Cleaning the mud: Integrative approach clarifies the taxonomy of mud crabs of the genus Panopeus H. Milne Edwards, 1834 (Decapoda: Brachyura: Panopeidae) along the Southwestern Atlantic

The crab genus Panopeus is characterized by a high morphological similarity among its species and the taxonomic problems that ensue. This complex situation can lead to misidentifications and mistaken comprehension of the phylogenetic relationships and species distributions. In this context, the present study morphologically and genetically (16S rRNA and COI) assessed the seven species of Panopeus recorded from the Southwestern Atlantic, with special focus on Panopeus occidentalis, whose occurrence in Brazilian waters has been under debate, and on the relationship between Panopeus austrobesus and Panopeus rugosus, where available genetic and morphological data were apparently contrasting. Although the investigated species are morphologically similar, this study describes some characters that can be used as diagnostic, such as a pigmented spot in the inner surface of the third maxilliped ischium. Phylogenetic inference and genetic distance reinforced the idea that the genus is non-monophyletic and were able to clarify the phylogenetic relationships within the genus. The integrative approach using morphological and genetic data confirmed the occurrence of six out of the seven Panopeus species reported along the Southwestern Atlantic coast, but with different geographic extents than what was previously assumed in some cases. We provide updated diagnoses for these species and also a key for their identification.