DISCRIMINATING BETWEEN POPULATION STRUCTURE AND PHENOTYPIC PLASTICITY IN LITTORINA SAXATILIS USING SHELL PHENOTYPE

Intraspecific variation may have many causes - genetic, developmental and environmental. The interactions of these causes and phenotypic consequences is a major theme in biology and paleontology. Littorina saxatilis, a common near-shore marine snail, is a model system for studying intraspecific variation because the shells are variable and response appears mediated by the near-shore environment (eg. ecotypy). In this study, we take a morphometric approach to phenotype in order to infer genetic-environmental interactions. In particular, we compare shell shape for L. saxatilis from different environments (eg. high wave energy, sheltered, etc...) and determine how these differences are built ontogenetically. Because L. saxatilis brood their young, we can determine if shell shape differences occur early in ontogeny before direct exposure to the environment or later in ontogeny after experiencing the environment.

In order to compare shape differences, we used landmark-based approaches applied to shells. Because L. saxatilis are indeterminate growers, we present a method to accurately compare whorls with each other. Our results show that one ecotypic group has completely shifted its whorl shape and whorl number suggesting a major developmental rearrangement. Also, genetic determination or maternal environmental affects are likely candidates for ultimate cause of shell shape differences in L. saxatilis because changes in shell shape happen early in ontogeny, before exposure to the environment.