A new Poptella from the rio Xingu basin, Brazil (Characiformes: Characidae)

Abstract A new species of Poptella is described from the rio Xingu basin, Pará and Mato Grosso states, Brazil. The new species is distinguished from all congeners by having a greater number of scales around the caudal peduncle (15–18 vs. 13–14). Furthermore, this species can be easily distinguished from P. brevispina, P. longipinnis, and P. fuscata by the lower number of branched rays in the dorsal fin (9 vs. 10–11). The description of new taxon represents the third species of the genus Poptella described in the last five years.

The genus Poptella currently consists of six valid species: P. compressa (Günther, 1864), P. longipinnis (Popta, 1901), P. paraguayensis (Eigenmann, 1907), P. brevispina Reis, 1989, P. actenolepis Garcia-Ayala & Benine, 2019, and P. fuscata Garcia-Ayala & Benine, 2021.It is widely distributed in all major cis-Andean South American hydrographic basins (Reis, 1989(Reis, , 2003)).Poptella is diagnosed by the saddle-shaped predorsal spine and the characin regular shape of the first anal-fin ray (Reis, 1989).Our comparative morphological analysis of Poptella specimens collected in various localities along the Amazon basin revealed a new species from the rio Xingu basin, which we formally describe herein.Fink, Weitzman, (1974) and Sidlauskas et al. (2011), andGarcia-Ayala, Benine (2019).Measurements were taken point to point with a digital caliper (precision of 0.1 mm) on the left side of the specimens.All measurements are presented as percent of standard length (SL), except those of the head, which are given as percent of the head length in the description, values in parentheses indicate the number of specimens with a particular count, and the asterisk indicates the values of the holotype.Measurements and counts of damaged and/or poorly preserved specimens were not included either in the text and/or in the Tab. 1. Counts of supraneurals, vertebrae, procurrent caudal-fin rays, branchiostegal rays, gill-rakers of the first branchial arch, tooth cusps, diminutive dentary teeth, and the position of pterygiophores were taken from four cleared and stained (c&s) paratypes prepared according Taylor, Van Dyke (1985).Radiographs were also used for eight paratypes taken using the X-ray system Faxitron LX60 DC12 at Laboratório de Ictiologia de Ribeirão Preto, Universidade de São Paulo, São Paulo, Brazil.Vertebral counts included the four vertebrae of the Weberian apparatus, and the terminal centrum counted as one single element.Institutional acronyms are described in Sabaj (2020).Diagnosis.Poptella fortispina can be distinguished from all congeners by having a higher number of scale rows around the caudal peduncle (15-18 vs. 13-14).It can be further distinguished from P. brevispina, P. longipinnis, and P. fuscata by having fewer branched dorsal-fin rays (9 vs. 10-11).The new species is distinguished from P. compressa, P. paraguayensis, and P. actenolepis by having a comparatively robust, thick, broad, and downward curved predorsal spine (vs.delicate, thin, and narrow straight predorsal spine) (Figs. 2, 4).Additionally, it can be distinguished from P. actenolepis by having more scale rows between the lateral line and dorsal-fin origin (8-10 vs. 7) and a comparatively elongated predorsal spine (4.3-5.5 vs. 2.8-4.2%mm SL).

Counts and measurements follow
Description.Morphometric data summarized in Tab. 1. Largest specimen examined 60.8 mm SL.Greatest body depth at dorsal-fin origin.Dorsal profile slightly convex between tip of snout and vertical through middle of orbit; slightly concave from this point to end of occipital process, and convex from tip of occipital process to dorsal-fin origin.Dorsal-fin base posteroventrally slanted.Profile straight or slightly convex from posterior terminus of dorsal-fin base to end of adipose fin.Caudal peduncle profile slightly concave both dorsally and ventrally.Ventral profile convex from tip of dentary to anal-fin origin.Body profile along anal-fin base straight and posterodorsally slanted.Prepelvic region compressed with median keel.
Mouth terminal.Distal tip of maxilla reaches vertical through anterior margin of pupil, not extending beyond third infraorbital.Premaxillary teeth in two rows; outer row with four*(110), rarely five (4) tricuspid teeth.Inner row with five*(113) or rarely six (1) pentacuspid teeth.Maxilla with one (37) or two*(75) conical or tricuspid teeth.Dentary with four teeth with five cusps, usually central cusp longest, and one small tricuspid tooth, followed by five to seven small conical teeth (Fig. 3).region to caudal peduncle.Caudal peduncle blotch absent.Pectoral fin hyaline, with chromatophores scattered throughout interradial membranes.Dorsal, pelvic, and anal fins with scattered dark chromatophores concentrated along first rays and interradial membrane.Adipose fin with scattered dark chromatophores.Caudal fin with scattered dark pigmentation on its distal margin (Fig. 1).
Sexual dimorphism.Mature males with small bony hooks on the unbranched and branched rays of the dorsal and anal fins.Dorsal fin with one to two hooklets on the distalmost segments of the posterior branch of the 2 nd to the 4 th branched rays.Anal-fin hooks unevenly distributed on the distalmost segments of the 1 st unbranched ray to the 26 th branched rays, being more numerous in the first twelve branched rays, with one to two hooks per segment.
Etymology.The name fortispina is from Latin "fortis" meaning strong and "spina" meaning spine, in reference to the robust predorsal spine.Remarks.As demonstrated by Garcia-Ayala, Benine (2020), the number of branched dorsal-fin rays is helpful for the taxonomy of the Stethaprion Clade in properly separating the species of Poptella and Brachychalcinus.Poptella may be split into three groups according to the number of branched rays in the dorsal fin: nine, ten or eleven (Garcia-Ayala, Benine 2020).Poptella fortispina is the seventh species described for the genus and shares with P. compressa, P. paraguayensis, and P. actenolepis the condition of nine branched dorsal-fin rays.Poptella brevispina and P. longipinnis have ten branched dorsal-fin rays and P. fuscata has eleven branched dorsal-fin rays (Tab.2).
Along with the number of branched dorsal-fin rays, the length of the predorsal spine has been employed in the taxonomy of Poptella (Reis, 1989;Garcia-Ayala, Benine 2019, 2020).Reis (1989) used the length of the predorsal spine as part of the diagnostic characters for the species of Poptella, indicating, however, overlapping ranges for P. compressa (3.1-6.9%mm of SL), P. longipinnis (2.5-3.4% mm of SL), P. paraguayensis (4.7-6.4% mm  of SL), and P. brevispina (2.6-4.2%mm of SL).The population of Poptella from the rio Xingu basin was first identified by Reis (1989) as P. compressa, due to the presence of nine branched dorsal-fin rays, adipose fin hyaline, and an elongated predorsal spine.
Owing to the broad range and probably in an attempt to identify a putatively hidden diversity, Reis (1989) subdivided P. compressa into four populations named Guyana, rio Madeira, NE Brazil, and rio Orinoco.This procedure substantially reduced the range in each subgroup, except for NE Brazil, which kept values between 4.0% and 6.9% mm of SL.Reis' (1989) NE Brazil included 32 specimens from tributaries of the rio Tocantins, rio Araguaia, rio Corda (Maranhão state), rio Parnaíba, and rio Xingu.Our analyses also showed more discrete subdivisions when these drainages were analyzed separately.Predorsal spines in specimens from the rio Tocantins-Araguaia vary between 5.4-7.0%mm of SL; from the rio Parnaíba, 3.9-4.9%mm of SL; and in specimens from the rio Xingu is 4.3-5.5% mm of SL.Therefore, these values indicate that the length of the predorsal spine may help distinguish specimens from the rio Xingu (P.fortispina) from those from the Tocantins-Araguaia basin.These differences, along with other diagnostic features (e.g., the number of scale rows around the caudal peduncle), indicate a hidden diversity in northeastern Brazil yet to be described.

ETHICAL STATEMENT
Not applicable.

COMPETING INTERESTS
The author declares no competing interests.
.br | scielo.br/niJames R. Garcia-Ayala and Ricardo C. Benine Conservation status.Considering that no imminent threats to the species were detected in area of occurrence, we suggest that Poptella forstispina be classified as least concern (LC) according to the International Union for Conservation of Nature (IUCN) categories and criteria (IUCN Standards and Petitions Subcommittee, 2022).

FIGURE 5 |
FIGURE 5 | Map showing the known distribution of Poptella fortispina.Red star indicates the type-locality.Symbols may represent more than one locality.

TABLE 1 |
Morphometric data of holotype (H) and paratypes of Poptella fortispina.Range includes values of the holotype.N = the total number of specimens examined.SD = Standard deviation.

TABLE 2 |
Species of Poptella grouped based on the number of branched dorsal-fin rays.
Poptella from the rio Xingu basin ni.bio.br| scielo.br/niA new species of