The integration of molecular data with morphological analysis has greatly advanced the understanding of parasitic diversity, particularly within groups like Dactylogyridae. While morphological features have long been used to classify genera and species, molecular studies have often revealed discrepancies that challenge traditional taxonomies. Urocleidoides (Mizelle & Price, 1964) is one such example, with previous research suggesting it may not be monophyletic. Meanwhile, Annulotrematoides (Kritsky & Boeger, 1995) has lacked any molecular data until now, limiting our understanding of its evolutionary relationships. In this study, we provide the first molecular sequences for Annulotrematoides, which were grouped with Urocleidoides spp. Our analysis of Annulotrematoides bonaerensis Rossin & Timi, 2016 and Urocleidoides surianoae Rossin & Timi, 2016 found parasitizing the gills of Cyphocharax modestus (Fernández–Yépez) revealed a strong phylogenetic relationship between these two species, despite their classification in different genera. This discovery challenges the current morphological–based taxonomy of Urocleidoides and suggests that U. surianoae may not be accurately placed within its genus. The clustering of Urocleidoides with Annulotrematoides for the first time highlights the need for a taxonomic re–evaluation of these groups. Additionally, this work underscores the importance of molecular data in resolving evolutionary relationships and advancing our understanding of parasitic diversity.
