A new species of Tetragonopterus ( Characiformes : Characidae ) from Central Amazon lowlands , Brazil

A new species of Tetragonopterus is described from lowland rivers of Central Amazon. It differs from congeners by having a vertically-oriented patch of dark pigmentation limited to posterior portion of the caudal peduncle and by bearing five thin and sharp teeth on dentary, along with other morphometric and meristic features. We hypothesize that the new taxon belongs to the herein named “Tetragonopterus anostomus clade” that includes T. anostomus, T. denticulatus, T. kuluene, and T. juruena.


Species of Tetragonopterus
are recognizable by the presence of two aligned rows of premaxillary teeth, presence of five tricuspidate to pentacuspidate teeth in the inner row of the premaxilla, complete lateral line that is bent downward anteriorly, or slightly bent as in T. georgiae (Géry, 1965), branched laterosensory canal in the sixth infraorbital, a flattened prepelvic area bounded laterally by well-marked angles, absence of predorsal spine and the possession of two or three supraneurals (Eigenmann, 1917;Mirande, 2010;Melo et al., 2011;Silva et al., 2016).The genus encompasses small-sized fishes (maximum 11.2 cm standard length) ranging from the Orinoco basin through most northern South America in the vast portion of the Amazon and Guianas and across the La Plata (except upper rio Paraná) and São Francisco basins and adjacent Atlantic coastal drainages, from lower Amazon basin to rio Itapecuru in northeastern Brazil (Silva et al., 2016).
The alpha taxonomy of Tetragonopterus has received effective advances since the description of several new species from craton-derived rivers of the Amazon basin (i.e., Brazilian and Guiana shields) (Melo et al., 2011;Silva, Benine, 2011;Silva et al., 2013;Araujo, Lucinda, 2014).This progress allowed the conclusion of a comprehensive taxonomic revision containing updated distribution information, redescription of poorly described species, formal descriptions of four new species and a molecular dataset of all mitochondrial lineages (Silva et al., 2016).Such improvement resulted in a total of twelve valid species of Tetragonopterus, in which eleven of them were included in the subsequent molecular, time-calibrated phylogeny (Melo et al., 2016).Despite the recent systematic revision and phylogeny, the continuous examination of Tetragonopterus collected along various localities along the Amazon basin revealed an undescribed species apparently endemic to lowland rivers of Central Amazon, which we formally describe herein.

Material and Methods
Counts and measurements follow Fink, Weitzman (1974) and Benine et al. (2004) and were taken point to point with a digital caliper (precision of 0.1 mm) on the left side of specimens whenever possible.All measurements of standard length (SL) and head length (HL) are expressed as percentage of SL or HL.Principal dentary teeth are the anteriormost similar teeth that decrease gradually followed 2 e170158 [2] by a series of distinctly smaller teeth.Cleared and stained (c&s) specimen was prepared according to Taylor, Van Dyke (1985).Radiographs were taking using the X-ray system Faxitron LX60 DC12 at LIRP.Vertebral counts included the four vertebrae of the Weberian apparatus and the terminal centrum as a single element.The gill raker centered between ceratobranchial and epibranchial was counted as for the ventral branch.Values in parentheses indicate the number of specimens and asterisks indicate the value of the holotype.Comparative material involves analyzed specimens cited in the taxonomic revision (Silva et al., 2016).Museum abbreviations are: Instituto Nacional de Pesquisas da Amazônia, Manaus (INPA); Laboratório de Biologia e Genética de Peixes, Instituto de Biociências, Universidade Estadual Paulista, Botucatu (LBP); Laboratório de Ictiologia de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto (LIRP); and Museu de Zoologia da Universidade de São Paulo, São Paulo (MZUSP).Convex from end of occipital process to dorsal-fin origin and slightly convex along dorsal-fin base.Slightly convex from rear of dorsal-fin base to rear of adipose fin.
Color in alcohol.Overall ground coloration yellowish tan.Dorsolateral portions of head and body darkly pigmented.Dorsolateral portion of body with few chromatophores along distal margins of scales.Ventrolateral portion of body unpigmented.Two pale brown humeral spots vertically elongate and separated by one and half scales from each other.Each humeral mark covering two to three scales vertically and two or three scales horizontally.Anterior humeral mark more evident than the posterior humeral mark; anterior mark separated by one and half or two scales from posteriormost margin of opercular bone.Caudal peduncle with a ventrally-oriented patch of dark pigmentation limited to posterior portion of caudal peduncle; dark mark often quite faint.Midlateral silver stripe broad, extending from supracleithrum to caudal peduncle.Distal portions of anal, dorsal, adipose and caudal, and more lateral rays of pectoral and pelvic fins densely scattered by dark chromatophores.Unbranched rays of all fins outlined by dark chromatophores (Figs.1a, 2).
Sexual dimorphism.Secondary sexual characters were not found in any analyzed specimen of Tetragonopterus manaos.

Geographic distribution.
Tetragonopterus manaos is known from the Central Amazon in the lower sections of Tefé, Negro, Uatumã and Trombetas rivers, near the mainstream of rio Amazonas (Fig. 4).
Etymology.The specific name manaos is in reference to the Manaós, indigenous tribe that inhabited the lower rio Negro, which includes the type-locality of the new species.A noun in apposition.

Conservation status.
Given the wide-ranging occurrence area throughout Central Amazon, the good environmental conditions and the lack of significant threats along most of the sampled localities, Tetragonopterus manaos should be categorized as Least Concern (LC) under the categories and criteria of the International Union for Conservation Nature (IUCN Standards and Petitions Subcommittee, 2017).Furthermore, sampled localities are proximate to several governmental protection zones, such as the Tefé National Forest, Anavilhanas National Park, Uatumã Biological Reserve, and Saracá-Taquera National Forest, northern Brazil.

Discussion
The most recent molecular phylogeny provided support for several monophyletic groups within Tetragonopterus (Melo et al., 2016).One of the highly supported clades includes T. denticulatus sister to a subclade with T. anostomus plus T. kuluene (referred there as T. sp.Xingu), hereafter named "T.anostomus clade".This three-species clade is morphologically supported by the exclusive possession of numerous (i.e., five or more) and relatively smaller dentary teeth (Silva, Benine, 2011;Silva et al., 2013;2016).Tetragonopterus juruena from the upper rio Tapajós, absent in that phylogeny (Melo et al., 2016), is another species that has the morphological conditions of the "T.anostomus clade".These exclusive morphological features are also evident in T. manaos, which allow us to hypothesize that it represents another member of that clade.Nevertheless, interspecific relationships between T. manaos and its congeners remain undefined.
Tetragonopterus manaos possesses a vertically-oriented patch of dark pigmentation that is limited to the posterior portion of the caudal peduncle (Figs.1a, 2), often quite faint in some specimens.A similar condition is also present in T. ommatus, a species from the middle rio Tapajós (Silva et al., 2016), which is sister to T. araguaiensis and not close related to the "T.anostomus clade" (Melo et al., 2016).Therefore, the pigmentation pattern observed in T. manaos and T. ommatus is likely homoplastic.These two species are distinguished by diagnostic characters (e.g., number of maxillary teeth, number and shape of dentary teeth, and relative body depth) that would support the hypothesis that they are unrelated taxa.However, the nature of these characteristics (teeth shape and number, and caudal peduncle color pattern) is a matter of further comparative research.