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| Astyanax nobre |
Abstract
A new species of Astyanax is described from the rio Salobra, tributary of rio Cuiabá, rio Paraguai basin. The new taxon can be distinguished from its congeners by having a well-defined dark midlateral stripe on body extending from the posterior margin of the opercle to the base of middle caudal-fin rays and a single vertical elongate humeral blotch. Although the new species is described in Astyanax, some specimens present an incomplete or a discontinuous series of perforated scales in the lateral line. Therefore, a discussion on its generic allocation is presented. Comments on different patterns of coloration among dark-striped species of Astyanax are also provided. The discovery of a new species in an underwater tourist point relatively near a large urban center underscores that even fish species daily observed by hundreds of people in limpid waters may lack a formal taxonomic identity. Such finding also highlights how the megadiverse Brazilian freshwater ichthyofauna still needs efforts and investments to identify and describe new taxa.
Keywords: Lateral line, Nobres, Longitudinal stripe, Taxonomy, Tourist destination.
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| Underwater photographs of wild specimens of Astyanax nobre at its type locality Recanto Ecológico Lagoa Azul, Bom Jardim district, Municipality of Nobres, Mato Grosso State, rio Salobra drainage, rio Paraguai basin: A. Pair of males; B. Pair of females. Details about differences between sexes can be found in Sexual dimorphism section. Photo by M. Melo. |
Astyanax nobre, new species
Diagnosis. Astyanax nobre can be distinguished from its congeners, except A. joaovitori Oliveira, Pavanelli & Bertaco, 2017 and A. scintillans Myers, 1928, by the presence of a well-defined, dark midlateral stripe on body extending from opercle to middle caudal-fin rays (vs. longitudinal stripe absent; stripe starting posterior the anterior humeral and never connected to it; stripe starting immediately posterior to the humeral blotch; narrower stripe starting at posterior margin of the opercle, becoming blurred posteriorly and not reaching the caudal fin). The new species can be clearly distinguished from A. scintillans by the presence of a vertical humeral blotch (vs. absence). Astyanax nobre can be distinguished from A. joaovitori by having the dark midlateral stripe ending at the base of the middle caudal-fin rays (vs. reaching the distal tip of the middle caudal-fin rays) and three scale series below the lateral line (vs. five or six). Another remarkable difference between these species is the body size. Astyanax nobre has mature individuals at about 25 mm SL and reaching up to 30 mm SL whereas A. joaovitori reaches a much larger body size, with up to 77 mm SL (see Oliveira et al., 2017). Astyanax nobre fits within the A. scabripinnis species complex sensu Bertaco, Lucena (2006). According to the authors, the group is characterized by species with body deepest and heaviest in area close to middle of pectoral fins, head heavy, snout short and abrupt by tapering, body depth smaller than 41% of SL (mean 30–33% of SL), reduced number of branched anal-fin rays (13–21, usually 17–18, rarely 22 or 23), presence of one or two humeral spots, and a dark, midlateral, body stripe extending to the tip of the middle caudal-fin rays. Except for the midlateral dark stripe that does not reach the tip of the middle caudal-fin rays, all characteristics are found in Astyanax nobre. According to Oliveira et al. (2017), it is impossible to infer about the dark midlateral stripe of A. scabripinnis (Jenyns, 1842) due to the loss of coloration in the holotype. Astyanax nobre differs from the holotype of A. scabripinnis by having 32–36 lateral line scales (vs. 38).
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Type locality of Astyanax nobre, rio Salobra at Lago Azul, tributary of rio Cuiabá, rio Paraguai basin, ..., Nobres, Mato Grosso State, Brazil. Photo by M. A. Junghans. Distribution map of Astyanax nobre in the rio Salobra, upper rio Paraguai basin, Brazil. Brown star (type locality). |
Geographical distribution. Astyanax nobre is so far known only from the rio Salobra, tributary of rio Cuiabá, rio Paraguai basin (Fig. 5).
Ecological notes. Astyanax nobre was collected in clear water river, with moderate water flow, over bottoms typically composed of rock and sand (Fig. 6). Vegetation includes areas with dense aquatic macrophytes and well preserved riparian forest. The species is one of the most abundant fish species in the locality. Individuals are very used to the human presence; frequently approaching the swimmers to nibble skin from the legs and arms.
Etymology. The specific name nobre refers to the municipality of Nobres (Mato Grosso State, Brazil) where the species occurs. Additionally, “nobre” means noble in Portuguese, in allusion to the beauty of the type locality and of being a noteworthy species of Astyanax. A noun in apposition.
Fernando C. P. Dagosta and Manoela M. F. Marinho. 2022. New Small-sized Species of Astyanax (Characiformes: Characidae) from the upper rio Paraguai Basin, Brazil, with Discussion on Its Generic Allocation. Neotropical Ichthyology. 20(1); e210127. DOI: 10.1590/1982-0224-2021-0127 ni.bio.br/1982-0224-2021-0127
