A new Silver Dollar species of Metynnis Cope, 1878 (Characiformes: Serrasalmidae) from Northwestern Brazil and Southern Venezuela

A new Metynnis is described from the rio Negro in Brazil and Venezuela, and from blackor clearwater tributaries in Brazil including the rios Parauari, Uatumã, Trombetas, and Sucunduri (the latter belonging to the rio Madeira basin). The new species can be distinguished readily from all congeners by having a high concentration of dark chromatophores on the lateral line scales. It can be further distinguished by the combination of head length 24.3-27.5% of SL, 13-18 gill-rakers on upper limb and 16-24 gill-rakers on lower limb. The new species is most similar to and likely most closely related to Metynnis hypsauchen. These two species share a similar color pattern, body shape and sexual dimorphism of the anal fin. However, they differ in that M. hypsauchen has a lightly pigmented lateral line. The new species is also distinguished from M. hypsauchen by having 56-65 predorsal scales and 90-104 lateral line scales (vs. 36-54, and 65-82, respectively). A detailed osteological description of the new species is provided.


Introduction
Metynnis Cope, 1878, is one of the most species-rich genera within Serrasalmidae and currently includes 15 valid species. The genus is widely distributed in cis-andean South America, including the Amazon, Orinoco, Paraguay, Paraná and São Francisco river basins, Northeastern Guiana Shield rivers, and coastal rivers from Northeastern Brazil (Zarske & Géry, 1999Jégu, 2003;Pavanelli et al., 2009;Ota, 2015). The taxonomy of the genus is still confused and impeded by high ontogenetic variability and sexual dimorphism that includes the presence versus absence of the anal-fin lobe, the elongation of the first dorsal-fin rays, and variation in body coloration (Zarske & Géry, 1999;Pavanelli et al., 2009;Ota et al., 2013).
The authors of previous taxonomic reviews involving Metynnis disagree on the number of valid species. Ahl (1923) described nine new species of Metynnis and redescribed others, recognizing a total of 18 species within the genus. Later, Gosline (1951), emphasizing similarities in body shape, counts and measurements among museum specimens, reduced the diversity of Metynnis to only six species. Zarske & Géry (1999), in the most comprehensive study of this genus, reviewed the type series of all nominal Metynnis taxa and recognized 11 species. Subsequently, Zarske & Géry (2008) described M. longipinnis Zarske & Géry, 2008 and M. polystictus Géry, 2008 andconfirmed M. goeldii Eigenmann, 1903 as a junior synonym of M. lippincottianus (Cope, 1870).
In spite of these taxonomic uncertainties, the monophyly of the genus is well accepted and has been corroborated by various authors in both morphological (Machado-Allison, 1982, 1983Jégu, 2004) and molecular phylogenetic analyses (Ortí et al., 1996(Ortí et al., , 2008Calcagnotto et al., 2005;Thompson et al., 2014), with the exception of Cione et al. (2009), which recovered a paraphyletic Metynnis. Species of Metynnis can be easily recognized within Serrasalmidae by having a long adipose-fin base, generally exceeding the distance between the dorsal-fin insertion and adipose-fin origin (Cope, 1878;Eigenmann, 1915;Jégu, 2003). Although representatives of the genus possess two rows of premaxillary teeth, like the herbivorous pacus (Machado-Allison, 1983;Ota et al., 2013), Metynnis has been recovered as being more closely related to piranhas, which are mainly characterized by a single row of premaxillary teeth (Machado-Allison, 1983;Ortí et al., 1996Ortí et al., , 2008Cione et al., 2009).
In the present paper, we describe a new species previously reported as Metynnis hypsauchen (Müller & Troschel, 1844), due to similarities in color pattern, body shape and sexual dimorphism of the anal fin. The new species is widespread throughout rivers draining the Guiana Shield, including the rio Negro in Brazil and Venezuela, the Uatumã and Trombetas river basins in Brazil. It also occurs inthe northern edge of Brazilian Shield in the Parauari and Sucunduri rivers in Brazil. All of these are black-or clear-water systems.
Description. Morphometric data summarized in Table 1. Body extremely compressed, overall aspect of body elliptical, body deepest at vertical through dorsal-fin origin. Predorsal and postdorsal origin equal in length. Dorsal profile of head convex from mouth to vertical through anterior portion of eye, slightly concave from latter point to base of supraoccipital process, and convex from that point to dorsal-fin origin. Dorsal body profile very steep from snout to dorsal-fin origin and with strong posteroventral slope after this point. Dorsal-fin base straight. Body profile almost straight from dorsal-fin insertion to adipose-fin origin. Adipose-fin base gently convex. Head short and oval with ventral half of opercle approximately twice as large as dorsal half, with posterior margin surrounded by broad opercular membrane, covering up to four scales horizontally at point of greatest head length.
Snout slightly rounded. Mouth terminal with molariform teeth; jaws isognathous. Premaxillary teeth in labial row contacting teeth in lingual row. Five teeth in labial row and two in lingual row (Figs. 3a,b). Premaxillary teeth 1-3 in labial row with slightly sharp edges; teeth 1 and 2 separated laterally (Fig. 3a). Dentary with four teeth decreasing in size posteriorly (Fig. 3c); pair of symphyseal dentary teeth always present behind main series of teeth (Fig. 3d). Maxillary edentulous.
-Olfactory region: Mesethmoid narrow, pointed and triangular anteriorly in dorsal view, dorsal profile convex in lateral view (Figs. 4a,b). Mesethmoid with narrow ventral process articulated to anterior region of vomer (Fig. 4a). Lateral wings of mesethmoid elongate, positioned on anterior half of bone. Lateral ethmoid elongate, dorsal portion reaching anteroventral surface of frontal, and lateral wing thin, pointed distally, and ventrally directed. Vomer excluded from limits of olfactory fossa, marked by longitudinal granulation on distal portion in ventral view (Fig. 4c). Nasal narrow, elongated, with well-defined canal (Fig. 4b).

Color in alcohol.
Overall body coloration light-brown to yellow, head and body darker dorsally. Scales of lateral line with dark chromatophores, forming thin dark stripe along lateral line. Flanks with inconspicuous rounded or vertically elongate dark blotches on middle portion of body and ventral to lateral line. Males with small dark spots scattered irregularly over the body, but most concentrated above the lateral line and below the lateral line on the anterior body (Fig. 1a). Chromatophores can form a large irregular blotch over the supracleithrum, not surpassing posteriorly Color in life. Based on observations of freshly collected specimens, overall body color silvery. Except for pectoral fin, all fins light yellow in base coloration. Anterior rays of anal-and pelvic-fins orange to an intense red. During breeding period, males with intense vertically elongate and irregularly-shaped blood-red blotch near supracleithrum, situated between lateral line and the region just dorsal to prepelvic spines. Conspicuous red dots occasionally present over anterodorsal region of body, extending posteriorly to vertical through dorsal-fin insertion (Fig. 7). Sexual dimorphism. Metynnis melanogrammus presents the same sexual dimorphism illustrated by Zarske & Géry (1999: 173) for M. hypsauchen and also recorded for M. guaporensis Eigenmann, 1915by Ota et al. (2013. This dimorphism involves the presence in mature males of an anteriorly rounded lobe formed by an extention of the tenth to thirteenth branched rays of the anal-fin. This lobe has a concavity in the fin margin after the first two branched rays. With the exception of M. fasciatus, M. guaporensis, M. hypsauchen, M. longipinnis, M. luna, and M. melanogrammus, the males of remaining congeners possesses a continuous anterior lobe lacking the concavity (Ota, 2015). The anal fins of females and juveniles have only the anterior unbranched rays elongate, conferring a characteristic falcate feature to this fin.
Mature males also have more intense body coloration than females, marked by a blood-red, vertically elongated blotch on the supracleithrum region, and dark spots scattered across the body (see Color in alcohol and Color in life). Females only possess notable coloration on the anterior anal-fin rays, which are orange to light red. Prolongation of anterior dorsal-fin rays, which is very common in other Metynnis species (e.g., Metynnis cuiaba Pavanelli et al., 2009) was not observed either in males or females of M. melanogrammus.

Distribution.
Metynnis melanogrammus is known from the rio Negro basin in Brazil and Venezuela, the Uatumã and Trombetas rivers, both of which are left-bank tributaries of the rio Amazonas, and from the rio Parauari, a right-bank tributary of the rio Amazonas. Zeinad & Prado (2012) recorded the new species (as Metynnis cf. hypsauchen) at lago Aracu, rio Sucunduri (c. 5°35'59.83"S 59°33'41.80"W, November 2009), a right-bank tributary of rio Madeira basin in Amazonas State, Brazil (Fig. 8). Despite a recent thorough survey of Serrasalmidae in the rio Madeira basin in Brazil (Ota et al., 2013), no preserved specimens of the new species from that basin are known. The red star represents the type locality, red circles represent the paratype localities, and the black circle represents the locality on the rio Surunduri at which the live photograph of the new species (Fig. 7) was taken.
Ecological notes. Specimens of Metynnis melanogrammus are primarily found in lakes and rivers with dark-stained waters, such as the rio Negro in Brazil and Venezuela, and the rio Uatumã in Brazil. The new species has also been collected in clear water lakes in the Parauari (type locality), Trombetas and Sucunduri rivers, but is not known from white-water systems.
Examination of a dissected female (INPA 18657, 169.2 mm SL) collected in December revealed the possession of small, numerous, yellowish oocytes, typical of mature female during reproduction. An image of a freshly collected Metynnis melanogrammus (Fig. 7) taken in November, shows the typical breeding coloration of the species.
Etymology. From the Greak melas (black or dark) and gramma (letter), referring to the well-marked dark lateral line present in the new species. An adjective in the nominative singular.

Conservation status.
Metynnis melanogrammus is relatively common in the rio Negro basin, and is widely distributed along rivers from Northwestern South America in Brazil and Venezuela, with an Extent of Ocurrence (EOO) of approximately 522.458 km 2 . No specific threats were detected. Accordingly, we suggest that the new species be categorized as Least Concern (LC) according to IUCN criteria (IUCN, 2014).

Remarks. The new species described herein is similar to
Metynnis hypsauchen in body shape, sexual dimorphism of the anal fin, and color pattern. Not surprisingly, analyzed material deposited at INPA and MCNG was misidentified as M. hypsauchen, and the species was recorded in the literature (Zeinad & Prado, 2012) as Metynnis cf. hysauchen. The first obvious feature used to recognize specimens described herein as M. melanogrammus as a potentially new species similar to M. hypsauchen, is the presence of a markedly dark lateral line, a feature that is absent in M. hypsauchen. In addition, Metynnis melanogrammus possesses distinctly smaller scales than M. hypsauchen which is reflected by higher scale counts, i.e., predorsal (56-65 vs. 36-54), rows above and below lateral line (52-66, vs. 35-55), between adipose-fin origin and lateral line (30-35 vs. 19-25), and total perforated scales of the lateral line (90-107 vs. 65-82). Ahl (1923) and Gosline (1951) regarded M. hypsauchen as the most common and widespread species of the genus. Indeed, Metynnis hypsauchen is widespread throughout the Amazon River system, in the Río Orinoco basin, and in some Guyanese rivers (e.g., Machado-Allison & Fink, 1995;Jégu, 2003;Ota et al., 2013). This species occcurs in sympatry with Metynnis melanogrammus in the rio Negro basin of Brazil and Venezuela, and also in the rio Uatumã and rio Trombetas basins in Brazil. The new species is known from the rio Madeira basin solely by a photograph of a single specimen from the rio Surunduri (Fig. 7). There are no confirmed specimen records of M. hypsauchen in rio Madeira tributaries (Ota et al., 2013). The record of M. hypsauchen at the Aripuanã and Madeira rivers by Rapp Py-Daniel et al. (2007) was based on misidentifications of specimens of M. guaporensis and M. luna (R. P. Ota, pers. obs.).

Discussion
Taxonomic and phylogenetic relationships. Taxonomic confusion is rife in the literature dealing with Metynnis species. There are 28 nominal species and only around half of them are recognized as valid. The elevated number of synonyms is probably a consequence of the enormous variability of body shape and color pattern, which in turn are highly influenced by ontogeny and sexual dimorphism (Zarske & Géry, 1999;Jégu, 2003;Pavanelli et al., 2009;Ota et al., 2013). As a consequence, there is little information about the phylogenetic relationships among species of Metynnis. Therefore, along with the taxonomic description of Metynnis melanogrammus, its osteological description was presently provided to contribute to the knowledge on the evolutionary relationships of Metynnis. Some exclusive synapomorphies of the genus, previously highlighted by Machado-Allison (1982), were observed in M. melanogrammus. For example, the presence of intercalar reduced (Fig. 4c) forming the posterolateral corner of the neurocranium (Machado-Allison, 1982: 208, character 181); and predorsal spine type II (Fig. 6a), rounded in shape, with its dorsal surface serrated and anteroventral cavity reduced or even absent (Machado-Allison, 1982: 206, character 176).
Phylogenetic relationships of the genus are currently under study by the first author (RPO). According to Ota's (2015)  melanogrammus and an undescribed species. The latter clade is supported by eleven synapomorphies, including the presence of a concavity on anterior lobe of males anal-fin (character 30: 325) (Fig. 1a), and 26 to 40 total number of gill-rakers (character 44: 334).
Inside this clade, M. melanogrammus is most closely related to M. fasciatus, M. hypsauchen, M. longipinnis, and Metynnis sp. based on eight synapomorphies including the long and strongly downturned parasphenoid (character 142: 399) (Fig. 4a), and the lack of fusion of hypurals 1 and 2 (character 187: 423) (Fig. 6c). Metynnis fasciatus was recovered as sister to a polytomy composed of M. hypsauchen, M. longipinnis, M. melanogrammus and Metynnis sp. on the basis of five synapomorphies, including the narrow, laminar, and semicircular opercle (character 109: 373) (Fig. 5b), and the solid epiotic (character 173: 416) (Fig. 4a). Finally, M. melanogrammus was recovered as more closely related to Metynnis sp. than to M. hypsauchen, with that placement supported by five synapomorphies, including a unique shape of ectopterygoid among all congeners, which is short, narrow and oblique (character 119: 383) (Fig. 5b). In M. hypsauchen and most other Metynnis species, that bone is well-developed. However, these relationships are still under study, and represent a first hypothesis of relationhips among the Metynnis species.
In turn, the narrow and elongated form of the hyomandibular with a ventrally directed triangular projection on the anterior margin of the plate (Fig. 5b) is autapomorphic for M. melanogrammus. Machado-Allison (1982, plate XXXI, Ic) pointed out this feature and showed a drawing with a similar condition in a specimen assigned by that author to M. mola, a species endemic to the Paraguay River basin. However, this ventrally directed projection was not found in any specimen of M. mola examined in the phylogenetic analysis. Therefore we atribute this inconsistency to a possible misidentification of the specimen utilized by Machado-Allison (1982), who might have examined a specimen of M. melanogrammus instead. This species occurs in the rio Negro within Venezuela, where most of the samples used by that author originated.
Distribution pattern. The new species possesses an unusual distribution throughout blackwater rivers as the rio Negro and rio Uatumã, as well as clearwater rivers such as the Parauari, Trombetas and Sucunduri rivers. With the exception of the Parauari and Sucunduri rivers, the remaining rivers drain the Guiana Shield. The occurrences of Metynnis melanogrammus in both right-and left-margin tributaries of the Amazon deserve consideration. The species presents a wide distribution throughout the rio Negro basin (from above the Río Casiquiare in Venezuela to the rio Padauari, and lower rio Negro basin), and is also recorded from the rio Branco drainage and from rio Uatumã and rio Trombetas. This distribution clearly demonstrates broad historical connectivity among the drainages of the Guiana Shield, such as the rivers Orinoco, Negro and Branco. On the other hand, there are few records of M. melanogrammus from Amazon right margin tributaries, and specimens from both river margins are morphologically undistinguishable. Therefore, Metynnis melanogrammus could be considered to have a central Amazonian distribution pattern (F.C.T. Lima, pers. comm.) that also occur in black-and clearwater tributaries but not in milky, sediment-rich rivers, such as the Amazon or Madeira mainstems. This pattern suggests that sediment-loaded waters might work as a current barrier to M. melanogrammus distribution.