Decomposing shell form into size and shape by geometric morphometric methods in two sympatric ecotypes of littorina saxatilis

Two sympatric snail ecotypes (RB and SU) of Littorina saxatilis from exposed rocky shores of NW Spain differ in many life history traits, but classical morphometric analysis has failed to reveal significant shell shape differences between them. We used geometric morphometric methods on landmark data from digitized shell images to study size and shape components in both ecotypes at two localities. The results showed significant differences between ecotypes in both shell size and shape (both uniform and non-uniform components). Allometry was also detected for some component of the local variation in shape, although it did not explain the observed differences between ecotypes. The SU ecotype had a relatively rounded shell shape with a big aperture, whereas the RB ecotype had higher spire and smaller aperture. We suggest that shape differentiation is correlated with adaptive differences between ecotypes.