Novel insights into the genetics, morphology, distribution and hosts of the global fish parasitic digenean Proctoeces maculatus (Looss, 1901) (Digenea: Fellodistomidae)

Challenges in the recognition of trematode species: Consideration of hypotheses in an inexact science

The description and delineation of trematode species is a major ongoing task. Across the field there has been, and currently still is, great variation in the standard of this work and in the sophistication of the proposal of taxonomic hypotheses. Although most species are relatively unambiguously distinct from their congeners, many are either morphologically very similar, including the major and rapidly growing component of cryptic species, or are highly variable morphologically despite little to no molecular variation for standard DNA markers. Here we review challenges in species delineation in the context provided to us by the historical literature, and the use of morphological, geographical, host, and molecular data. We observe that there are potential challenges associated with all these information sources. As a result, we encourage careful proposal of taxonomic hypotheses with consideration for underlying species concepts and frank acknowledgement of weaknesses or conflict in the data. It seems clear that there is no single source of data that provides a wholly reliable answer to our taxonomic challenges but that nuanced consideration of information from multiple sources (the 'integrated approach') provides the best possibility of developing hypotheses that will stand the test of time.

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.