Date Published: July 10, 2019
Publisher: Public Library of Science
Author(s): Naércio A. Menezes, Manoela M. F. Marinho, Roberto E. Reis.
Knodus nuptialis n. sp. is described from the Rio Curuá drainage, Rio Xingu basin, Brazil. It can be diagnosed from its congeners by having dentary teeth decreasing gradually in size posteriorly, outer premaxillary teeth row with five cusps, 12–15 branched anal-fin rays and a single humeral spot. The species presents notable sexual dimorphism consisting of densely concentrated nuptial tubercles on head, body, and fins, gill-gland, and bony hooks in the anal fin of mature males. It was found that these sexually dimorphic features are useful and functional in males of the new species only during the reproductive season and after this period, they become atrophied, and eventually disappear. The list of characiform species presenting breeding tubercles is updated and nine species and two genera of the Characidae, Deuterodon and Bryconacidnus, are for the first time reported to have breeding tubercles.
The combination of two premaxillary tooth rows, inner row with four teeth, and caudal-fin scaled has been traditionally used to diagnose the characid genus Knodus Eigenmann, defined as being “a Bryconamericus in all but is scaled caudal” . The recognition of Knodus has long been controversial, some consider it as synonym of Bryconamericus Eigenmann in Eigenmann (see discussion in [2–4]). In recent phylogenetic analyzes there is consensus that Knodus is not a natural assemblage as traditionally defined [5–7], and  further defined a “Knodus sensu stricto” based on molecular data, encompassing species of Knodus, Bryconamericus and Bryconadenos. Therefore, the genus still lacks a phylogenetic definition.
Counts and measurements are those described in  and  except for the number of longitudinal scale rows below the lateral line, which are counted from the pelvic-fin origin to the lateral line. The number of scale rows below lateral line includes the smaller and notched scale at the base of the lateralmost pelvic-fin ray. The pattern of radii was examined on scales sampled from the third horizontal scale row from dorsal-fin base to the lateral line. Numbers of vertebrae and vertebral elements, supraneurals, procurrent caudal-fin rays, teeth cusps and unbranched anal-fin rays were obtained from five cleared and stained (CS) specimens prepared according to . In the list of specimens, mol indicate molecular tissue preserved. Vertebral counts include the four vertebrae of the Weberian apparatus as well as the first pre-ural and first ural centrum of the caudal region, counted as single element. In the description, the range of meristic data are followed by the frequency for each count in parenthesis, with an asterisk (*) indicating the value of the holotype. Institutional abbreviations follow . Analyses for differences between sexes were performed using T-Test Calculator for 2 Independent Means in  using measurements not transformed into body proportions. Difference was considered significant when p ≤ 0.05. Correlation of sexual dimorphic traits were confirmed upon examination of the gonads of most of the specimens, under the stereomicroscope. Color in life was based on photographs of freshly collected specimens.
Knodus nuptialis share the traditional morphological features that define the genus, especially the possession of scales on the caudal fin, sometimes slightly elongate extending to about two thirds over base of caudal-fin rays. Therefore, the new species is here included in Knodus. It is noteworthy, however, that K. nuptialis shares some similarities with Myxiops aphos Zanata and Akama.
Astyanax guaricana: MZUSP 112225, 10 paratypes, Rio Cubatão, Paraná, Brazil Astyanax jenynsii: MZUSP 110321, 6, Rio Paraíba do Sul, São Paulo, Brazil Astyanax jordanensis: MZUSP 99137, holotype, Rio Iguaçu basin, Paraná, Brazil Astyanax scabripinnis: MZUSP 45927, Rio Paraíba do Sul, São Paulo, Brazil. Bryconacidnus ellisi: MZUSP 121041, 8, Rio Madre de Dios basin, Peru. Bryconacidnus hemigrammus: MZUSP 121060, 20, lower Rio Urubamba, Cuzco, Peru. Bryconamericus microcephalus: MZUSP 80012, 40, Rio Ribeira de Iguape, São Paulo, Brazil. Deuterodon iguape: MZUSP 114835, Rio Ubatubinha, costal drainage, São Paulo, Brazil. Knodus geryi: MZUSP 78863, 28 paratypes, upper Rio Paraguai, Mato Grosso, Brazil. Knodus tiquiensis: MZUSP 81166, 1 paratype, MZUSP 85086, 1 paratype, MZUSP 85042, 3, Rio Negro basin, Brazil. Myxiops aphos: MZUSP 81026, holotype, MZUSP 81025, 17 paratypes, 4 c&s, tributary of Rio Paraguaçu, Lençóis, Bahia, Brazil.