A new species of Poptella (Characiformes: Characidae: Stethaprioninae) from the Rio Juma, Rio Madeira basin, Brazil

A new species of Poptella is described from the Rio Juma, a tributary of the lower Rio Aripuanã, Rio Madeira basin, Amazonas, Brazil. The new species is distinguished from all congeners, except P . brevispina , by having a lower number of scale rows between the lateral line and dorsal-fin origin (7 vs . 8-10). The new species can be readily distinguished from P . brevispina by the lower number of branched dorsal-fin rays (9 vs . 10). This is the first description of a new species of Poptella since the revisionary study of the Stethaprioninae, published 30 years ago.


Introduction
The subfamily Stethaprioninae was first proposed by Eigenmann (1907). It is morphologically defined by the presence of a bony, anteriorly directed spine, preceding the first dorsal-fin ray, and by the presence of very small, thin anal-fin hooks facing in all directions on mature males (Eigenmann, 1907;Reis, 1989). Recently, Mirande (2018) proposed a new phylogenetic classification based on morphological and molecular characters. This author expanded Stethaprioninae and divided it into four tribes: Stethaprionini Eigenmann, 1907, Grundulini Fowler, 1958, Gymnocharacini Eigenmann, 1909, and Rhoadsiini Fowler, 1911. Mirande also expanded the Stethaprionini to comprise, along with Brachychalcinus, Orthospinus, Poptella, and Stethaprion (original genera of the Stethaprioninae Eigenmann, 1907;Reis, 1989), species of Aphyodite, Bryttanichthys, Bryconella, Ectretopterus, Gymnocorymbus, Hasemania, Hemigrammus, Hyphessobrycon, Jupiaba, Moenkhausia, Petitella, Pristella, and Stichonodon. Poptella is widely distributed in all the major cis-Andean tropical South America river drainages, and small rivers flowing into the Atlantic from South America, between the Río Orinoco and the Rio Parnaíba, except the upper Rio Paraná, Rio Uruguay and Rio São Francisco (Reis, 1989(Reis, , 2003. The genus Poptella was proposed by Eigenmann (1907) to allocate Tetragonopterus compressus Günther, 1864, based on syntypes from Guyana, Suriname, and Peru. Poptella is characterized by having the anterior end of the predorsal spine rounded and ventrally concave and saddleshaped, and first anal-fin ray not expanded (Reis, 1989).
Poptella currently comprises four valid species: P. brevispina Reis (1989), P. compressa (Günther, 1864), P. longipinnis (Popta, 1901), and P. paraguayensis (Eigenmann, 1907). The diagnostic characters of these species rely on the number of branched dorsal-fin rays, length of predorsal spine, number of rows of scales between the lateral line and dorsal-fin origin, and degree of pigmentation on adipose fin. The Stethaprioninae Eigenmann (1907) was the subject of a systematic study (Reis, 1989) in which P. brevispina was the last species of Poptella to be described. After almost three decades of Reis's paper, the continuous examination of Poptella collected in various localities along the Amazon basin revealed a new species, apparently endemic to the Rio Juma, Rio Madeira system, which we formally describe herein.

Material and Methods
Counts and measurements follow Sidlauskas et al. (2011) andFink, Weitzman (1974), except for the number of scale rows between lateral line and midventral scale series, which were counted at the vertical just ahead of the pelvicfin insertion, and head depth, which was measured at the vertical through the tip of supraoccipital spine; the predorsal spine length is added, measured as the greatest length along its longitudinal axis, taken point to point. The number of scale rows between dorsal-fin origin and lateral line was counted in all paratypes, considering their diagnostic nature. Measurements were taken point to point with a digital caliper (precision of 0.1 mm) on the left side of specimens. Values between parentheses indicate the number of specimens with a particular count and the asterisk indicates values of the holotype. For osteological observations, cleared and stained (c&s) specimens were prepared according to Taylor, Van Dyke (1985). Radiographs were taken using the X-ray system Faxitron LX60 DC12 at LIRP (FFCLRP-USP). Vertebral counts included the four vertebrae of the Weberian apparatus and the terminal centrum counted as one single element, and were taken from two c&s specimens. Color description in life was based on field photographs of one freshly collected specimen. Measurements are presented as proportions of standard length, except for subunits of the head, which are given as proportions of head length. Specimens examined herein belong to the following institutions: Instituto Nacional  Diagnosis. Poptella actenolepis can be distinguished from all congeners, except from P. brevispina, by having a lower number of scale rows between lateral line and dorsal-fin origin (7 vs. 8-10). It can be distinguished from P. brevispina and P. longipinnis by having a lower number of branched dorsal-fin rays (9 vs. 10). Poptella actenolepis can also be differentiated from P. longipinnis by having the first rays of the dorsal and anal fin darker and much longer than the following rays (vs. first dorsal and anal fin rays slightly longer than the following rays). The new species differs from P. paraguayensis by having a lower number of branched anal-fin rays (27-29 vs. 30-35) and a comparatively shorter predorsal spine (2.8-4.3 vs. 4.7-6.4% mm SL).

Poptella actenolepis, new species
Description. Morphometric data summarized in Tab. 1. Largest specimen examined 72.4 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, 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 of head and body convex from tip of dentary to anal-fin insertion. Body profile along anal-fin base straight and posterodorsally slanted. Prepelvic region compressed with a median keel.
Mouth terminal. Maxilla reaching vertical passing through anterior margin of pupil; its posterior end not extending beyond third infraorbital. Premaxillary teeth in two rows. Outer row with 4*(40) or 5(3) teeth with three cusps. Inner row with 5(43) teeth with five cusps. Maxilla with 1(11) or 2*(32) teeth with three cusps or conical. Dentary with five pentacuspidate teeth, usually central cusp longest, followed by a series of 7 to 9 small conical teeth (Fig. 2).
Color in life. Dorsal portion of head and body green brownish. Infraorbitals and opercular area silvery. Iris orange on upper portion, silvery ventrally, anterior and posterior portions of eye with faint silvery pigmentation. Middorsal and ventral portions of body silvery. First humeral blotch conspicuous, second humeral blotch inconspicuous. Dorsal, adipose, pelvic, and anal fins orange. Pectoral and caudal fins yellow (Fig. 4).

Sexual dimorphism.
Mature males of Poptella actenolepis over 60.0 mm SL exhibit a series of small bony hooks on all fins. Hooks usually concentrated on postero-lateral margin of each segment of distal half of fin rays. Hooks distributed on 1 st to 7 th branched dorsal-fin rays, with one to two hooks per ray segment. Pectoral and pelvic fins with one or two hooks per ray segment, distributed from 1 st to 5 th branched fin rays. Anal fin with with one to three hooks per ray segment of most branched rays (1 st -27 th ), being more numerous on the first seventh branched rays. Caudal fin with one hook on per ray segment, more concentrated on middle and inferior lobe rays. Further secondary sexual dimorphisms, such as differences in standard length or sexual dichromatism, were not detected in P. actenolepis.
Etymology. The specific epithet "actenolepis" is from the greek akten, meaning without lot, poor; and from the greek lepis, meaning scale, in reference to the lower number of transversal scale rows of the new species. An adjective.

Conservation status.
Poptella actenolepis is so far known only from the rio Juma and one of its tributaries, rio Aripuanã drainage, and its conservation status is uncertain based on the currently available data of its geographic distribution. However, considering that no imminent threats to the species were detected in the area of occurrence of the new species, we suggest that P. actenolepis be classified as least concern (LC) according to the International Union for Conservation of Nature categories and criteria (IUCN, 2017).
Remarks. Reis (1989)  only differentiated by the number of longitudinal series of scales above the lateral line (7 vs. 8-9, respectively (Reis, 1989(Reis, , 2003. In the Rio Madeira, it is known to occur only in its upper portion (Rio Guaporé and Rio Jamari). Poptella actenolepis is only known from the Rio Juma, a tributary of the the highly endemic Rio Aripuanã (see Kullander, 1995;Ohara et al., 2017). Ohara et al. (2017) discussed that the fish endemism is not only characteristic to the upper portion of the Rio Aripuanã, as first discussed by Kullander (1995), but, actually, patchily distributed in its upper and middle portions. P. actenolepis, as several examples cited by Ohara et al. (2017), is only known from its type locality, and most probably restricted to this area. Garcia-Ayala et al. (2017) discussed that small bony hooks on all fins of adult males are present in Poptella compressa, P. brevispina, P. longipinnis, and in Brachychalcinus reisi. Adult males of P. actenolepis also bear bony hooks on rays of all fins. Although the occurrence of bony hooks on all fins of mature males has been considered an uncommon characteristic in Characidae (Bertaco, Lucinda, 2006;Bertaco et al., 2007;Mirande, 2010;Camelier, Zanata, 2014), it seems to be well distributed in the Stethaprioninae (sensu Reis, 1989), as already indicated in the description of B. reisi (Garcia-Ayala et al., 2017).