A new species of Moenkhausia Eigenmann ( Characiformes : Characidae ) from the rio Xingu basin , Brazil

A new species of Moenkhausia from the upper rio Xingu basin is described. Moenkhausia eurystaenia is distinguished from all congeners, except M. heikoi and M. phaeonota, by the presence of a dark, broad longitudinal stripe from the opercle to the end of caudal peduncle. The new species is distinguished from M. heikoi mainly by having 18-21 branched anal-fin rays (vs. 2426) and the absence of a round blotch at the end of the caudal peduncle (vs. present). Moenkhausia eurystaenia can be distinguished from M. phaeonota mainly by the anterior portion of longitudinal band darker at horizontal septum (vs. darker below horizontal septum), the deep dark pigmentation along horizontal septum extending anteriorly up to humeral region (vs. extending up to vertical through pelvic-fin origin), number of vertebrae 34 (vs. 36-37), and others additional measurements.


Introduction
Moenkhausia Eigenmann, 1903 was redefined by Eigenmann (1917) to include species with a combination of premaxillary teeth in two rows, the inner with five teeth, complete lateral line, and partially scaled caudal fin.At the present time, Moenkhausia is represented by 70 valid species occurring in South American drainages (Lima et al., 2003;Eschmeyer, 2010).
The new Moenkhausia species described below was collected sympatrically with a similar congener, M. phaeonota Fink, in a clear water stream in the upper rio Xingu basin, Brazil.Both species share a dark broad and straight longitudinal band on body, extending to middle caudal-fin rays, with a pale longitudinal line immediately above, and concentration of dark chromatophores behind the opercle.Similar color pattern is found in other small characids such as Moenkhausia heikoi Géry & Zarske, Tucanoichthys tucano Géry & Römer, and some Hyphessobrycon Durbin in Eigenmann species.

Material and Methods
Counts and measurements were taken with digital calipers under a stereo microscope, according to Fink & Weitzman (1974), with the addition of head depth, measured at vertical through the posteriormost tip of bony opercle.Horizontal scale rows below lateral line were counted to the pelvic-fin insertion.Counts are followed by their occurrence in parentheses; asterisks indicate the count of the holotype.Measurements are given as percents of standard length (SL), except subunits of the head, which are given as percents of head length (HL).Supraneurals, gill-rakers of first arch, tooth cusps, unbranched anal-fin rays, procurrent caudal-fin ray counts and position of pterygiophores were taken from cleared and stained (c&s) specimens, prepared following Taylor & van Dyke (1985).Vertebrae count was taken from c&s and radiographs from alcohol preserved specimens.Vertebrae of the Weberian apparatus were counted as four elements and the fused PU1+U1 of the caudal region as a single element.
Jaws for Scanning Electron Micrographs (SEM) images were removed from c&s specimens, immersed in weak (less than 1%) sodium hypochlorite solution and dried by a successively more concentrated solutions of alcohol, and then acetone.Catalog numbers are followed by the number of specimens in alcohol and the SL range, the number and SL range of measured specimens in parenthesis, when different, and if any, the number of c&s specimens and their SL range.Measured specimens of Moenkhausia phaeonota comes from the lots MZUSP, 13793, 13794, 13795, 45301, 61043, and  Moenkhausia eurystaenia is distinguished from M. phaeonota (Fig. 3) by its more dispersed dark stripe, not well delimited (vs.less disperse, relatively well delimited), the anterior portion of the stripe darker at horizontal septum (vs.darker below horizontal septum), the deep dark pigmentation along horizontal septum extending anteriorly up to umeral region (vs.extending up to vertical through pelvic-fin origin), the melanophores of interradial membranae between first and fifth branched dorsal-fin rays restricted to the edge of lepidotrichia (vs.spread over interradial membranae), the distance between snout to pelvic-fin origin 46.9-50% SL, n = 35 (vs.43.2-46.6,n = 30), the distance from snout to analfin origin 62.1-65.4% SL, n = 35 (vs.55.9-61.2,n = 30), the analfin base length 25.1-28.4%SL, n = 35 (vs.28.5-33.8,n = 30), and the number of vertebrae 34, n = 13 (vs.36-37, n = 12).
Description.Morphometrics in Table 1.Largest specimen examined 36.8 mm SL.Body compressed, greatest body depth slightly ahead of vertical through dorsal-fin origin.Dorsal profile of body convex from upper lip to vertical through anterior edge of nostril; straight from latter point to tip of supraoccipital spine; convex from supraoccipital spine to dorsal-fin origin; straight to slightly convex and posteroventrally inclined along dorsal-fin base; straight to slightly convex from posterior terminus of dorsal-fin base to adipose-fin origin; slightly concave along caudal peduncle.Ventral profile of body convex from tip of lower jaw to pelvic- fin origin; straight from pelvic-fin origin to anal-fin origin; straight and posterodorsally inclined along anal-fin base and slightly concave along caudal peduncle.Jaws vertically aligned, mouth terminal.Premaxillary teeth in two rows, outer with 3*(13) or 4(22) tricuspid teeth, inner with five tri-to pentacuspid teeth.Maxilla reaching vertical through anterior fourth of orbit, with 2(14), 3*(17), or 4(1) unito tricuspid teeth.Dentary with three pentacuspid teeth, followed by smaller tricuspid and series of 5-10 small conical teeth decreasing gradually in size (Fig. 2).
Etymology.The specific name eurystaenia comes from the Greek eurys, broad, and the Latin taenia, band, a reference to the broad dark stripe.

Discussion
Several authors agree that the species currently included in the genus Moenkhausia most likely do not form a monophyletic group (Fink, 1979;Costa, 1994;Lucena & Lucena, 1999;Lima et al., 2007;Benine et al., 2009).According the most comprehensive phylogenetic analysis of Characidae to date (Mirande, 2009) Fink (1979) suggested a close relationship between Moenkhausia phaeonota and H. loretoensis, H. metae, H. peruvianus, and perhaps H. agulha and H. herbertaxelrodi.These species share a dark broad and straight stripe, which generally extends from the eye to the caudal-fin base, usually more conspicuous above the anal-fin base.These species also present a pale longitudinal line immediately above the dark stripe, and a concentration of dark chromatophores near the opercle.This color pattern is similar to the one found in Moenkhausia eurystaenia, except by the fainter overall black pigmentation, presented by the later.
), 34*(8), or 35(4), perforated scales.Longitudinal scale rows between dorsal-fin origin and lateral line 5; longitudinal scale rows between lateral line and pelvicfin origin 3. Single row of 9*(15), 10(19) or 11(1) predorsal scales.Single row of 3-5 scales overlying base of anteriormost anal-fin rays.Scale rows around caudal peduncle 14.Small scales along first third of upper caudal-fin lobe and first half of lower lobe.Vertebrae 34(13).Branchiostegal rays 4. First gill arch with 17(2) or 19(1) gill rakers: six on epibranchial, one on intermediate cartilage, eight(2) or 10(1) on ceratobranchial, and two on hypobranchial.Gill rakers without small denticles.Color in alcohol.Overall ground color yellowish.Infraorbital, opercular and gular areas silvery.Dorsal portion of head and body dark.Dark chromatophores scattered at infraorbital and opercular areas, upper and lower jaws.First three dorsal horizontal scale rows on body with slightly reticulated pattern, formed by scales bordered by dark pigment.Faint pale stripe posterodorsally from eye to caudal peduncle.A broad stripe on body, from opercle to end of caudal-peduncle, darker over horizontal septum, fading ventrally, and extending over middle caudal-fin rays as faint stripe.Smaller specimens with clearer stripe.Narrow midlateral dark stripe at horizontal septum conspicuous, formed by deeper dark chromatophores, extending from humeral region to end of caudal peduncle.Concentration of dark chromatophores immediately posterior to opercle, on first to fourth lateral scales, forming inconspicuous triangular humeral blotch.Scattered dark chromatophores above anal-fin base.All fins with dark chromatophores along edge of lepidotrichia.

Moenkhausia eurystaenia, new species Figs. 1-2 Diagnosis.
Moenkhausia eurystaenia is distinguished from all congeners, except M. heikoi and M. phaeonota, by the presence of a dark, broad stripe, from the opercle to the end of the caudal peduncle (vs.stripe, when present, narrow and frequently originating approximately at vertical through dorsal-fin origin).The new species is distinguished from M. heikoi by having 18-21 branched anal-fin rays (vs.24-26), by the absence of a round blotch at the end of the caudal peduncle (vs.blotch present), and by the dark stripe formed by relatively small chromatophores (vs.large chromatophores).