This investigation quantifies some aspects of the parasite–host relationship between the digenean Microphallus piriformes and its intermediate host Littorina saxatilis, the rough periwinkle. M. piriformes has an abridged life-cycle with no free-living stages, metacercariae remain within host viscera. Noticeable differences in shell shape of parasitized and uninfected periwinkles were investigated. These differences in shell shape were defined by growth parameters of height, diameter and β angle. The relationship between these parameters was examined together with their impact on parasite reproduction. All 3 shape parameters were altered in periwinkles infected by M. piriformes. The alteration in β angle and height increased the available volume for parasites in the shell spire by about 12%. As metacercarial production per sporocyst has been shown to depend on host size, the increased volume enables considerable additional life-time reproduction by the parasite, of approximately 550–850 additional metacercariae in hosts of the usual size range. The form of gigantism found in this study is discussed in relation to previous concepts. It is suggested that gigantism in permanently castrated hosts is adaptive parasite manipulation of host physiology, favoured in parasites with abbreviated life-cycles, when host viability increases parasite transmission, and when an initially small host individual is infected.

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.