Size does not matter: molecular phylogeny reveals one of the largest trematodes from vertebrates, the enigmatic Ithyoclinostomum dimorphum, as a species of Clinostomum (Trematoda: Clinostomidae)

Despite the recent advances raised in the molecular era to the taxonomic knowledge of species of the family Clinostomidae, especially those belonging to the specious genus Clinostomum, some groups of these vertebrate parasites remain poorly studied. This is the case of species of the enigmatic genus Ithyoclinostomum Witenberg, 1926, until recently monotypic and restricted to South America, but with its occurrence expanded to North America after the description of I. yamagutii Rosser et al., 2020. Nevertheless, molecular data for the type species of the genus, Ithyoclinostomum dimorphum (Diesing, 1850), is lacking so far. In the present study, large clinostomid metacercariae morphologically indistinguishable from I. dimorphum were obtained from two erythrinid fishes from the Rio Doce River, Southeast Brazil. Samples of the parasites were subjected to a multigene (28S rDNA, ITS and cox1) molecular characterization followed by phylogenetic reconstructions. Phylogenies based on single-gene and concatenated datasets revealed unequivocally that I. dimorphum falls in a well-supported clade together with species of the genus Clinostomum. Moreover, the molecular divergences observed in relation to Clinostomum spp. [ranges of 2.4–6%, 2.4–3.8% and 14.7–19.3% for the ITS, 28S and cox1 genes, respectively] are compatible with a congeneric status with these species. Therefore, the genus Ithyoclinostomum is here synonymized with Clinostomum Leidy, 1856 and C. dimorphum (Diesing, 1850) Braun, 1899 re-established. In the phylogenetic analysis, the recently described 'Ithyoclinostomum' yamagutii, presented as an isolated, independent lineage, showing significant molecular divergences to C. dimorphum (12.6%, 7.6%, 18,6% for the ITS, 28S and cox1 genes, respectively). However, given the complex scenario raised in the morphology-based taxonomy of Clinostomidae, we took a conservative approach by not proposing a new genus to 'I.' yamagutii until molecular data of other clinostomid genus from birds, Clinostomatopsis, become available. Data here presented reveals that body size is not a useful criterion for higher-level classification in Clinostomidae. Finally, we highlighted the importance of the availability of molecular data for the type species of trematode genera proposed from South America to support a trans- or intercontinental distribution.