A new large species of Myloplus (Characiformes, Serrasalmidae) from the Rio Madeira basin, Brazil

Abstract Myloplus zorroi sp. n. is described from the Rio Madeira Basin in Amazonia. The new species had been treated as an undescribed Tometes species because of the absence of a marked abdominal keel and few small spines forming its prepelvic serrae, features commonly found in the species of the Myleus clade of the Serrasalmidae (species of genera Myleus, Mylesinus, Ossubtus and Tometes) and also in species of Utiaritichthys. Myloplus zorroi sp. n. shares the following characters with its congeners and Utiaritichthys: molariform teeth (versus incisiform teeth in Myleus clade members); a labial row of premaxillary teeth separated from lingual row by an internal gap (versus absence of internal gap between premaxillary teeth rows); and an ascending process of premaxilla wide from its base to the tip (versus ascending process tapering from its base to the tip). Like other Myloplus species, Myloplus zorroi sp. n. differs from Utiariticthys by having a deeper body, approximately 60% of standard length (versus usually less than 50% of standard length). Considering all the morphological evidence, including the presence of 13–19 low spines forming the prepelvic serrae in Myloplus zorroi sp. n. versus more than 20 high spines forming a marked prepelvic keel in other species of Mylopus, the new species is here assigned to Myloplus. Comparisons of the new species with nominal species of Myloplus, representatives of the Myleus clade, and other related taxa are provided.


Introduction
Myloplus Gill, 1896 comprises large Serrasalmidae fishes that can reach up to 475 mm standard length . The species of this genus, commonly known as 'pacu' in Brazil and 'asitau' or 'kumaru' in French Guiana, are of high commercial value, particularly in the Amazon (Jégu 2003, Meunier et al. 2004. They inhabit slowor rapid-flowing rivers and have specialized dentition for crushing seeds (Goulding 1980, Ota et al. 2013. The Serrasalmidae members are traditionally classified according to the morphology and arrangement of teeth (Ortí et al. 2008). Géry (1972) classified the species with premaxillary teeth weakly incisiform, two rows of teeth separated by an internal gap, premaxillary labial row forming a gentle arc, and symphyseal teeth always present in the subgenus Myloplus of the genus Myleus Müller & Troschel, 1844, and recognized three species: Myleus (Myloplus) asterias (Müller & Troschel, 1844), Myleus (Myloplus) rubripinnis (Müller & Troschel, 1844), and Myleus (Myloplus) knerii (Steindachner, 1881). However, Jégu and Santos (2002), in their revision of the taxonomic status of Myleus (Myloplus) knerii, distinguished this species from the former two species in having abutting premaxillary teeth rows versus premaxillary teeth rows separated by a gap. Later, Jégu et al. (2004) elevated Myloplus to the generic level and allocated to it the seed-eating Myloplus asterias and Myloplus rubripinnis, both of which, in addition to having two rows of premaxillary teeth that are set apart from each other, have molariform teeth, whereas the other species have incisiform teeth.
From the material collected in the Rio Madeira Basin, Brazil, a previously undescribed species was identified by Camargo and Giarrizzo (2007) as a member of the genus Tometes Valenciennes, 1850, probably based on the very small prepelvic serrae of the specimens and because some of them have been collected in rapids, the preferred environment of Tometes. However, based on morphology, these specimens are assigned to Myloplus and described as a new species, thus bringing the total number of Myloplus species currently recognized to 12.

Methods
Counts and measurements were performed as described by Jégu et al. (2003). All measurements were calculated as proportions of the standard length (SL), and the subunits of the head are presented as proportions of the head length (HL). Measurements were taken with a digital caliper to the nearest 0.1 mm. The frequency of examined specimens with a particular count is provided within parentheses after the respective count, and the values for the holotype are indicated by an asterisk. Vertebrae and supraneural counts were made from radiographs of specimens MPEG 30663, INPA 48546 and ZUEC 10776. Additional description of dentition was performed from analysis of the dissected specimen ZUEC. The osteological terminology used is that proposed by Weitzman (1962). The total number of vertebrae includes those of the Weberian apparatus, counted as four elements, and the fused PU 1 +U 1 counted as a single bone.
The institutional abbreviations follow Andrade et al. (2016)    lobatus, M. schomburgkii, and M. rhomboidalis by having two rows of premaxillary teeth forming a slight arc (e.g., Fig. 3a) versus two rows of premaxillary teeth forming a shape that resembles the uppercase letter "A" (Fig. 3b) Table 1. Body laterally compressed, ovoid profile, greatest body depth at dorsal-fin origin (Fig. 1a, b). Dorsal profile of head distinctly convex from upper lip to vertical through anterior nares, nearly concave or gently straight from that point to distal margin of supraoccipital spine, and distinctly convex from that point to dorsal-fin origin. Dorsal-fin base slightly convex. Profile straight from dorsal-fin terminus to adipose-fin origin. Ventral profile of head slightly concave; ventral profile of body distinctly convex. Caudal peduncle relatively short, profile of lower caudal peduncle slightly concave. Anal-fin base distinctly convex at its basal third.
Color in alcohol. Ground coloration silver brownish to yellowish silver, with pale hues. Darker coloration on humeral region. Overall pigmentation of head above eye somewhat darker than that of adjoining areas. Body more yellowish postero-ventrally on anal-fin region. Darker blotches, irregular in size and shape, scattered on the flanks (Fig. 1a, b) mainly in males. Dorsal, anal, and caudal fins somewhat yellowish, with distal margins darker, most conspicuous on the caudal fin. Pectoral and adipose fins overall hyaline. Pelvic fin hyaline with distal margin darker. Edge of teeth brownish (Fig. 4a-c).
Color in life. Based on photos of specimens collected by sport fishermen at Rio Aripuanã, M. zorroi sp. n,. has ground coloration reddish silver, inconspicuous darker marks distributed on flanks, dorsum and head more darkened, and belly pale yellow. Dorsal, adipose, anal, and caudal fins yellowish brown.
Sexual dimorphism. The main secondary feature in mature males of Myloplus zorroi sp. n. is the additional anal-fin lobe centered on the 14 th branched ray (Fig. 1b). Darker and irregularly shaped blotches are present over flanks at maturity (Fig. 1a, b). Filamentous extensions on dorsal fin and stiff hooks laterally curved on anal fin found in species of Tometes, Myleus, Mylesinus, and other Myloplus species were not present in three examined males of Myloplus zorroi sp. n.
Distribution. Myloplus zorroi is known from Aripuanã and Roosevelt rivers, two tributaries of the Rio Madeira basin (Fig. 5). The presence of the new species within a conservation unit was confirmed from the records for Rio Roosevelt in the area of the Campos Amazônicos National Park (formerly known as: Marmelos Conservation Area BX044), located on the boundaries of the Amazonas and Rondônia States, Brazil.
Habitat. The type locality of Myloplus zorroi is a moderately to rapidly flowing, clear-water river running over rocky and sandy bottoms (Fig. 6), with a depth ranging from approximately 2 m to at the most 8 m, and a mean width of 320 m. The river is surrounded by extensive riparian vegetation that is mainly composed of ombrophilous forest and is located at an elevation of approximately 78 m above sea level. Water flow in the main channel is significantly reduced during the dry season (June-September), with most of the inflow restricted to small channels with rapids and extensive spread of rock outcrops scattered along the course of the main river. The records of Myloplus zorroi in Rio Roosevelt were collected close to the vegetated edge, while the specimens collected in Rio Aripuanã were made around the rapids of Corredeira dos Periquitos and Salto de Dardanelos.
Etymology. The specific name 'zorroi' is dedicated to Mauricio Camargo-Zorro, a researcher at the Instituto Federal de Educação, Ciência e Tecnologia, in recognition of his invaluable contribution to the fish fauna inventory from the Marmelos Conservation Area. The name 'zorroi' also alludes to the Latin-American fictional character 'Zorro', which was the secret identity of Don Diego de la Vega, because the special features "masked" this fish as Tometes, concealing its true identity.

Discussion
Myloplus zorroi differs markedly from its congeners in having a rounded abdominal region that lacks a marked keel and has a low number of small spines forming the prepelvic serrae (Fig. 2). This configuration is common to species of Utiaritichthys and the Myleus clade sensu Ortí et al. (2008), the latter mainly including representatives of the genera Myleus, Mylesinus and Tometes. The low number of small prepelvic spines in the Myleus clade was considered a derived character state by Jégu (2004: 352, character 28). The reduction of prepelvic serrae in number and size in Myloplus zorroi is most likely an autapomorphic character state amidst Myloplus species, but a phylogenetic study is needed to better characterize prepelvic features and the relationships among the species sharing them.
Myloplus zorroi was incorrectly classified as an unknown species of the genus Tometes by Camargo and Giarrizzo (2007). A more detailed analysis of its dentition and osteological features suggests that it is better classified as a species of Myloplus. The most important characters placing the species in Myloplus and not in any genera of the Myleus clade are as follows: molariform teeth (versus incisiform teeth); rows of premaxillary teeth separated by a gap (versus rows abutting); and ascending process of the premaxilla broad from its base to the tip, with a rounded edge that is strongly attached to the neurocranium (versus ascending process of the premaxilla narrowing from its base, with an acute edge that is weakly attached to the neurocranium). Goulding (1980) and Boujard et al. (1990) suggested that these features are possibly anatomical modifications allowing Myloplus specimens to crush seeds. In contrast, members of the Myleus clade with their incisiform teeth are specialized to cut leaves.
Several classifications of Myloplus have been proposed. Gill (1896) Eigenmann (1915) resurrected Myloplus, distinguishing it from Myleus by features corresponding to sexual dimorphism. Gosline (1951) considered as irrelevant the characters proposed by Norman (1929) to differentiate Myleus, Myloplus, and also Paramyloplus, and he combined all of these genera in Myleus. Gosline (1951) also considered the species Utiaritichthys sennaebragai Miranda Ribeiro, 1937, which has reduced prepelvic serrae, as a possible species of Myleus. Gosline (1951) described 11 species within Myleus, many of which were synonymized later. Géry (1972), following the classification of Gosline (1951), then proposed Prosomyleus Géry 1972 as a subgenus of Myleus and recognized four subgenera of Myleus based mainly on the arrangement and shape of the teeth: Myloplus, Myleus, Paramyloplus, and Prosomyleus. Based on molecular data, Ortí et al. (2008) indicated that Myleus sensu Géry (1972) forms a paraphyletic group and suggested that species of Myleus Müller & Troschel, 1844 are more closely related to Mylesinus and Tometes [the latter previously identified as 'N. gen. A' by Ortí et al. (1996)] than to species of Myleus sensu Géry (1972) and the subgenera Myloplus and Prosomyleus. However, a more comprehensive study involving anatomical and molecular analysis is required to further explain the relationships among these Serrasalmidae genera. Géry (1977), as well as Gosline (1951), proposed that U. sennaebragai should be treated as Myleus sensu Géry (1972) due to its reduced prepelvic serrae; however, until a study directed to solve this question is conducted, Utiaritichthys is considered distinct from Myleus. Jégu et al. (1992) redescribed the types of U. sennaebragai and showed that reports of the species made by several authors since Gosline (1951) were actually of species not belonging to Utiaritichthys. Additionally, Jégu et al. (1992) described Utiaritichthys longidorsalis Jégu, Morais & Santos, 1992 from the Rio Aripuanã, Madeira river basin. This species can be distinguished from its syntopic M. zorroi mainly by having an elongate body, body depth usually less than 50% of SL (versus deeper body, around 60% of SL in M. zorroi), 24-25 branched dorsal-fin rays (versus 20-22), and 26-30 spines forming the prepelvic serrae (versus 13-19). Utiaritichthys sennaebragai differs from M. zorroi, as well as U. longidorsalis by possessing a deeper body and by having 9-10 spines forming the prepelvic serrae (versus 13-19). Note that the counts of 9-13 prepelvic spines for U. sennaebragai observed by Gosline (1951), Géry (1979), and Pereira and Castro (2014) are actually from specimens of the newly described Tometes acylorhynchus Andrade, Jégu & Giarrizzo, 2016. In the morphological phylogeny of Jégu (2004), the two species of Utiaritichthys form a polytomy with the Myloplus clade. Pending further study, Utiaritichthys, which shares most of the diagnostic features of Myloplus such as teeth morphology and arrangement of premaxillary teeth rows, remains a separate genus.