Periwinkles and parasites: the occurrence and phenotypic effects of parasites in Littorina saxatilis and L. arcana in northeastern England

Littorina saxatilis is a common intertidal gastropod on shores of the North Atlantic, and an important study species for evolutionary investigations. Its congener L. arcana is much less widely distributed, but both species are common at Old Peak, Yorkshire, UK. The parasite profiles of L. saxatilis and L. arcana from this shore were determined histologically, revealing a ciliated protist, Protophrya ovicola, an unidentified apicomplexan (present in <1% L. saxatilis) and three trematode parasites tentatively assigned to Renicola sp., Microphallus similis and M. pygmaeus. The profile data include prevalence information and associated histology. Protophrya ovicola associated predominantly with the wave ecotype of L. saxatilis (65%) rather than the crab ecotype (16%). Microphallus similis occurred at a higher prevalence in the L. arcana population (38%) in comparison with the L. saxatilis population (11%). Overall, there appeared to be a lower prevalence of trematodes in the high-shore L. saxatilis. By modelling the occurrence of individual parasites and shell morphometrics, an assessment of parasite-associated morphological change was conducted. We conclude that parasitism appeared not to cause shell-shape change, but rather that snails of a certain shell shape were more likely to display infection. Records of parasites in L. saxatilis and L. arcana are briefly reviewed, showing that the diversity of parasites reported here is relatively low.