Unveiling hidden diversity in Cosmetocleithrum (Monogenoidea: Dactylogyridae) from thorny catfishes (Doradidae) endemic to South America: Two new species, morphological variation, and molecular phylogenetic insights.

Abstract

The Cosmetocleithrum represents the most diverse group of monogenoids parasitizing Neotropical doradid catfishes. In this study, based on morphological and molecular data, we describe two new species, Cosmetocleithrum guamaensis n. sp. and Cosmetocleithrum taeniophallum n. sp., and assess their phylogenetic relationships within the Dactylogyridae using partial 28S rDNA sequences. Our analyses support the validity of these new taxa and suggest their phylogenetic affinity with Cosmetocleithrum species parasitizing auchenipterid catfishes. Additionally, we confirm the presence of Cosmetocleithrum falsunilatum and Cosmetocleithrum bulbocirrus in Megalodoras uranoscopus and Pterodoras granulosus, respectively, with C. falsunilatum exhibiting notable morphological variation across river basins, possibly influenced by environmental factors. Our phylogenetic results corroborate previous studies indicating two distinct Cosmetocleithrum lineages, one exclusively associated with doradids and another with both doradids and auchenipterids, with weak support for the genus' monophyly. Furthermore, we discuss morphological diversity within Cosmetocleithrum, highlighting significant variation in key diagnostic features, including haptoral structures, copulatory complex morphology, and vaginal opening orientation. These findings reinforce previous hypotheses suggesting that Cosmetocleithrum may not constitute a natural group and that taxonomic revisions are needed. The limited molecular data available for Cosmetocleithrum species (with only 13 out of 31 described species studied) and the disparity between the number of known doradid hosts and those surveyed for monogenoids highlight the urgent need for further integrative taxonomic and phylogenetic studies. This approach will be crucial for providing valuable insights and advancing our understanding of the complex evolutionary puzzle of one of the largest genera of monogenoids parasitizing Neotropical Siluriformes.