INSIGHTS FOR A NEW LINEAGE OF THE MONOTYPIC GENUS Hemiodontichthys BLEEKER 1862 (SILURIFORMES: LORICARIIDAE) IN THE EASTERN AMAZON

The armored catfish Hemiodontichthys acipenserinus is a monotypic species of Neotropical freshwater fish belonging to the family Loricariidae. In the last decade, several studies have questioned the monotypic status of this species, proposing the occurrence of cryptic species and the existence of different lineages under the ‘umbrella’ of a single species. Recent studies have described the occurrence of this species in the hydrographic basins of northeastern Brazil. Based on this, this study aimed to verify, through molecular analyses, whether specimens from a population in the Turiaçu River (State of Maranhão, Eastern Amazon) belong to this species or to one of the known molecular lineages. To this end, sequences of the Cox1 gene were produced and compared with sequences from the five lineages of this species available in the BOLD Systems and NCBI (GenBank). The genetic distance between the populations was calculated, in addition to performing five species delimitation analyses: BIN, ASAP, ABGD, bPTP, and GMYC. The relationships between the obtained lineages were analyzed using a phylogenetic tree through Bayesian Inference (BI), and a molecular clock was constructed to identify the divergence time between the identified lineages. The results of all analyses revealed that the Turiaçu River (State of Maranhão, Eastern Amazon) population constitutes a different lineage from the previously identified lineages, with genetic distances between 3.89% and 6.21% from the other lineages, the greatest distance being to H. acipenserinus and the smallest to the Guamá River lineage, the closest geographically. The phylogenetic tree also indicates greater proximity to the Guamá River lineage, while the molecular clock indicates that these lineages separated from the others approximately 1.4 Ma ago. The results obtained here confirm the monophyly of Hemiodontichthys, as well as the existence of different evolutionary lineages, including the Turiaçu River population. Although the species has a wide distribution throughout the Brazilian Amazonian rivers, this distribution is fragmented. This discontinuity in habitats may be a factor in the isolation of these populations, consequently promoting vicariance and allopatric speciation. However, more integrative studies with morphological and molecular data are needed to identify characters that can differentiate these populations into species.