A GEOGRAPHICALLY-BASED STUDY OF SHELL SHAPE IN SMALL ROUGH PERIWINKLES

A study using principal component analysis and discriminant analysis was carried out on shell shape variation in 3093 specimens of rough periwinkles, 2500 of which were below 5.5 mm in columella length, from around the North Atlantic. Using a combination of colour plus sculpture, and life history trait, the snails were classified by inspection and examination into Littorina nigrolineata, L. arcana, L. saxatilis and L. neglecta. Principal component analyses indicated that similar aspects of variation were important in the different taxa, but these were sometimes of differing levels of importance between L. saxatilis and L. neglecta. Crossvalidation in a discriminant analysis showed classification of shells larger than 5.5 mm to have at least an 88% accuracy. That of shells below 5.5 mm showed an accuracy of 49% in L. arcana, increasing to 54% in L. saxatilis and 63% in L. neglecta, with 76% accuracy for small L. nigrolineata. This last was a special case as only one site was sampled, therefore comparative data are not available. This geographically-based study reveals that L. neglecta is more homogeneous over its range than recently reported by other workers and shows greater differences from L. saxatilis than the latter does from either L. nigrolineata or L. arcana. Size effects do not account far these differences because L. neglecta is morphometrically distinct from both large and small L. saxatilis. Furthermore, small, mid-shore L. saxatilis classify with large high-share L. saxatilis in discriminant analysis, not with L. neglecta. These results provide evidence that the taxon L. neglecta is more distinct than has sometimes been suggested.