A new species of Moenkhausia ( Characiformes , Characidae ) from the Içá River , Amazon Basin , northern Brazil

A new species of the genus Moenkhausia is described from the Içá River, Amazon Basin. Moenkhausia britskii sp. n. is most similar to M. grandisquamis and M. xinguensis. From these two species it is readily distinguished by the pigmentation pattern of the humeral spot. Moenkhausia britskii further differs from M. xinguensis by the pattern of radii on the scales, which curve upward and downward (vs. scales with straight radii). Discussions on the putative relationship of the new species with M. grandisquamis and M. xinguensis, and on the peculiar upward and downward arched scale radii are provided.

During a taxonomic study of M. grandisquamis (Müller & Troschel, 1845) and M. xinguensis (Steindachner, 1882), we verified the existence of an undescribed species, very similar morphologically to the Academic editor: Peter Bartsch formers, which is formally described herein.Given such similarity, the lack of phylogenetic definition for the genus, and the fact that the new species bears all the still used traditional generic diagnostic features (see Eigenmann 1903, Eigenmann 1917), we conservatively allocate our new species in Moenkhausia.

Material and methods
Measurements were taken following Fink and Weitzman (1974) and using a digital caliper (0.1 mm of precision).Counts were also performed according to Fink and Weitzman (1974) and taken at the left side of the specimens, whenever possible.Counts are followed by their frequency in parentheses; asterisks indicate values for the holotype.Detailed analysis of teeth, fin rays, and vertebrae were made from cleared and stained (c&s) specimens prepared according to Taylor and Van Dyke (1985), which was dissected following Weitzman (1974).Vertebral counts were taken from x-ray images and cleared and counterstained (c&s) specimen.Weberian apparatus was counted as four vertebrae; and the fused preural and ural centrum PU1+U1, located in the caudal region, as one element.
Diagnosis.Moenkhausia britskii sp.n. differs from all congeners, except M. grandisquamis (Müller & Troschel, 1845), M. tergimacula Lucena &Lucena, 1999 andM. xinguensis (Steindachner, 1882), by the presence of a single humeral spot, presence of a silvery lateral stripe, absence of a dark line at the base of the anal-fin rays, absence of a caudal lobule mark, dorsal fin hyaline, five series of scales above lateral line and four series of scales below lateral line.Moenkhausia britskii differs from M. tergimacula by the absence of a dark spot located anteriorly to the dorsal-fin origin; and from M. xinguensis by the scales showing upward and downward bent radii (vs.scales with straight radii) (Fig. 2).Finally, it differs from M. grandisquamis by having a larger humeral spot, located over the third to fifth or fourth to sixth lateral-line scales and extending vertically over three or four horizontal scale rows above the lateral line (vs.smaller humeral spot, located over second to third or third to fourth scale of the series immediately above the lateral line) (Fig. 3).The new species also differs by having four or five cusps on the second tooth of the inner row of the premaxilla (Fig. 4) (vs.eight, seven, occasionally six cusps on the   second tooth of the inner row of the premaxilla in M. grandisquamis).Table 1.Greatest body depth at dorsal-fin origin.Dorsal profile of body slightly convex from tip of supraoccipital spine to dorsal-fin origin.Dorsal-fin base posteroventrally inclined.Profile straight or slightly convex from end of dorsal fin to adipose-fin origin; adipose-fin base posteroventrally inclined; caudal peduncle concave both dorsally and ventrally; ventral profile of body convex from tip of lower jaw to anal-fin origin; anal-fin base posterodorsally inclined.Prepelvic region transversally flattened, mainly at pelvic-fin insertion.
Scales cycloid with striae curved upward and downward.Lateral line slightly curved, with 30 (3), 31 (6), 32* (6) or 33(2) pored scales.Five transversal scales above lateral line and four transversal scales below lateral line.A single series of scales covering base of second to eleventh branched anal-fin rays.Small scales covering the proximal two-third of caudal-fin lobes.
Etymology.The specific epithet britskii is in honor of Dr. Heraldo Antônio Britski, collector of this new species and for his enormous and valuable contributions to our knowledge of the Neotropical Ichthyology.

Discussion
Aside from an overall similarity in proportions and morphometric and meristic data, Moenkhausia britskii sp.n. and M. grandisquamis share with M. xinguensis the unique combination of a single humeral spot, presence of a silvery lateral stripe, absence of a dark line at the base of the anal-fin rays, absence of a caudal lobule mark, dorsal fin hyaline, and five series of scales above and four series of scales below lateral line.Moenkhausia tergimacula also fits in this combination except for the humeral spot, which is represented by a field of sparse dark chromato-    phores at the humeral area, and an unusual conspicuous dark blotch located just ahead of the origin of the dorsal fin (see Lucena and Lucena 1999 for more details).Thus, these probably closely related species should be necessarily considered in a phylogenetic analysis of the genus, since the group encompasses its type species, M. xinguensis.
The peculiar upward and downward arched scale radii observed in Moenkhausia britskii sp.n., is also present in M. grandisquamis.Indeed, this feature was first described by Eigenmann (1917: 67) as "some striae of the scales diverging from the middle line of each scale in nearly opposite directions, up and down" to directly differentiate M. grandisquamis from M. ovalis (Günther, 1868).Moreover, Eigenmann (1917: 85) emphatically argued that this feature actually would distinguish M. grandisquamis "from all others of the genus", evidencing the uniqueness of such character.Thus, considering the coloration pattern, and overall morphometric and meristic similarities between M. grandisquamis and M. britskii, it is plausible to consider such a condition synapomorphic and indicative of close relationship between these two species.Nonetheless, our comparative study also demonstrates that Stichonodon insignis (Steindachner, 1876) has scales with upward and downward curved radii.However, this is a highly modified deep-bodied species currently allocated at the Stethaprioninae (Nelson et al. 2016).Accordingly, occurrence of this trait in distantly related groups must be considered convergent/homoplasic for the time being.Distribution and significance of this rare character for systematics of Characidae shall be evaluated by more detailed comparison and a phylogenetic analysis encompassing more taxa.
Infraorbital and opercular series light silvery due to the scarce remaining guanine pigmentation.Dark chromatophores more densely concentrated along dorsal midline.Sparsely distributed dark chromatophores dorsal of horizontal septum.A dark line marks the horizontal septum.A silvery-gray midlateral stripe extending from posterior margin of humeral spot to caudal peduncle.Irregularly shaped, humeral spot located over third to fifth or fourth to sixth lateral-line scales and extending vertically over three or four horizontal scale rows above lateral line.Paired fins and anal fin hyaline.Adipose with very few dark chromatophores.

Figure 6 .
Figure 6.Partial map of South America with yellow circle indicating the distribution of Moenkhausia britskii.