Detecting shape differences in species of the Littorina saxatilis complex by morphometric analysis

We investigated variation in shell size and shape of marine snail species of the Liltorina saxatilis complex (L. saxatilis, L. compressa and L. arcana) using geometric morphornetric methods. These morphologically similar periwinkle species that are common in the European intertidal have presented many problems for diagnosis based on morphology alone. A discriminant analysis demonstrated that geometric morphometrics is very efficient for diagnosing individuals to species among sympatric populations. We successfully diagnosed an average of 96% of the specimens (with 85.7-100% correct diagnosis for specific comparisons). The diagnosis capability of this method is absolute at the population level. This makes the technique potentially useful for the design of manipulative field or laboratory experiments. Moreover, a geometric-morphometric analysis was also accomplished in two snail ecotypes (H and M) of L. saxatilts from rocky shores of NE England which are apparently adapted to different degrees of wave exposure. We found that the H (exposed) ecotype has a relatively rounded shell shape with a bigger aperture; whereas the M (protected) ecotype has a smaller aperture.