First Report of Proctoeces maculatus (Digenea: Fellodistomidae) Infecting the Ribbed Mussel, Geukensia demissa: Detection of a Unique Haplotype in New England, USA

Parasites play critical roles in ecosystems, influencing host populations and community dynamics. Despite their ecological significance, the diversity and genetic structure of parasites in the ribbed mussel, Geukensia demissa (Dillwyn), a key species in salt marsh ecosystems, remain poorly understood. This study provides the first record of the trematode Proctoeces maculatus (Looss, 1901) infecting G. demissa in the Herring River estuary, Cape Cod, Massachusetts. Among 50 mussels examined, P. maculatus prevalence was 66%, with infection intensities ranging from light (less than 20 sporocysts per individual) to over 300 sporocysts per individual. Heavily infected mussels displayed mantle discoloration, suggesting potential pathological impacts. Molecular analysis of the 28 S ribosomal RNA gene identified a unique haplotype, GD1, restricted to G. demissa populations in Cape Cod. Haplotype network analysis revealed GD1’s distinctiveness within P. maculatus populations, while genetic divergence (K2P: 0.001–0.003) indicated incipient host-associated differentiation rather than cryptic speciation. These findings highlight the role of ecological partitioning, such as the salt marsh-specific habitat of G. demissa, in shaping parasite genetic structure.