Systematics of Neotropical electric knifefish <i>Tembeassu</i> (Gymnotiformes, Apteronotidae)

A new species of the poorly known and critically endangered ghost knifefish <i>Tembeassu</i> is described from the upper Paraná and Araguari rivers, Brazil, using external anatomy and X-ray microcomputed tomography (µCT scan). <i>Tembeassu titanicus</i> sp. nov. is distinguished from its sole congener, <i>T. marauna</i>, by a unique set of morphometric and meristic characters, in addition to the absence of a tooth patch at the anterior portion of the roof of the oral cavity and the external corner of the mouth slightly passing the vertical through the posterior margin of the posterior nare. To test the monophyly of <i>Tembeassu</i> and reassess its phylogenetic position, a total-evidence approach was performed through a Bayesian inference (BI) and maximum-parsimony analysis (MP). Our results indicate that <i>Tembeassu</i> is monophyletic and the sister taxon of a clade formed by <i>Apteronotus</i> s.s., <i>Megadontognathus</i>, and <i>Parapteronotus</i> (BI); or as part of a large polytomy at the base of Apteronotidae (MP). Species of <i>Tembeassu</i> co-occur in the Paraná River basin, and the absence of the patch of accessory teeth in <i>T. titanicus</i> sp. nov. may indicate that this species accesses a different food resource, and also putatively occupies a different habitat than <i>T. marauna</i>. Comments on the evolution of the mandibular lobe in Gymnotiformes, dentition pattern in <i>Tembeassu</i>, and apteronotid diversity in the Paraná River are provided. http://zoobank.org/urn:lsid:zoobank.org:act:691CB9A5-5CDB-4BF9-8A6FEAE877563C12