The effects of seasonality and parasitism on diet and habitat selection in the common periwinkle

The common periwinkle Littorina littorea is an ecologically important grazer, known for its strong influence on algal communities and its role in structuring ecosystems. It serves as the first intermediate host for several trematode species in the Baltic Sea, especially for the fluke Cryptocotyle lingua . Parasitism and seasonality can have direct single and interactive effects on the grazer's fitness and activity, as well as indirect effects on the surrounding benthic habitat. To understand whether infection with C. lingua in combination with seasonal variation can alter the diet and habitat choice of the common periwinkle, 1) the structural and biological composition of the habitat – including substrate type (e.g. stone, sand and macroalgae) and presence of epibionts (e.g. barnacles and mussels) – was investigated, 2) the habitat choice of the periwinkle was observed in‐situ, and 3) the composition of L. littorea's diet (based on stable isotope composition of carbon and nitrogen isotopes) was analysed in spring, summer and autumn. The results indicate that habitat selection and diet composition were primarily dependent on the availability of habitats and food sources, demonstrating a strong connection to seasonal variations. A consistently higher contribution of Fucus vesiculosus to the diet of L. littorea was observed during spring and summer, compared to other food sources, regardless of infection status. The effects of parasitism were only evident in summer, when infected periwinkles exhibited decreased δ 15 N values, indicating compromised digestive or metabolic capacities. This study highlights the need to consider combined effects of parasitism and seasonality in ecological studies, as it is possible that they exert distinct and interactive effects on the physiology of abundant and ecologically relevant grazers and ecosystem dynamics, especially in host–parasite systems.