Shell-shocked: parasite-induced behaviour and development in an invasive dead-end snail host

Parasites with complex life cycles produce large numbers of free-living infectious stages to overcome the low odds of successful transmission between hosts. These stages often infect non-competent or 'dead-end' hosts, which cannot support parasite development or transmission. While typically viewed as ecological cul-de-sacs, dead-end hosts may still experience meaningful effects from parasite exposure. Here, we examined how exposure to Paragordius varius hairworm larvae influences behaviour and development of Physella acuta, an invasive freshwater snail likely functioning as a dead-end host in this system. Using a dose-response design under controlled conditions, we exposed juvenile snails to increasing larval concentrations and tracked activity over 24 h in relation to water, total home range and core activity areas. Snails were then reared to assess impacts on shell development. Infection intensity scaled with larval dose. At the highest exposure, snails showed an almost one-third reduction in home range, spent substantially more time submerged and entered water less frequently but for longer durations. These shifts indicate reduced exploratory behaviour and altered water use, potentially limiting ecological flexibility. Although shell size and shape were unaffected, shell crush resistance increased markedly with dose, suggesting altered properties tied to behavioural or physiological responses. Our results show that parasites can impose sublethal yet ecologically significant costs on dead-end hosts. This challenges traditional views of dead-end hosts as passive endpoints and highlights their potential role in shaping host-parasite dynamics. For invasive species such as P. acuta, even non-transmissive infections may carry fitness and ecological consequences.