The role of Miocene-Pleistocene environmental change in diversification of the genus Hypoclinemus 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. However, the biogeography of Hypoclinemus mentalis , initially described as Solea mentalis and subsequently assigned to the genus Achirus , was eventually reclassified into its current monotypic genus due to its limited distribution in freshwater environments, in contrast to the species of Achirus . The broad distribution of a single species across multiple South American river basins positions H. 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. 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. ### Competing Interest Statement The authors have declared no competing interest.