Is Saccocoelioides bacilliformis Szidat, 1973 (Digenea: Haploporidae) a valid species? Genetic and morphological studies of this controversial species

Saccocoelioides is a genus within the family Haploporidae and is distributed throughout the Americas. The recent application of molecular techniques has facilitated the reorganization of this genus and the description of new species, resulting in a current total of 28 species. In Argentina, 11 species have been identified; however, the validity of Saccocoelioides bacilliformis and Saccocoelioides octavus remains controversial. Recently, specimens of digeneans were collected from two distinct geographical locations: the Paraná River basin (Misiones province) and the Río de la Plata basin (Buenos Aires province). These specimens were found parasitizing two different fish species: Bryconamericus sylvicola and Bryconamericus iheringi. Sequences of the 28S and COI genes were obtained for both digeneans from the two collection sites and hosts. The morphological study, combined with genetic sequences, confirmed their assignment to the genus Saccocoelioides. The COI and 28S gene sequences obtained from specimens at both collection sites exhibited genetic distances of 0.13% and 3.41%, respectively, indicating that these Saccocoelioides specimens represent a single species. The comprehensive taxonomic study of the newly collected specimens, along with an examination of the type material of S. bacilliformis and S. octavus, allows us to confirm the validity of both species. Argentina is home to more than 550 species of freshwater fauna, and it is likely that new species of Saccocoelioides remain to be discovered in underexplored regions of the basin, such as the Uruguay, Pilcomayo and Bermejo rivers.