The Role of Miocene‐Pleistocene Environmental Change in Diversification of the Genus <i>Hypoclinemus</i> Chabanaud, 1928

The Neotropical region stands out as one of the most taxonomically diverse areas on the planet, garnering significant attention in the context of marine incursions and their role in shaping this diversity. Among marine-derived taxa, pleuronectiform fishes exhibit distinctive morphological characteristics that have attracted significant scientific interest. The broad distribution of a single species across multiple South American river basins positions Hypoclinemus mentalis as an ideal candidate for biogeographic studies within South America, with an emphasis on the detection of cryptic lineages associated with major drainage basins. This study aimed to investigate the monotypic status of Hypoclinemus, its evolutionary history, and whether historical paleogeographical events could play a role in the dispersion of H. mentalis in the Neotropical region, utilizing mitochondrial and nuclear markers. In our study, we employed mitochondrial and nuclear markers to investigate the potential existence of such lineages within the broader context of a molecular phylogeny that encompasses all valid genera in the flatfish family Achiridae. Our findings reveal that Hypoclinemus comprises seven operational taxonomic units (OTUs), as deduced from specimens collected across the majority of its documented range. Furthermore, our phylogeographic analyses support the hypothesis that colonization of freshwater habitats occurred through connections between the Caribbean Sea and Lake Pebas approximately 21.28 million years ago. Moreover, we observed that differentiation of lineages within the Hypoclinemus genus was significantly influenced by pronounced sea level fluctuations during the Plio-Pleistocene epoch, underscoring the impact of glaciations and interglacial periods on the biogeographic patterns.