Hemigrammus ataktos : a new species from the rio Tocantins basin , central Brazil ( Characiformes : Characidae )

A new species of Hemigrammus is described from the middle rio Tocantins basin, central Brazil. The new species can be distinguished from all congeners by having a black midlateral stripe on the body extending from the posterior margin of the eye to the median caudal-fin rays. Mature males possess dorsal-, pelvic-, and anal-fin rays elongate, features that also help to recognize the new species. Although the new species is described in Hemigrammus, some specimens present a complete series of pored scales in the lateral line. A discussion about the generic allocation of the new species is presented.


Introduction
The Characiformes is one of the largest orders of fishes with approximately 2000 valid species exclusively distributed in Africa, South of North America, Central, and South America (Eschmeyer, 2013).The Characidae itself encompasses slightly more than half of the number of species in the order, and includes mostly small-sized species generally referred to as "tetras", many of which just recently described (e.g., Ingenito et al., 2013;Mattox et al., 2013) or still waiting to be discovered.
During a recent expedition to upper and middle portions of the rio Tocantins, a new species of Hemigrammus Gill, 1858 was discovered.The examination of representatives of Characidae deposited in collections (CAS, MZUSP, and ZUEC) evidenced that this species had been previously collected several times but remained unknown for nearly ninety years.The new species is similar in coloration to a group of species of Hyphessobrycon Durbin, 1908 referred by Géry (1977) as the Hyphessobrycon heterorhabdus group, characterized by having a dark straight and relatively narrow midlateral stripe on the body, from the opercle to the caudal peduncle.However, the new species possesses small scales largely covering the caudal-fin lobes, which are absent in all species of Hyphessobrycon.The presence of scales covering the caudal-fin lobes, however, is one of the features that traditionally define the genera Hemigrammus and Moenkhausia Eigenmann, 1903(e.g., Eigenmann, 1917;Géry, 1977).Furthermore, the new species presents variable lateral-line perforation, an unusual or poorly documented condition among the species of Characidae.
Based on its unique combination of morphological features, the new species is herein formally described and diagnosed from all other species of the family.Its generic assignment is discussed according to the current knowledge of species of the Characidae.

Material and Methods
Counts and measurements follow 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 pelvicfin insertion, not including the small scale at pelvic-fin insertion, which may be absent.Counts are followed by their absolute frequency of 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.Supraneurals, branchiostegal, gill-rakers of first branchial 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 according to Taylor & Van Dyke (1985).Vertebrae of the Weberian apparatus were counted as four elements and the fused PU1+U1 of the caudal region as a single element.Color in life was based on Fig. 1a.Catalog numbers are followed by the number of specimens in alcohol and the SL range, and if any, the number of c&s specimens and their SL range.Males and females or immature were sexed based on the elongation of the fins.The positive correlation between sex and fin elongation was confirmed on four specimens, with examination of the gonads under a stereomicroscope.Data from species of Hemigrammus, Hyphessobrycon, and Moenkhausia were taken directly from the specimens (see Comparative Material), or from original descriptions and redescriptions: Géry (1977) (for Hyphessobrycon scholzei), Zarske & Géry (2004)   New Hemigrammus from the rio Tocantins basin 260 raker on intermediate cartilage, and 5(2), 6(8), or 7(1) rakers on epibranchial.Gill rakers with small spines along its length.
Color in alcohol.Overall ground color light tan.Infraorbital, opercular and gular areas with guanine (Fig. 1b, c).Dorsal portion of head dark.Snout, maxilla, and lower jaw scattered with dark chromatophores.Dorsalmost three horizontal scale rows on body with slightly reticulated pattern, formed by dark pigment on middle portion of exposed area of scales.Deep black midlateral stripe on body, extending from posterior margin of eye to median caudal-fin rays.Stripe one-and-ahalf scales deep.Narrow longitudinal dark line at horizontal septum, formed by embedded dark chromatophores, extending approximately from vertical through dorsal-fin origin to end of caudal peduncle.Scattered dark chromatophores above anal-fin base.All fins with dark chromatophores along edge of lepidotrichia.Distal margin of third to fifth branched dorsal fin-rays and smallest branched anal-fin rays dark.1. Smallsized species, largest examined specimen 37.7 mm SL.Body compressed, moderately elongate.Greatest body depth slightly anterior to dorsal-fin origin.Dorsal profile of head convex from upper lip to vertical through anterior nostril; straight to slightly convex from that point to tip of supraoccipital spine.Dorsal profile of body slightly convex along predorsal region, straight and posteroventrally inclined along dorsal-fin base, straight to slightly convex from terminus of dorsal-fin base to adipose-fin origin, and concave along caudal peduncle.Ventral profile of head and body straight to slightly convex from tip of lower jaw to pectoral-fin origin, convex from that point to anal-fin origin, straight and posterodorsally inclined along anal-fin base, and concave along caudal peduncle.
Remarks.Specimens from CAS collection were designated as non-type material due to their poor condition.

Discussion
The traditional classification of the Characidae, established by Eigenmann (1917) and later followed by Géry (1977), greatly influenced the current classification of both subfamilial and generic levels.Several genera of the Characidae were proposed based on combination of morphological characters, Color in life.Dorsal portion of body yellowish tan above black midlateral stripe (Fig. 1a).Opercular area and ventral portion of body below black midlateral stripe with guanine.Narrow bright yellow line above black midlateral stripe.Black midlateral stripe over middle portion of eye.Dorsal portion of eye red, ventral portion silver.Tip of dorsal, pelvic, and anal fins creamy white.Adipose fin, most of dorsal and pelvic fins, and proximal portion of anteriormost rays of anal fin yellow.Proximal two-thirds of caudal-fin lobes red on mature males (no information available for females and juveniles).
Sexual dimorphism.Mature males with dorsal (31.6-44.9% of SL), pelvic (18.8-23.8% of SL) and anal fin (23.1-27.3% of SL) longer than females or immatures (28.1-32.9%;15.8-20.3%;19.5-24.4%,respectively) (Table 1, Fig. 3).Tip of pelvic fin extending from base of first to fifth branched analfin ray in mature males and not reaching anal fin or extending to base of first branched anal-fin ray in females.Bony hooks were not observed on fins of any analyzed specimen.

Geographic distribution.
Hemigrammus ataktos is known from middle rio Tocantins basin, from rio Santa Tereza, Goiás State, from the rio Manoel Alves basin, and from smaller tributaries of the rio Tocantins immediately downstream of the mouth of rio Manoel Alves, Tocantins State, Brazil (Fig. 4).
Etymology.From the Greek ataktos, meaning disordered or irregular, in allusion to the variation in the perforation of lateral-line scales present in the new species.An adjective.discussed in Eigenmann (1917)] and Odontostilbe dialeptura Fink & Weitzman, 1974.Lateral line reduction is hypothesized to have been independently acquired within several lineages in Characidae (Mirande, 2010), probably as a result of independent events of miniaturization, which may advance the presence of paedomorphic characters (Buckup 1998;Weitzman & Fink, 1983;Weitzman & Vari, 1988).Variation of lateral line perforation within a species may represent a key to understand mechanisms of truncation of the laterosensory canal system on body.
According to Mirande (2010), Hemigrammus, Hyphessobrycon, and Moenkhausia are non-monophyletic including characteristics now known to have independently evolved within different lineages of the family (e.g., length of poring on lateral line, tooth-cusp morphology, caudal-fin squamation).Although this procedure was in general effective for alpha taxonomy, it resulted in an artificial delimitation of many non-monophyletic groups of species.This is the case for the most diverse characid genera, such as Hemigrammus (54 species), Hyphessobrycon (130 species) and Moenkhausia (76 species) (Weitzman & Fink, 1983;Lima et al., 2003;Eschmeyer, 2013).According to this classification, which is still being used to classify species (e.g., Carvalho et al., 2010;Marinho, 2010;Sousa et al., 2010;Bertaco et al., 2011aBertaco et al., , 2011b;;Ingenito et al., 2013), the genus Hemigrammus is diagnosed from Hyphessobrycon uniquely by the presence of small scales covering the proximal portion of the caudal-fin lobes (vs.absence), and from Moenkhausia by having an incompletely pored lateral line (vs.completely pored lateral line).Based on this system, part of the specimens of the new species would fit into Hemigrammus, whereas part would be best identified as Moenkhausia.Of a hundred examined specimens, 65 have incomplete, 31 have discontinuous, and 4 have complete lateral line (see Description).Variation of lateral line perforation within a species was also described for other small characids, including Hemigrammus barrigonae Eigenmann & Henn, 1914, Moenkhausia celibela Marinho & Langeani, 2010, Moenkhausia cotinho Eigenmann, 1908 [see also Eigenmann (1918) for more information on lateral line scale counts for M. cotinho], M. sanctaefilomenae (Steindachner, 1907) [lateral line scale counts of M. sanctaefilomenae   and H. luetkenii which were hypothesized to be more closely related to species of Astyanax).The unique synapomorphy proposed by Mirande (2010) for Tetragonopterinae, the presence of two pairs of uroneurals (character 306), is not shared by the new species, which has only one pair.However, several other species nested in Tetragonopterinae also has only one pair (e.g., clade 288 of Mirande, 2010).Although Hemigrammus ataktos probably belongs to this subfamily, a phylogenetic analysis is necessary to confirm this hypothesis.
A cladistic-based classification of Hemigrammus, Hyphessobrycon (Weitzman & Palmer, 1997;Weitzman & Malabarba, 1998), and Moenkhausia (Mirande, 2010) will probably result in generic reallocation of many species currently assigned to these genera.Pending further studies on the relationships of these genera, we tentatively assign the new species to Hemigrammus rather than Moenkhausia, because most of the examined specimens fit the traditional definition of the former genus.The new species can be distinguished from all species of Moenkhausia, except M. eurystaenia Marinho, 2010, M. heikoi Géry &Zarske, 2004, andM. phaeonota Fink, 1979, by having a dark midlateral stripe from opercle to caudal peduncle (vs.stripe, when present, originating approximately at vertical through dorsal-fin origin).It is distinguished from Moenkhausia eurystaenia, M. heikoi and M. phaeonota by having elongate anteriormost dorsal and anal-fin rays on mature males (vs.not elongate).The new species is further distinguished from M. heikoi and M. phaeonota by the absence of humeral blotch (vs.present) and from M. eurystaenia by having a conspicuous black relatively narrow midlateral stripe on body (vs.midlateral stripe broader and not conspicuous).

Fig. 4 .
Fig. 4. Distribution of Hemigrammus ataktos in the middle rio Tocantins basin, central Brazil.Type locality represented by red circle.