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

We investigated variation in shell size and shape of marinesnail species of the Littorina saxatilis complex (L. saxatilis,L. compressa and L. arcana) using geometric morphometric methods.These morphologically similar periwinkle species that are commonin the European intertidal have presented many problems fordiagnosis based on morphology alone. A discriminant analysisdemonstrated that geometric morphometrics is very efficientfor 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 populationlevel. This makes the technique potentially useful for the designof manipulative field or laboratory experiments. Moreover, ageometric-morphometric analysis was also accomplished in twosnail ecotypes (H and M) of L. saxatilis from rocky shores ofNE England which are apparently adapted to different degreesof wave exposure. We found that the H (exposed) ecotype hasa relatively rounded shell shape with a bigger aperture, whereasthe M (protected) ecotype has a smaller aperture. (Received 23 March 2006; accepted 25 January 2007)