Hyphessobrycon nicolasi ( Teleostei : Characidae ) a new species from the Uruguay River basin in the Mesopotamian Region , Argentina

Hyphessobrycon nicolasi is described from the Uruguay River basin in the Mesopotamian Region, Entre Ríos, Argentina. The new species can be distinguished from all congeners by the presence of a well-defined oblique and marginal black stripe on each lobe of the caudal fin. Other characters defining H. nicolasi are the possession of 1-3 teeth with 3-10 cusps on maxilla; outer premaxillary row with 2-3 small teeth, with 5 or 7 cusps; iv-vi, 27-36 anal-fin rays; 33-36 scales on the longitudinal series; two vertical dark spots on the humeral region; dorsal and anal fins dappled in black and base of caudal fin bearing conspicuous black spot; and the presence of bony hooks on the rays of dorsal, anal and caudal fins of the mature males.


Introduction
The genus Hyphessobrycon Durbin includes more than 100 valid species, occurring from Mexico and Central America to the Mar Chiquita Lagoon in Buenos Aires, Argentina and reaching its highest richness in the Amazon basin (Lima et al., 2003;Miquelarena & López, 2006;Bertaco et al., 2007).Authors that have recently addressed the systematics of the genus are unanimous in pointing out that the group is not well defined and its monophyly is yet uncertain (e.g., Bertaco & Carvalho, 2005;Bertaco & Malabarba, 2005;Carvalho & Bertaco, 2006;Bertaco et al., 2007).
During field trips to tributaries of the Uruguay River in Entre Ríos province, Argentina, the authors collected a new species of Hyphessobrycon that presents unique characters among the members of the genus.

Material and Methods
Measurements to the nearest 0.01 mm were made using a digital caliper following Fink & Weitzman (1974).Teeth, lateral line perforated scales, gill rakers, infraorbitals, procurrent caudal-fin rays and vertebral counts were made on 13 specimens (9 females and 4 males) cleared and stained (c&s) for bone and cartilage following the protocol by Taylor & van Dyke (1985).Vertebral counts include the four vertebrae of the Weberian apparatus and the terminal centrum was counted as one vertebra.Gill raker count was made on the upper and lower branches of the first branchial arch (epibranchial/ ceratobranchial, hypobranchial).
All measurements are expressed as percentage of standard length (SL), except for head measurements, which are expressed as percentage of head length (HL).For all counts, frequencies are given in parentheses and an asterisk indicates the holotype.Institutional abbreviations follow Ferraris (2007).
Diagnosis.Hyphessobrycon nicolasi can be distinguished from congeners by the presence of a well-defined, oblique and marginal black stripe on each lobe of the caudal fin.In addition, the following combination of characters permits the differentiation of the species: maxilla with 1-3 teeth, bearing 3-10 cusps; outer premaxillary row with 2-3 small teeth, bearing 5 or 7 cusps; iv-v, 27-36 anal-fin rays; 33-36 scales on the longitudinal series; two vertically elongated dark humeral spots; black caudal spot rounded or fan-shaped; dorsal and anal fins with the first rays dappled in black; and presence of minute bony hooks on the dorsal, anal and caudal fins of the mature males.
Description.Morphometric data are shown in Table 1.Body compressed and rhomboidal, greatest body depth at dorsalfin origin.Predorsal body profile convex, slightly concave at level of supraoccipital process.Body profile slightly straight between dorsal-fin origin and adipose-fin origin.Ventral profile almost symmetrical with dorsal counterpart.Dorsal and ventral margins of caudal peduncle straight or slightly concave.
Caudal peduncle relatively high.Head short.Snout short.Eye relatively large.Frontals broad at interorbital area, its width slightly less than or equal to eye diameter.Mouth terminal.Maxilla short not reaching anterior edge of orbit.Dorsal-fin origin almost equidistant from tip of snout and base of caudal-fin rays.Tip of pectoral fin not surpassing pelvic-fin origin.Pelvic-fin tip not surpassing the anal-fin origin.Presence of bony hooks on rays of dorsal, anal and caudal fins of mature males.Dorsal-fin rays ii,8 (5), i,9 (2), ii,9* (30), or iii,9 (6).In the latter specimens, first unbranched ray very small and only visible in cleared and stained specimens.Posterior margin of dorsal fin rounded, last unbranched ray and first two branched rays longest.Adipose fin well developed.Pectoral-fin rays i,10 (19), i,11* (17), or i,12 (5).Pelvic-fin rays i,6 in all specimens.Pelvic fin with axillary scale.
Color in alcohol.Dorsal and dorsolateral portions of head and body light brown.Dense concentration of chromatophores along dorsal profile, more conspicuous from end of dorsal fin to dorsal procurrent rays.Scales above lateral longitudinal stripe darkened by higher concentration of small chromatophores.Infraorbital and opercular areas with scattered, dark chromatophores.Two dark blotches above and below the eye's pupil, which are reddish in living and 1-2 horizontal scale rows below lateral-line perforated scales.Second humeral spot, usually well developed, separated from anterior spot by three less pigmented scales.Posterior to latter spot, sparse melanophores form a well defined narrow midlateral stripe (one scale high); stripe more conspicuous upon caudal peduncle and broader at caudalfin base and connected to, a dark rounded or fan-shaped spot somewhat expanding over caudal-fin rays.Pectoral and pelvic fins translucent, with few melanophores on rays.Dorsal fin usually with first seven rays black.Proximal half of first anal-fin rays black.Caudal fin with two oblique black stripes on dorsal and ventral margins of each lobe.These stripes variably developed, reaching tips of fin lobes in some specimens.Snout, lips, and maxilla with scattered dark chromatophores.Ventral body light brown.Scales above the base of anal fin with dark chromatophores.Interradial membrane of paired and unpaired fins with small dark chromatophores, distributed along borders of the rays (Fig. 1).

Color in life.
Ground color in life iridescent with yellow and turquoise hues.Iris with two reddish blotches above and below, the upper one with a dark lower edge.Dorsal fin yellow with first seven rays black.Generally, first two unbranched rays and first two branched rays more heavily pigmented.On remaining rays, dark pigmentation only reaching proximal third of ray length.Anal fin reddish yellow with anterior third intense red and proximal half of first rays black.Pectoral fin translucent with yellow hues and pelvic fin reddish yellow.Adipose fin yellowish.Caudal fin red with tips of lobes and middle caudal rays yellow; two oblique black stripes of variable length.Stripe on upper lobe of caudal fin involving upper marginal and first branched principal rays; stripe on lower lobe involving last branched and lower marginal principal rays.Two dark humeral spots vertically elongated and a rounded or fan-shaped dark spot on caudal-fin base (Fig. 3).Small dark chromatophores scattered over body and often on rays of all fins.Upper and lower lips with fine dark dotting.
Sexual dimorphism.Males with minute and blunt bony hooks on dorsal, anal and caudal fins.Dorsal fin with one or two bony hooks at the distal end branched 6 th or 7 th rays.Bony hooks especially numerous on anal fin, with one bony hook in each ray segment, distributed from last unbranched ray to usually, the 26 th branched ray.In one male specimen (39.7 mm SL) bony hooks anal and dorsal fins only.In specimens larger than 40 mm SL hooks also on central principal rays of caudal fin.Morphology of anal fin similar for both sexes, somewhat more concave in females.
Distribution.Hyphessobrycon nicolasi is known from the type locality El Pelado stream, tributary of Uruguay River in the vicinity of the city of Colón, Entre Ríos Province, Argentina (Fig. 4).
Habitat notes.At the type locality, El Pelado stream has clear water and rocky/sandy bottom, with irregularly distributed small stones, cobbles and boulders.The stream has shallower sectors with fast-running water up to 60 cm deep, and deeper portions with around 2 m depth and abundant submerged vegetation (Fig. 5).Species collected with H. Etymology.This species is dedicated to Nicolás Bonelli, whose affection and company we have enjoyed for the last few years.

Discussion
Within characids Hyphessobrycon is diagnosed by the combined possession of incomplete lateral line, naked caudal fin, presence of adipose fin, two series of premaxillary teeth (the inner row with 5 or more teeth), and second infraorbital not contacting the preopercle inferiorly (Eigenmann, 1917;Géry, 1977).We assign the new species to Hyphessobrycon in accordance to that diagnosis of the genus.
The presence of oblique black stripes on the caudal-fin lobes of H. nicolasi is a unique character, not described before for any known species of the genus.Among characids the presence of dark symmetric blotches on the caudal fin has been mentioned for Brittanichthys axelrodi Géry (Géry, 1965;Lima et al., 2004).In these species the distribution of blotches corresponds to four patterns: apical in M. dichroura, M. intermedia and M. bonita (Benine et al., 2004); medial in H. rhodostomus, H. bleheri and P. georgiae (Géry & Mahnert, 1986); basal in K. geryi, M. elongates and M. casiquiare (Bohlke, 1953;Géry, 1973;Lima et al., 2004) and marginal in B. axelrodi and H. nicolasi.The arrangement of the stripes on the main caudal-fin rays described by Géry (1965) for B. axelrodi is similar to the condition of H. nicolasi, however, we found no other characters that would indicate further similarity between these taxa.
Hyphessobrycon nicolasi is similar to H. meridionalis Ringuelet, Miquelarena & Menni by its high total anal-fin ray count (31-40 vs. 30-36) and by the morphology and number of teeth with numerous cusps however, it differs from that species by having shorter pectoral fin that does not reach the base of the pelvic fin (vs.surpassing the origin of the pelvic fin) and a different coloration pattern.Hyphessobrycon pytai Géry & Mahnert also has a high total anal ray count (32-36) but differs from H. nicolasi by the following morphometric characters: smaller eye (35.0-39.0%vs. 36.4-47.0%HL), longer snout (20.0-30.0%vs. 15.4-21.7%HL), longer head (26.0-28.0%vs. 20.9-26.1% SL), and a different coloration pattern.
The presence of minute bony hooks on the dorsal-fin rays also occurs in some species of the "rosy tetra clade" (Weitzman, 1977;Weitzman & Palmer, 1997).Hyphessobrycon nicolasi differs from these species (e.g.H. erythrostigma and H. socolofi) by the number of teeth in the inner row of the premaxilla (4 or 5 vs. 5 to 7) and the absence of the remarkable "rosy tetra clade" color pattern.
According to the ichthyogeographical schemes of Ringuelet (1975) and Arratia et al. (1983), the geographical distribution of H. nicolasi is within the Paranoplatensean Province and according to López et al. (2008) it corresponds to the Great Rivers Province.Within the biogeographical units proposed by Abell et al. (2008), this species occurs in the Lower Uruguay ecoregion, along with H. anisitsi (Eigenmann) Miquelarena, 2008) supports the assertions of Lucena & Kullander (1992) and Miquelarena & López (2004), who characterized the Uruguay River basin as a significant area of endemism.

Table 1 .
Morphometric data of holotype and paratypes (n) of Hyphessobrycon nicolasi from the El Pelado stream, Uruguay River basin.