A new miniature of Xenurobryconini ( Characiformes : Characidae ) from the rio Tapajós basin , Brazil

A new species of Xenurobrycon is described from tributaries of the rio Tapajós, Pará, Brazil. It is diagnosed from all congeners by the dark color of the posterior half of both caudal-fin lobes, the presence of infraorbitals one, two, and three, the presence of a set of lamellar processes on eighth principal caudal-fin ray of mature males, the presence of bony hooks similar in size or decreasing posteriorly on last unbranched and first 9-12 branched anal-fin rays of mature males, the presence of only conical teeth on both jaws, the lack of the adipose fin and the presence of 15-18 predorsal scales. The description of the new species increases the number of species in Xenurobrycon to six. An updated key to the species of the genus is provided.


Introduction
The Xenurobryconini (Characidae: Stevardiinae) traditionally comprises seven inseminating genera (Weitzman & Fink, 1985;Weitzman & Menezes, 1998) Weitzman & Fink (1985), based on sexually dimorphic characters.Such phylogenetic definition was subsequently adjusted by Weitzman et al. (1994) to include the genus Ptychocharax in the group, although any explicit phylogenetic hypothesis was provided.The later addition of Chrysobrycon restricted the definition of the Xenurobryconini to the presence of a single pouch scale with or without hypertrophied radii in adult males, an ambiguously optimized character in the phylogenetic hypothesis by Weitzman & Menezes (1998), giving little support to the new composition of the group.With effect, the recent study by Thomaz et al. (2015), questioned the monophyly of the Xenurobryconini, since their data strongly refuted Chrysobrycon (nested within the Stevardiini) and Argopleura (now an incertae sedis taxon) as members of the tribe, as previously proposed by Weitzman & Fink (1985), Weitzman et al. (1994) and Weitzman & Menezes (1998).
The definition of the genus Xenurobrycon is based on the presence of several secondary sexual characters in mature male specimens (Weitzman & Fink, 1985).This involves mainly their conspicuously larger size and the anterior displacement position of the pelvic fin, along with modifications to the pelvic girdle; the double ligamentous attachment of the pouch scale; and the interrupted principal caudal-fin rays (sensu Weitzman & Fink, 1985 usually the median-most ray of upper and lower caudal-fin lobes).The genus currently consists of five valid miniature species (sensu Weitzman & Vari, 1988), and a sixth is herein described from the rio Tapajós basin.Its putative relationships among congeners are discussed and an updated key to identify all Xenurobrycon species is also provided.

Material and Methods
Counts and measurements follow Fink & Weitzman (1974).Measurements were taken point-to-point with aid of scaled eyepiece reticule.Standard length (SL) is presented in mm and other measurements are expressed as proportions of SL, except for subunits of head which are expressed as proportions of head length (HL).Descriptions of meristic data are accompanied by frequency counts in parentheses; an asterisk indicates the values of the holotype.
The number of scales along contralateral pelvic fins were counted between the pelvic-fin insertions (at the origin of the first unbranched ray).Numbers of vertebrae, supraneurals, procurrent caudal-fin rays, branchiostegal rays, gill rakers, and teeth were determined from cleared and stained paratypes (c&s) prepared according to Taylor & Van Dyke (1985).Vertebrae of the Weberian apparatus were included in the vertebral counts and the compound centrum formed by the first pleural and first ural centra (PU1 + U1) of the caudal region was counted as a single element.All examined specimens were alcohol preserved, except when indicated by c&s, specimens preserved in glycerin.The non-type series is represented by damaged specimens.The term "set of lamellar processes" is used to indicate the sum of the paired flange-like processes of Weitzman & Fink (1985), which are ventral, plus the dorsal processes of the same lepidotrichia segments, or more than one lepidotrichium bearing a flange-like process.Institutional abbreviations follow Ferraris (2007) with addition of the Museu de Ciências Naturais Pontifícia Universidade Católica de Minas Gerais, Belo Horizonte (MCNIP).An identification key of the Xenurobrycon species was modified from Weitzman & Fink (1985) and Weitzman (1987).Statistical analyses were carried out in R v.3.1.1 software (R Core Development Team, 2012): the packages npsm, Rfit and xlsx were additionally employed (Dragulescu 2014; Kloke & McKean, 2012;2014a,b).A nonparametric robust ANCOVA and the Wilcoxon regressions were used as the assumptions of normality, required for use of their parametric counterparts, were not met.
Diagnosis.Xenurobrycon varii is readily distinguished from its congeners by the dark posterior half of both caudal-fin lobes (vs.hyaline) and the absence of infraorbitals 4 and 5 (vs.absence of infraobitals 1, 4 and 5 in X. coracoralinae Moreira and presence of infraorbitals 1-5 in X. heterodon Weitzman & Fink, X. macropus Myers & Miranda Ribeiro, X. polyancistrus Weitzman and X. pteropus Weitzman & Fink).The new species is further differentiated from its congeners, except X. coracoralinae and X. macropus, by the presence of a set of lamellar processes on the eighth principal ray of the lower caudalfin lobe (Figs. 3a,3b,vs.absence in X. heterodon, X. polyancistrus and X. pteropus).The new species differs from X. coracoralinae by the presence of hooks on the last unbranched plus the first 9 th -12 th branched anal-fin rays of mature males (Fig. 4a, 4b, vs. hooks present only on last unbranched and first 4 th -5 th branched rays).Xenurobrycon varii can be additionally distinguished from X. heterodon by the presence of only conical dentary teeth (Fig. 5 vs. anterior dentary teeth tricuspid).The new species is also differentiated from X. polyancistrus by the posterior anal-fin hooks approximately equal or reduced in size posteriorly in mature males (Fig. 4a, 4b, vs. posterior larger anal-fin hooks).Finally, it further differs from X. pteropus by the absence of adipose-fin (vs.presence) and greater number of predorsal scales (15-18 vs. 13).Description.Morphometric data for examined specimens in Table 1.Body profile convex along dorsal portion from tip of snout to posterior end of supraoccipital spine; slightly convex from that point to dorsal-fin origin (more prominent in females) approximately straight along dorsal-fin base, and becoming almost completely straight from terminus of dorsal-fin to first dorsal procurrent caudal-fin ray.Ventral profile of head convex from lower lip to pectoral-fin origin.Ventral profile of body slightly convex from that point to anterior margin of urogenital papillae, straightening out along anal-fin base, and becoming slightly convex from anal-fin terminus to first ventral procurrent caudal-fin rays.Mouth terminal; upper and lower jaws of equal length.Teeth conical with slightly posteriorly recurved tips.Premaxillary teeth 8(2), 9(4), 10(2) or 11(3).Maxillary teeth 7(6) or 8(6).Posterior tip of maxilla reaching anterior margin of lateral ethmoid.Dentary teeth 11(3) or 12(1); second and sometimes third tooth anterodorsally aligned.Branchiostegal rays 4(12); first to third attached to anterior ceratohyal and fourth attached to cartilage between anterior and posterior ceratohyal (n=8) or to posterior ceratohyal (n=4).Only antorbital and infraorbitals 1, 2 and 3 present.Supraorbital, pterotic laterosensory canal and epioccipital bridge absent.Extrascapular and posttemporal bones absent.Supracleithrum laterosensory canal absent.

Color in alcohol.
Body pale yellow with small chromatophores on posterior margin of scales.Chromatophores highly concentrated on lips, but more disperse on dorsal portion of head, infraorbital and mandibular areas.Postorbital area with scattered chromatophores, more concentrated anteriorly to dark longitudinal stripe.Narrow, dark longitudinal stripe extending from fifth or sixth longitudinal scale to end of caudal peduncle.Narrow, dark dorsal stripe extending from posterior margin of supraoccipital to dorsal caudalfin procurrent rays, whitish at dorsal fin base.Narrow, dark ventral stripe from first anal-fin ray to ventral caudal-fin procurrent rays.Usually, only base of first pectoral-fin ray, and rarely others, pigmented.Dorsal, anal and posterior half of caudal-fin rays densely pigmented.
Sexual dimorphism.Anal-fin base slightly concave in females, and nearly straight in males.Number of scales between pelvic-fin insertions in mature males 5(7) or 6(4), in females 1(6) or 2(4).Scales between mesial pelvic-fin rays larger with more radii than other body scales (except for pouch scale) in mature males, and equal in size in females and juveniles.Number of scales between mesial pelvic-fin rays 2(2) or 3(9) in mature males (a single male with one large scale, almost as large as the pouch scale), and 1 (10) in females.Pouch scale somewhat teardrop shaped, similar to that of X. polyancistrus (Weitzman 1987: fig. 3) but with numerous, loosely positioned radii extending over posterior, posterodorsal, and posteroventral margins, being more numerous on latter portion.
Tip of pectoral fin at vertical through 9 th (11) or 11 th (1) scale of lateral line longitudinal series in mature males, whereas in females and juveniles, tip of pectoral fin located at vertical through 11 th (7) scale.Pelvic fin hyaline in females and juvenile specimens inserted at vertical through 10 th (9) scale of lateral line longitudinal series, with vii(16) rays.In mature males, pelvic fin pigmented from first to fourth, or sometimes fifth ray, inserted at vertical through 5 th (11) or 7 th (l) scale of lateral line longitudinal series; pelvic-fin rays i,6(1), ii,5*(11) or ii,6(5).Mature males with two unpaired antrorse hooks per segment on ventral margin of unbranched and branched pelvic-fin rays; hooks increasing in size until middle of hooked portion, then decreasing distally; hooks on first unbranched ray larger than on branched ones.Anterior tip of pelvic bone located anterior to vertical through pectoralfin rays insertion (5), posterior tip of pelvic bone between 2 nd and 3 rd (5) ribs in mature males.In female and juvenile specimens, anterior tip of pelvic bone between 1 st and 2 nd (6) ribs, posterior tip of pelvic bone at 3 rd (2) or between 3 rd and 4 th (4) ribs.Pelvic -fin of mature males significantly longer than in females (32.2-38.9%SL vs. 9.1-13.3%SL p<0.001;Fig. 6).Distance between snout to pelvic-fin insertion between male and female or juvenile specimens (28.0-36.0%SL vs. 33.9-43.2%SL; Fig. 7) not significantly different.Anterior anal-fin lobe formed by 3 rd unbranched ray and anteriormost four branched rays (Fig. 4a), longer in mature males than in females.Lamellar process of pterygiophores well developed in 1 st proximal radial, gradually increasing in size from 7 th to 16 th or 17 th proximal radials but poorly developed or, absent from 2 nd to 6 th pterygiophores (Fig. 4b).Bilateral, antrorse anal hooks located from 3 rd unbranched ray to 8 th branched rays, and strongly recurved hooks on branched rays 9 th -12 th ; usually one pair on each segment.Anal hooks roughly equal in size, but larger on posteriormost rays.Caudal-fin musculature of mature males same as that described for X. macropus (Weitzman & Fink, 1985: 77, 81-83, and fig. 11).Caudal-fin rays with bilateral antrorse hooks, usually one pair per lepidotrichia segment; approximately equal in size on most rays but lager on unbranched rays (Fig. 3a).In mature males, set of lamellar processes present on eighth principal caudal-fin ray of lower lobe composed by ventral, paired lamellar expansions on 3 rd to 5 th lepidotrichia segments, and an unpaired lamellar expansion from 4 th to 5 th lepidotrichia segments (Fig. 3b).
Distribution.Xenurobrycon varii is known from tributary streams of rio Tapajós and rio Jamanxim (Fig. 8).Etymology.This species is named in honor of Richard P. Vari, an esteemed person and ichthyologist, for his contributions to the systematics of fishes and his continuous support and aid to the authors.

Conservation status.
Considering that present relevant threats to the species were not detected in its distribution area, Xenurobrycon varii could be classified as Least Concern (LC), according to the International Union for Conservation of Nature (IUCN) categories and criteria ( IUCN Standards and Petitions Subcommittee, 2014).On the other hand, at least five major hydroelectric power plants denominated Complexo Tapajós (São Luiz do Tapajós, Jatobá, Cachoeira dos Patos, Cachoeira do Caí and Jamanxim) are expected to be built in the occurrence area of the species by the Brazilian government, causing highly plausible impacts to the population of Xenurobrycon varii, leading to a revaluation of the conservation status of the species, along with others, in the medium term.

Discussion
Xenurobrycon varii processes all eight synapomorphies recognized for that genus by Weitzman & Fink (1985), justifying its inclusion in Xenurobrycon.Similarly to its congeners, X. varii is a miniature species (sensu Weitzman & Vari, 1988), with the largest known specimen reaching 17.3 mm SL.In addition to their diminutive size, the lack of infraorbital bones 4 and 5 in X. varii might also be associated with miniaturization events, as suggested by Weitzman & Vari (1988) and further discussed by Toledo-Piza et al. (2014) and Mattox et al. (2016).Comparatively, the infraorbital series is almost complete in X. heterodon, X. macropus, X. pteropus and X. polyancistrus, which present infraorbitals 1 to 5 (Moreira, 2005).On the other hand, X. coracoralinae shares with X. varii, the lack of those bones, but also lacks infraorbital 1, a unique condition among Xenurobrycon species (Moreira, 2005) The presence of a set of lamellar processes on segments 3-5 on the eighth ray of the caudal-fin lower lobe (see "Sexual Dimorphism") is an unusual feature among xenurobyconin species.Besides X. varii, similar structures were found only in X. coracoralinae and X. macropus, however, in the latter species the dorsal, unpaired process is lacking, and the paired, ventral processes are distinctly larger, forming a conspicuous flange-like process that extends across two segments, as described in Weitzman & Fink (1985: fig. 11).Despite Moreira (2005) reported the absence of the flange-like process in X. coracoralinae, analyses of paratypes of that species (MNRJ 24887) revealed the presence of these processes on the eighth ray of the caudal-fin lower lobe.Xenurobrycon heterodon, X. pteropus, and X. polyancistrus lack any of these processes.Xenurobrycon varii shares the presence of a teardrop shaped pouch scale with X. coracoralinae, X. heterodon and X. polyancistrus (Weitzman, 1987: fig. 3).In contrast, X. macropus and X pteropus have more rounded pouch scales, with short blunt posterior protuberances (Weitzman & Fink, 1985: fig. 28) and numerous ventral radii in the later (Weitzman & Fink, 1985: fig. 29).Such conditions contradict, at least in part, the idea that X. varii would share a close common history with X. macropus, but seem to be an additional indicative of its closely relatedness to X. coracoralinae.However, despite the aforementioned considerations, the relationships among Xerunobrycon species remains to be investigated, since this point is out of the scope of the present contribution.

Fig. 3 .
Fig. 3. Xenurobrycon varii, MPEG 26591, male, 15.0 mm SL, left side of caudal-fin rays (pouch-scale removed): (a) bilateral, antrorse hooks, usually one pair per lepidotrichium segment (rectangle indicates magnified area represented in b); (b) set of lamellar processes on eighth principal caudal-fin ray of lower lobe, dashed lines represent the delimitation of left elements of paired ventral processes and dorsal unpaired process along ray segments 3 to 5 and 4 to 5, respectively.Scale bars = 1 mm and 0.25 mm respectively.

Fig. 6 .
Fig. 6.ANCOVA plot showing the difference between pelvic-fin length of mature males and females of Xenurobrycon varii.

Fig. 7 .
Fig. 7. ANCOVA plot showing relation between snout to pelvic-fin length and SL of mature males and females of Xerunobrycon varii.

Fig. 8 .
Fig. 8. Map of portion of rio Tapajós basin, and adjoining areas, showing collecting sites (dark circles) and the type locality (white circle) of Xenurobrycon varii.

Table 1 .
Morphometric data of Xenurobrycon varii.Values for the holotype included in number of specimens and range of values.Max = maximum; Min = minimum; N = number of specimens; SD = standard deviation.