A new species of Knodus (Characiformes: Characidae: Stevardiinae) from the rio Aripuanã, rio Madeira basin, Brazil

A new species of the characid genus Knodus is described from the rio Aripuanã (rio Madeira basin). It can be distinguished from its congeners by its very low body depth, the presence of trito pentacuspid teeth on the outer premaxillary series, with the median cuspid larger than the lateral ones, the teeth of the inner premaxillary series pentacuspid, distinctly larger than those of the outer series, the maxillary teeth trito pentacuspid, with the median cusp slightly larger than the lateral ones, the four anteriormost dentary teeth pentacuspid, the smaller posterior teeth trito pentacuspid, a complete lateral line with 36–38 scales, 3 longitudinal scale series from pelvic fin origin to lateral line, and 11– 12 circumpeduncular scales. The new species is also compared to incertae sedis species of ‘Bryconamericus’ from northern South America since phylogenetic studies suggest a closer relationship of those species with Knodus.


INTRODUCTION
The speciose genus Knodus Eigenmann, 1911 currently includes 28 valid species (Fricke et al., 2020) of small-sized fishes (30 to 90 mm SL; van der Sleen et al., 2018), mainly occurring in the Amazon basin, but with representatives distributed in all main drainages of cis-Andean South America (Lima et al., 2004;Ferreira, Netto-Ferreira, 2010;Esguícero, Castro, 2014). The traditional diagnosis of Knodus from other Characidae is very similar to that of Bryconamericus Eigenmann, 1907 in its broad sense. Both genera are recognized based on a generalized combination of characters following the Eigenmanian classification scheme (Eigenmann, 1917). Besides being plesiomorphic characters within the Stevardiinae or the Characidae (Malabarba, Weitzman, 2003;Weitzman et al., 2005;Mirande, 2010), a considerable plasticity has been documented within both genera for each of the diagnostic characters, and the sole character allowing the distinction from each other is based on the presence of scales covering half the extent of caudal-fin rays in Knodus, or being restricted to the base of that fin in Bryconamericus (Eigenmann, 1911;Schultz, 1944;Ferreira, Netto-Ferreira, 2010;Esguícero, Castro, 2014;Dagosta, Netto-Ferreira 2015). Despite presenting a broad variation (Schultz, 1944), the extent of scales onto caudal-fin rays has been oversimplified into a binary character (caudal fin "scaled" or not), causing ambiguous interpretations of various authors, which resulted in conflicting opinions concerning the allocation of the species to the proper genus, also recurrent in other speciose Characidae genera (i.e. Hyphessobrycon Durbin, 1907vs. Hemigrammus Gill, 1858Astyanax Baird, Girard, 1854vs. Moenkhausia Eigenmann, 1913. Given the controversy involving the extent of caudal fin squamation distinguishing Bryconamericus and Knodus, some authors have rejected its validity as a diagnostic character between these genera, or even proposing the synonymy of Knodus with Bryconamericus (i.e. Schultz, 1944;Taphorn, 1992;Román-Valencia, 2000, 2003, 2005; Román-Valencia 3/14 scielo.br/ni | sbi.bio.br/ni et al., 2009). Contradicting that proposal, Thomaz et al. (2015) and Mirande (2019) formally corroborated the distinction of Knodus and Bryconamericus as separate lineages, justifying the maintenance of both genera as valid members of the Diapomini.
In the present contribution, a new species tentatively assigned to Knodus from the rio Aripuanã in the rio Madeira basin is described. The new species is also compared to representatives of Bryconamericus from northern South America, since the composition of both genera is likely to be further modified in future studies, considering the possible closer relationship between 'Bryconamericus' from northern South America with Knodus, suggested by the aforementioned studies (Thomaz et al., 2015;Mirande, 2019).

MATERIAL AND METHODS
Counts and measurements are those described in Fink, Weitzman (1974) and Menezes, Weitzman (1990) except for the number of longitudinal scale series below the lateral line, which are counted from the pelvic-fin origin to the lateral line. To determine the disposition and number of radii on body scales, a scale was taken from the third horizontal series down from the dorsal-fin base to the lateral line. Numbers of vertebrae and vertebral elements, supraneurals, procurrent caudal-fin rays, teeth cusps and unbranched analfin rays were obtained from 5 cleared and stained (CS) specimens prepared according to Taylor, Van Dyke (1985). Vertebral counts include the vertebrae of the Weberian apparatus as four elements, as well as the complex caudal ossification PU1+U1 with the associated hypural bone and "half vertebra" counted as a single element. Institutional abbreviations follow Ferraris (2007). Meristic characters are presented in the descriptions, with the range of each count followed by their frequency in parentheses, and an asterisk (*) indicating the values of the holotype. In determining the position of the last supraneural, and dorsal-and anal-fin pterygiophores in relation to neural spines, counts of the vertebral centra, include those that are part of the Weberian apparatus.

Description.
Morphometric data of holotype and paratypes in Tab. 1. Body comparatively small (largest examined specimen 73 mm SL). Head and body elongate and laterally compressed; greatest body depth at dorsal-fin origin. Profile distinctly convex from upper jaw to posterior nostril, slightly convex from latter point to dorsalfin origin, straight along dorsal-fin base, slightly concave from latter point to adiposefin origin, and slightly concave to nearly straight from latter point to anterior most dorsal procurrent caudal-fin ray. Ventral body profile convex from tip of lower jaw to isthmus, nearly straight from that point to vertical through pectoral-fin origin, convex from latter point to pelvic-fin origin, and straight from that point to anal-fin origin, and straight along anal-fin base and concave along caudal peduncle.

Coloration in alcohol.
Ground color pale yellowish to brown. Color pattern poorly counter shaded. Upper portion of head from tip of snout to end of supraoccipital spine and predorsal scales only slightly more pigmented, and somewhat darker than lateral and ventral portions of body; minute dark chromatophores scattered around eye extending laterally over maxilla, first, second, fourth, fifth and sixth infraorbitals, upper half of third infraorbital, upper half of opercle and interopercle; anterior part of lower jaw with scattered dark chromatophores. Postorbital portion of the head  with disperse chromatophores forming a discrete darker pigmentation from the infraorbitals 5 and 6, and opercle to the pectoral girdle. Dark chromatophores onto distal margins of scales, distinctly lighter at focus; scattered dark chromatophores on lower part of body below lateral line series, darker above anal-fin base. A vertically elongate humeral blotch present onto scales of 3 to 4 longitudinal series, becoming narrower downward (Fig. 1). Diffuse stripe with scattered chromatophores present on the body, from vertical through pectoral-fin midline to caudal-fin base, becoming more conspicuous near vertical through anal-fin origin, being slightly enlarged over caudal peduncle (Fig. 1). All fins hyaline with scattered dark chromatophores on intervening membranes.
Sexual dimorphism. Mature males of Knodus angustus present bilateral hooks on first unbranched and the following six branched pelvic-fin rays (Fig. 3). Bilateral bony hooks present on last unbranched plus four to six anteriormost branched anal-fin rays (Fig. 4). Mature males also with a gill gland on the lower portion of the first gill arch involving 8 gill filaments. Hooks and gill gland absent on mature females.
Etymology. The species epithet angustus is from the Latin, meaning "narrow" in allusion to the narrowness of the body of this species. An adjective.
Geographical distribution. Knodus angustus is known so far from the rio Aripuanã, a tributary of the rio Madeira, Amazonas, Brazil (Fig. 5)   Conservation status. Although Knodus angustus is known only from its type locality in the rio Aripuanã, the area where the specimens were sampled is adjacent to the Floresta Nacional do Aripuanã. Despite the recent, on-going deforestation peak in the Brazilian Amazon, it is likely that anthropic impacts on the forest will take several years to generate impacts on the population of K. angustus, therefore, the species is classified herein as Least Concern (LC) according to the International Union for Conservation of Nature (IUCN) categories and criteria (IUCN Standards and Petitions Subcommittee, 2019).

DISCUSSION
The present allocation of the new species in Knodus is based both on the presence of scales covering over one third of caudal-fin rays, following the Eigenmannian classification scheme, but also in the strong evidence provided by Thomaz et al. (2015) and Mirande (2019) that small Stevardiinae from the Amazon basin and cis-Andean northern South America fitting the general diagnosis for Knodus and Bryconamericus would be more closely related to Knodus meridae. That relationship pattern was largely corroborated by García-Melo et al. (2019) with most samples originating from cisandean Northern South America being nested with species of Knodus, the only exception being the specimens identified as 'Bryconamericus' pachacuti Eigenmann, 1927 from the Amazon, which grouped with Bryconamericus. That result contradicts the hypotheses of Thomaz et al. (2015) and Mirande (2019), in which 'B'. pachacuti grouped with Attonitus Vari, Ortega, 2000. Considering the oversimplified phylogenetic framework of García-Melo et al. (2019), lacking representatives of several Diapomini genera (i.e. Attonitus, Diapoma Cope, 1984, Odontostoechus, Piabarchus Myers, 1928, and the effect of taxon sampling in phylogenetic trees topology, the close relation of 'B'. pachacuti with B. exodon may be an analytical artifact.  Besides the multiple taxonomic recombinations within the Diapomini, the aforementioned phylogenetic studies partially refute the validity of the caudal-fin squamation as a diagnostic character between Bryconamericus and Knodus, as the extended sense of Knodus includes a broad variation of that character, partially corroborating the criticism from several authors (Schultz, 1944;Taphorn, 1992;Román-Valencia, 2000, 2003, 2005Román-Valencia et al., 2009). In addition, as both genera lack unambiguous morphological diagnostic characters, it is not possible, at this moment, to unambiguously assign which species of 'Bryconamericus' would be, in fact, more closely related to Knodus.
Knodus angustus is the third species of the genus described from tributaries of the Rio Madeira, the other species being K. smithi and K. jacunda. The first occurs upstream of the Teotônio rapids in Porto Velho, Rondônia (Queiroz et al., 2013), whereas Knodus jacunda, also described from near Porto Velho, is only known from the holotype and its validity is still questionable. Knodus angustus differs from K. smithi by having 3 longitudinal scale series from the lateral line to the pelvic fin-origin (vs. 4), 11-12 circumpeduncular scale rows (vs. 14); 15-19 branched anal-fin rays (vs. 23-27); the presence of pentacuspid teeth on the inner premaxillary series (vs. teeth heptacuspid); and the lateral line with 36-38 perforated scales, (vs. 38-40 perforated scales).
Despite tentative, the comparisons between K. angustus and the species above are an attempt to facilitate the distinction between them in future studies, especially considering the great impact of recent phylogenetic hypotheses in the number of species presently included in the genus Knodus (Thomaz et al., 2015;Mirande, 2019;García-Melo et al., 2019). Although the relationships of several species from northern South America with other Stevardiinae are yet to be adequately tested, those studies also provide strong evidence suggesting the composition of Knodus is likely to be further adjusted by future, more comprehensive phylogenetic hypotheses.