Phylogenetic Structure and Morphological Diagnosis of Species Groups in the Species‐Rich Genus Rineloricaria (Siluriformes: Loricariidae: Loricariinae)

The Neotropical freshwater ichthyofauna harbours exceptional diversity, yet many groups remain taxonomically challenging due to the lack of robust phylogenetic frameworks. The genus Rineloricaria (Siluriformes: Loricariidae: Loricariinae), one of the most species‐rich and widely distributed loricariid genera, exemplifies this issue, with numerous undescribed taxa and cryptic diversity. In this study, we present a comprehensive molecular phylogeny of Rineloricaria , including most described species and multiple potentially new taxa, based on one nuclear and four mitochondrial markers. Phylogenetic analyses using maximum likelihood and Bayesian inference recovered Rineloricaria as monophyletic with strong support and revealed five major clades broadly congruent with South American freshwater ecoregions. Within these clades, we identified multiple monophyletic species groups supported by exclusive morphological characters or unique combinations of characters. These include the R. teffeana , R. lanceolata , R. pentamaculata , R. aurata , R. platyura , R. phoxocephala , R. latirostris , R. nigricauda , R. microlepidogaster and R. catamarcensis groups. Some lineages remained unassigned, reflecting morphological ambiguity or data limitations, and likely represent undescribed diversity. Our results demonstrate that, despite extensive homoplasy in external morphology, consistent diagnostic characters can be identified when interpreted within a robust phylogenetic framework. The integration of molecular and morphological data provides a stable basis for delimiting natural groups and offers a practical framework to accelerate taxonomic revisions in this hyperdiverse genus. This study represents a significant step towards overcoming the taxonomic impediment in Rineloricaria and contributes to a more comprehensive understanding of diversification patterns in Neotropical freshwater fishes.