Shell shape in gastropods is generated by the spiral path of shell material laid down at the aperture. Many aspects of shape will therefore be associated. Such associations may indicate constraints, limiting the range of variation seen in populations as a response to different environmental pressures. Associations of shell variables were studied using the sibling species Littorina saxatilis and Littorina arcana. The same closely associated subset of three variables was found in both species, related to the maintenance of a constant aperture shape, despite variation in overall morphology. These closely associated variables describe similar variation in the two species. The less closely associated variables may contribute more to inter-specific differences.