Infection Dynamics of Two Echinostomatoidea Cercariae from Echinolittorina peruviana to Secondary Intermediate Hosts

Digenean of the superfamily Echinostomatoidea exhibit complex life cycles that involve up to three host species. Several echinostomatoid species have been reported in snails of the family Littorinidae. In the zebra snail Echinolittorina peruviana from central Chile, two cercarial species resembling Echinostomatoidea were recognized based in their morphological characteristics. Molecular characterization was carried out to identify the species. Key aspects of their infection dynamics were quantified and compared between the two cercarial morphologies, with particular emphasis on transmission to secondary intermediate hosts. We evaluated: (i) swimming activity, (ii) the success of cercarial settlement in a mussel host across different body sizes, and (iii) host selection among three potential secondary intermediate hosts (a mussel, a snail, and a crab). Molecular analyses, based on the 28S gene, indicated that the examined cercariae corresponded to two distinct species from the superfamily Echinostomatoidea. However, species-level identification could not be confirmed, and they were therefore designated as species 1 and species 2. Species 1 exhibited a significantly faster decline in swimming activity compared to species 2, whose active period was approximately twice as long. Mussels were successfully infested in all trials, with no significant differences among shell-size groups. In host preference experiments, all host types were selected. However, only the cercaria species 2 showed a significant preference for the crab, suggesting that this may be an alternative secondary intermediate host. Despite their similar body size, the two cercarial species displayed distinct infection dynamics and host-use strategies, indicating divergent biological traits and transmission pathways within a limited time window.