How does urbanization shape shell phenotype, behavior, and parasite prevalence in the snail Cornu aspersum?

Urbanization is a complex and multivariate environmental change, leading to habitat fragmentation and loss, changes in local climate, soil imperviousness, pollution, etc. This is likely to exert pressures simultaneously on various dimensions of organisms' multivariate phenotypes, leading to trait shifts with potential ecological consequences. However, responses to urbanization are often studied one (type of) trait at a time. In this context, we studied how, in the brown garden snail Cornu aspersum, shell phenotype (shell size and color/reflectance), behavior (food intake, mobility), and metazoan parasite prevalence respond to urbanization. Within an urbanization gradient spanning the French city of Rennes, we found that snails from more urbanized sites closer to the urban center were smaller, whereas urbanization had no detectable effect on shell reflectance, parasite prevalence, or behavior. Larger snails and snails with paler shells were more likely to be infected by trematode metacercariae and sexually transmitted nematodes (Nemhelix bakeri), respectively. Snails harboring trematode sporocysts ate typically less, while those infected by N. bakeri moved more slowly. We discuss the decrease of snail size along the urbanization gradient in relation to the Urban Heat Island effect and the potential decrease of resource quality and availability in urban sites. The absence of detectable effects of urbanization on shell reflectance, mobility, and parasite prevalence may be due to scale mismatches between how urbanization is measured and how snails experience microhabitats. We propose further experimental and field studies to decipher interactions between urbanization effects, shell phenotype, life-history traits, and parasitism.