity of growth. Associated with these changes in growth rates was convergence of shape. Under conditions of rapid growth, the dwarf snails became relatively flatter, and hence more similar to the native snails at that site. These results indicate that the dwarf phenotype is largely a plastic stunting in response to conditions of little submersion time. The snails from the exposed shore also grew faster in the sheltered, wet pond than at their native site. However, they not only retained their flat shape, but actually became flatter (and hence divergent from the pond snails) when grown in the pond. Thus, variation in shell shape was due to interactions between source population and a common plastic association of flatter growth profile with more rapid growth. Previous experiments had demonstrated high heritability of the flat phenotype, while the present results show that the expression of the genetically different types is affected substantially by the conditions of growth, and that phenotypic differences among populations may either overestimate or underestimate the underlying genetic differences. This unpredictability of the relationship between variation in shell form and its underlying genetic basis complicates interpretations of geographical variation or palaeontological sequences based on shell form. |
