Redescription of Moenkhausia megalops ( Eigenmann , 1907 ) , a widespread tetra from the Amazon basin ( Characiformes , Characidae )

Moenkhausia currently comprises 90 valid species and represents one of the very species-rich genera in Characidae. Most of these species need to be revised since their original descriptions are out-of-date and are uninformative. In this sense, Moenkhausia megalops is redescribed based on the examination of the holotype and additional specimens from drainages of the Amazon basin and the type locality. The species is distinguished from its congeners by the combination of the following characters: a conspicuous bootshaped humeral spot, ventrally curved to anterior region of the body, followed by a second inconspicuous vertically elongated humeral spot; a dark longitudinal midlateral stripe on body extending from immediately posterior to the second humeral spot to the middle caudal-fin rays; a dark blotch on the upper caudal-fin lobe; and teeth with seven cusps on inner series of premaxilla and dentary.


Introduction
Moenkhausia (Eigenmann, 1903) represents one of the very species-rich genera in Characidae, currently comprising 90 valid species (Eschmeyer et al. 2017).The genus is widely distributed in the Neotropical cis-Andean river basins, notably occurring in the Amazon basin (e.g., a polyphyletic group (Mirande 2010, Oliveira et al. 2011, Mariguela et al. 2013).Nevertheless, Moenkhausia is still diagnosed by the same combination of characters as proposed by Eigenmann (1917): premaxillary teeth in two series, with five multicuspid teeth in the inner series; caudal-fin covered by small scales; and complete lateral line.
Eigenmann, in Eigenmann and Ogle (1907) (hereafter Eigenmann, 1907), proposed the name Astyanax megalops based on a single specimen from Itaituba, Pará, Brazil.Later, Eigenmann (1910) listed the species in the genus Moenkhausia and expanded its geographical distribution from Gurupá to Itaituba, and Guyana.
The short and uninformative original description of Astyanax megalops Eigenmann (1907) allied to the poor condition of its preserved holotype has hindered a satisfactory understanding and definition of the species, currently valid as Moenkhausia megalops.The more detailed redescription provided by Eigenmann (1917) is still general and with no definitive diagnostic character or even a workable character combination.These ineffective descriptions allied to Géry's (1977) popular and straightforward identification key are possibly leading to inconsistent identifications throughout fish collections and inventorial publications (e.g., Mojica et al. 2005, Castellanos and Sánchez-Duarte 2007, and Lima et al. 2013).Géry and Zarske (2004: 42), in a brief discussion, stated that this is "a rare species" from the rio Tapajós basin and Guyana, and did not provide new features for its identification.Recently, Oliveira and Marinho (2016) did shed some light on the identity of M. megalops when distinguishing it from their new species, Moenkhausia abyss, by the shape of the humeral mark and by the pigmentation of the longitudinal midlateral stripe.
During an extensive analysis of lots of Moenkhausia deposited in the fish collection of the INPA (Instituto Nacional de Pesquisas da Amazônia), we recognize specimens of Moenkhausia megalops identified with different names, and here these are properly re-identified.Thus, we herein present a redescription of M. megalops, based on the examination of the holotype and additional material, including topotypes, and discuss our arguments concerning its taxonomic definition.

Material and methods
Counts and measurements were taken as described by Fink and Weitzman (1974) and Benine et al. (2015) on the left side of specimens whenever possible using a digital caliper with precision of 0.01 mm.Measurements are given as percents of standard length (SL), except subunits of the head, which are given as percents of head length (HL).Vertebral, supraneurals and procurrent caudal-fin ray counts were taken from cleared and stained specimens (c&s) prepared following Taylor and Van Dyke (1985).Vertebral counts include Weberian apparatus, counted as four elements, and the fused PU1+U1 of the caudal region counted as a single vertebral element.The gill-raker at the junction of the ceratobranchial and the epibranchial is included in the counting of gill-rakers of the lower limb.Scanning electron micrography (SEM) of teeth and jaws was taken from cleared and stained dissected specimens.The specimens were sexed by the visual examination of their gonads under a stereomicroscope since no externally dimorphic feature was observed.Counts and measurements from examined material were given whenever possible.In the description, counts are followed by the number of specimens examined in parentheses; asterisk indicates the counts of the holotype; unbranched rays are presented in lower-case Roman numerals.In the examined material account, the total number of specimens of each lot is listed first, followed by the number of analyzed specimens in parentheses (when different from total number of the lot), and then by those c&s.Specimens examined are deposited in the CAS, California Academy of Sciences, San Francisco; INPA, Instituto Nacional de Pesquisas da Amazônia, Manaus; and GEA, Laboratório de Ictiologia do Grupo de Ecologia Aquática, Universidade Federal do Pará, Belém.Ferreira 1993: 69 [list].Maldonado-Ocampo 2001: 65 [list].Lima et al. 2003: 148 [list].?Arbeláez et al. 2004: 102 [list;new record].?Camargo et al. 2004: 134 [list].Géry and Zarske 2004: 42 [compared to M. xinguensis; placed in M. xinguensis group; identification key].?Lasso et al. 2004: 115 [list].?Ferreira et al. 2007: 180 [list].?Maldonado-Ocampo et al. 2008: 171 [list].?Hercos et al. 2009: 48 [list].?Miller-Hurtado et al. 2009: 167 [list;new record].?Ferreira et al. 2011: 280 [list].Description.Morphometric data presented in Table 1.
Dorsal-fin rays ii(60) 8(1) or 9*(59).Dorsal-fin origin slightly anterior to middle of body.First unbranched ray approximately one-half length of second unbranched ray.Adipose fin present.Pectoral-fin rays i(61), 11(6), 12*(29), 13(24) or 14(2); tip of adducted longest ray generally not reaching vertical through pelvic-fin origin.Pelvic-fin rays i(58), 6(2), 7*(55) or 8(1).Pelvic-fin origin slightly anterior to vertical through dorsal-fin origin; tip of adducted longest ray generally not reaching vertical through anal-fin origin.Anal-fin rays iii(34) or iv(19), 25( 5  Color in alcohol.General ground color pale yellow.Body dorsal surface with dark chromatophores from tip of snout to caudal peduncle, resulting in a dark dorsal stripe on the dorsum.Dorsolateral portion of head and body with scattered dark chromatophores.Two vertically elongated humeral spots; the first humeral spot very conspicuous, boot-shaped, located over third to fourth lateral line scales and extending over two horizontal series of scales above lateral line; the second humeral spot, less conspicuous, located over seventh to eighth lateral line scales and extending over two horizontal series of scales above lateral line.Broad band from the second humeral spot to middle caudal-fin rays, covering two series of scales above lateral line.Abdominal region almost devoid of chromatophores.Dorsal fin with scattered dark chromatophores.Adipose fin with scattered dark chromatophores sometimes concentrated on the edge of the fin.Pectoral and pelvic fin hyaline.Anal fin with a dark band in the mid-basal region, not reaching tip of the rays.Posterior margin of upper caudal-fin lobe with a dark blotch, reaching tip of middle caudal-fin rays or, sometimes, reaching the lower caudal-fin lobe.

Color in life.
Overall body color silvery on flanks.Dorsal and lateral surface of head, dorsolateral portions of  Sexual dimorphism.None of the adult specimens examined had hooks on fins or any other apparent sexually dimorphic features.

Material
Another character not mentioned by Eigenmann (1907) is the shape of premaxillary and dentary teeth.Indeed, one strong evidence to associate the name M. megalops to this specific form is that the holotype of Moenkhausia mega- lops has premaxillary and dentary teeth with seven cusps.In addition, another evidence to link the specimens examined to the name M. megalops is that this is the unique form in the rio Tapajós with large eyes (42.6-50.8mm HL), a broad silvery lateral stripe, and scales with noncurved radii, as seen in the holotype of M. megalops.For these reasons, we are convinced that this population must be linked to the name M. megalops, confirming previous identifications [e.g., Souza et al. (2012), and Oliveira and Marinho (2016)].
Among the congener species, Moenkhausia grandisquamis, another species quite abundant in the rio Tapajós basin with a conspicuous silvery lateral band, also has premaxillary and dentary teeth with or eight cusps (see Azevedo-Santos and Benine 2016).Eigenmann (1917) included a form from Itaituba identified by Ulrey (1895) as Tetragonopterus grandisquamis in the synonymy of M. megalops and distinguished the true M. grandisquamis (Müller & Troschel, 1845) from M. megalops by differences in morphometric proportions and scale striae.We partially agree with Eigenmann, given that M. megalops, in fact, does not present scales with curved radii, as those in M. grandisquamis, but we observed considerable morphometric overlapping between these species.However, our comparative examinations evidenced that M. megalops has a narrower, somewhat rectangular second infraorbital versus a wider and trapezoidal bone in M. grandisquamis.Eigenmann (1917: 92) pointed out that the specimen of M. megalops "from the Essequibo 2488C is deeper and has a narrower second suborbital than the type".The examinations of the holotype (CAS 71433) and the specimen from the Essequibo (now FMNH 53966 -photograph) confirm the deeper body of the latter, even though it clearly has a wider second infraorbital that fits to M. grandisquamis, contrary to Eigenmann's statement.For these reasons, we treated Eigenmann's (1910;1917) material as in part synonyms.Hardman et al. (2002) listed Moenkhausia megalops to Guyana, although solely based on Eigenmann's material, since this species was not collected in their comparative survey.Watkins et al. 2004 listed M. megalops for both Essequibo and Burro-Burro river drainages in Guyana.We examined material from Essequibo (ANSP 175609) and Burro-Burro (ANSP 176990 -photograph) and confirmed both as M. grandisquamis.We also had the opportunity to examine images of the material listed as M. megalops by Souza et al., 2012 and confirm this identification.These authors tested the influence of the Rupununi Portal on freshwater fish distribution on Rupununi District, Guyana, and their results showed that M. megalops is restricted to rio Takutu, Rio Branco drainage.Thus, we still do not have evidence of occurrence of M. megalops in the Essequibo river, Guyana.
Although we were not able to confirm these informations, there are also records of occurrence of M. megalops for other rivers in the Amazon basin that were not included in our Geographic distribution section, such as rio Purus (Duarte et al. 2013), and from Colombia (Maldonado-Ocampo 2001, Arbeláez et al. 2004, Maldonado-Ocampo et al. 2008;Miller-Hurtado et al. 2009), and Venezuela (Lasso et al. 2004, Gonzáles et al. 2012).However, we analyzed photographs of specimens from Rio Orinoco, Venezuela (ANSP 190778;ANSP 191245) that presents the diagnostic humeral and caudal marks of M. megalops and could be tentatively identified as this species.We were not able to examine teeth and scale morphology.Nevertheless, we herein provide sufficient elements for the unequivocal differentiation of M. megalops from congeners, which subsequent inventorial and revisionary works may count on.

Figure 5 .
Figure 5. Distribution of Moenkhausia megalops.Circles represent lots examined; star indicates type locality in the rio Tapajós basin.