Monday, July 29, 2019

[Ichthyology • 2019] Knodus nuptialis • A New Species of Knodus Eigenmann (Characiformes: Characidae: Stevardiinae) from Rio Xingu Basin, Brazil, with Comments on Nuptial Tubercles and Gill Gland in Characiform Fishes

Knodus nuptialis 
Menezes & Marinho, 2019

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.

Fig 1. Type specimens of Knodus nuptialis. Brazil, Pará, Altamira, Rio 13 de Maio at PCH Salto do Três de Maio, tributary of Rio Curuá, Rio Xingu basin
 (A) holotype, preserved coloration, MZUSP 124829, 46.5 mm SL, male;
(B) paratype MZUSP 124828, female, 50.8 mm SL.

Knodus nuptialis, new species

Diagnosis: Knodus nuptialis can be distinguished from all congeners, except K. deuteronoides Eigenmann in Eigenmann and K. tiquiensis Ferreira and Lima, by having the dentary teeth arranged in a continuous series, with teeth decreasing gradually in size posteriorly (versus arranged in a discontinuous series with the anterior teeth conspicuously larger, followed by abruptly smaller teeth posteriorly). Knodus nuptialis differs from K. deuteronoides by having 3–5 (rarely 3) premaxillary teeth in the outer row (versus 2–3 (rarely 3)), 4 scale rows below lateral line (versus 3), the origin of the dorsal fin closer to snout tip than to caudal-fin base (versus dorsal-fin origin in the middle of the distance between snout tip and caudal-fin base), the origin of the anal fin posterior to vertical crossing base of last dorsal-fin ray (versus anal-fin origin anterior to vertical crossing base of last dorsal-fin ray in K. deuteronoides; data from [13], and midlateral dark stripe reaching humeral spot (versus not reaching, humeral spot with a pale area behind; data from [14]. The new species can be distinguished from K. tiquiensis by having a single humeral spot (versus two) and the relatively narrow midlateral stripe (versus broad stripe). Knodus nuptialis can be further distinguished from all congeners, except K. deuterodonoides, K. figueiredoi Esguícero and Castro, K. geryi Lima, Britski and Machado, K. meridae Eigenmann, K. orteguasae Fowler, and K. tiquiensis, by having 12–15 branched anal-fin rays (versus 16–26). It can be further distinguished from K. figueiredoi by having inner premaxillary teeth with 5 to 8 cusps (versus 3), from K. meridae and K. orteguasae by having 4 scale rows between lateral line and pelvic-fin origin (versus 2 ou 3) and from K. deuterodonoides and K. tiquiensis by the features aforementioned. The presence of densely concentrated nuptial tubercles in mature males may also help to diagnose the new species.

Fig 10. Type locality of Knodus nuptialis. Rio 13 de Maio, tributary of Rio Curuá, upper Rio Xingu basin at Serra do Cachimbo.

Fig 9. Geographic distribution of Knodus nuptialis.
 Rio 13 de Maio, tributary of Rio Curuá, upper Rio Xingu basin at Serra do Cachimbo, state of Pará, Brazil. Shaded area corresponds to rio Tapajós basin.

Etymology: The species name nuptialis is from the Latin meaning pertaining to marriage, in allusion to the presence of a series of sexual dimorphic traits (hooks, gill glands and nuptial tubercles) during the breeding season of this species. 

Distribution: Knodus nuptialis is so far known from the Rio 13 de Maio, tributary of Rio Curuá, upper Rio Xingu basin in the state of Pará, Brazil (Figs 9 and 10).

Naércio A. Menezes and Manoela M. F. Marinho. 2019. A New Species of Knodus Eigenmann (Characiformes: Characidae: Stevardiinae) with Comments on Nuptial Tubercles and Gill Gland in Characiform Fishes. PLoS ONE. 14(7): e0217915. DOI: 10.1371/journal.pone.0217915


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