From coast to interior: Trematodes completing freshwater life cycles are dominant in gulls recorded inland.

Large gulls (Laridae) have expanded from coastal regions into inland Central Europe, where they increasingly exploit fishponds, landfills, and urban habitats. This shift alters their exposure to trematode parasites and their role in linking parasite transmission across ecosystems. We examined the trematode communities of 5 gull species (Larus argentatus, Larus cachinnans, Larus canus, Larus hyperboreus, and Larus marinus) collected in the eastern Czech Republic between 2022 and 2025. Complete helminthological examinations were performed on 207 individuals. Trematodes were detected in 82% of the birds, but the infection patterns differed among host species. Assemblages were overwhelmingly dominated by freshwater-transmitted taxa associated with cyprinid fish, particularly Diplostomum spp. and related genera. Pond-foraging gulls (L. argentatus and L. cachinnans) presented high prevalence, richness, and infection intensity, with frequent co-infections indicating repeated exposure within freshwater food webs. In contrast, L. canus and landfill-associated birds were often uninfected or lightly infected, reflecting limited participation in aquatic transmission cycles. Marine and brackish trematodes occurred sporadically and at low intensities, which is consistent with carry-over infections acquired in coastal regions without inland transmission. Community assemblages at the individual-host level were aggregated and consistent with stochastic exposure once the host species were considered. Maturity had little influence on helminth richness or diversity in gulls. Trematode assemblages closely reflected gull foraging strategies and habitat use, indicating that freshwater pond systems are the primary settings in which inland gulls may influence parasite transmission.