Clinal variation in the shell morphology of intertidal snail Acanthina monodon in the Southeastern Pacific Ocean

In this study we determine clinal variation of Acanthina monodon shell morphology along the southeastern Pacific coast. Our goals were to describe the clinal variation between biogeographical areas, and between localities along the latitudinal gradient between 30 degrees and 54 degrees S. Specimens were collected from 12 localities throughout the entire geographical distribution range of the species. Geometric morphometric analyses were used to evaluate the morphological variation of A. monodon along the Chilean coast. Relative warps and centroid size scores were used to compare the shape and size and their morphological variation and similarity was evaluated through multivariate analyses between all populations. The first two relative warps explained 78% variance. Size and shape scores decreased southward showing high biogeographical and latitudinal variation, principally due to both the thickness of the lip and the spire length. Multivariate analyses showed two main morphotypes: northern populations with short spires, thick and smooth lips and robust shells; the other with longer spires, thinner ornamented lips and weak shells associated with southern populations. However, three southern populations showed a merging of characteristics from both morphotypes, which could correspond to an intermediate morphotype or a morphological transitional zone. The results suggest the existence of clinal variation along the southeastern Pacific coast showing two morphotypes to north and south of 41 degrees S, with a feature merging zone that probably extends to 45 degrees S, related to biogeographical areas proposed for the Chilean coast. We suggest that the morphotypes reported are the result of site-specific eco-phenotypes responding to local variation.