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The coastline of Argentine Patagonia in the Southwest Atlantic provides highly productive habitats for a variety of aquatic birds. Both resident birds and migratory birds coming for wintering from the high arctic tundra, converge in these wetlands for resting and feeding, which favours parasite transmission. In the course of helminth diversity survey in Patagonian birds, we found a new digenean species belonging to the genus Parorchis (Philophthalmidae). Herein, we describe its complete life cycle and provide the morphological description accompanied by DNA sequence data. Adult specimens were obtained from the resident Kelp Gull Larus dominicanus (Laridae), and Nearctic migratory shorebirds, the Baird's Sandpiper Calidris bairdii and the White-rumped Sandpiper Calidris fuscicollis (Scolopacidae). Rediae of the new species were found in the muricid gastropod Trophon geversianus (Muricidae), whose distribution is restricted to the intertidal and subtidal marine south coasts of Argentina and Chile. Sequences of complete ITS and partial 28S regions confirmed the conspecificity of the rediae from T. geversianus and adults specimens recovered from L. dominicanus and C. fuscicollis. Cercariae emerge from these molluscs and quickly encyst on the substrate including surfaces of the mollusc itself; shorebirds become infected by eating snails. We examine phylogenetic affinities of the new species using partial 28S sequences. Our analysis placed the new species in a clade with other Parorchis spp., which also use muricid gastropods as intermediate hosts. This study contributes to the knowledge of the diversity of marine trematodes in the subpolar region of South America. [ABSTRACT FROM AUTHOR]