Priocharax britzi, Mattox & Souza & Toledo-Piza & Oliveira, 2021

Mattox, George Mendes Taliaferro, Souza, Camila da Silva de, Toledo-Piza, Mônica & Oliveira, Claudio, 2021, A new miniature species of Priocharax (Characiformes: Characidae) from the upper rio Ipixuna, Purus drainage, Brazil, Neotropical Ichthyology (e 210048) 19 (2), pp. 1-18 : 5-13

Priocharax britzi , new species urn:lsid:zoobank.org:act:A2309042-25EE-4849-8488-77B2A5FDE955

( Figs. 1 –5; Tab. 2 )

Holotype. MZUSP 125822 , 13.7 mm SL, Brazil, Amazonas , Canutama , small lake approximately 400 m from the rio Ipixuna , after bridge crossing this river, on the right side of the Transamazônica Road ( BR –230) going from Humaitá to Lábrea , rio Ipixuna drainage, upper Purus basin, 07°31’11.46’’S 63°20’59.58’’W, 3 Sep 2018, G. M. T. Mattox & S. Souza.

Paratypes. All collected with holotype . LBP 28493 , 5 , 11.7–12.7 mm SL . MZUSP 125823 , 43 , 9.3–14.1 mm SL (6 c&s, 11.0– 12.9 mm SL) . UFRO-ICT 27662 , 5 , 10.2 – 12.0 mm SL .

Diagnosis. Priocharax britzi is distinguished from all congeners except P. pygmaeus by the lower number of teeth in the maxilla (21–30 vs 32–58) and dentary (26–29 vs 33– 55). It differs from P. pygmaeus in the presence of two postcleithra (vs absence) and by having 22–27 (modes 25 and 26) branched anal-fin rays (vs 19–22, mode 19). The new species is further distinguished from P. nanus and P. varii by having i,5 pelvic-fin rays (vs i,6) and a slender and sinuous postcleithrum 3 (vs postcleithrum 3 relatively thick and straight), from P. ariel by the presence of 22–27 branched anal-fin rays (vs 16–22) and the presence of two postcleithra (vs absence). Priocharax britzi is further distinguished from P. varii by the absence of the adipose fin (vs presence) and from P. nanus by the absence of the claustrum (vs presence). Complementarily, Priocharax britzi has a shorter snout relative to the orbital diameter when compared to all congeners except P. nanus . This difference is reflected in the range of the proportion of snout length in relation to orbital diameter and its mean which is 45–59 (mean = 53.6; SD = 3.8) in P. britzi vs 60–76 (mean = 65.5; SD = 4.9) in P. pygmaeus , 54–81 (mean = 67.0; SD = 5.1) in P. varii and 54–84 (mean = 68.4; SD = 6.3) in P. ariel . The molecular analyses provide additional support towards the recognition of the new species.

Description. For overall appearance, see Fig. 1 . Morphometric data are presented in Tab. 2 . Body laterally compressed and elongated, greatest depth at vertical through dorsal-fin origin. Dorsal-fin origin approximately at midbody, at vertical slightly anterior to anal-fin origin. Pectoral-fin bud at vertical through anterior portion of pseudotympanum. Pelvic-fin origin approximately midway between posterior margin of opercle and anal-fin origin. Dorsal profile of head and body slightly convex from tip of snout to dorsal-fin origin. Dorsal profile of body along dorsal-fin base nearly straight, gently sloping posteroventrally; sloping more conspicuous from latter point to caudal peduncle. Dorsal profile of caudal peduncle slightly concave to base of dorsal procurrent rays. Ventral profile of head and body slightly convex from symphysis of lower jaw to vertical through pectoral-fin origin; straight to slightly convex from latter point to pelvic-fin origin. Ventral profile of body posteroventrally sloping from pelvic-fin to anal-fin origin; straight and posterodorsally rising along anterior one-half of anal-fin base, gently concave from latter point to base of ventral procurrent rays. Caudal peduncle short. Pseudotympanum located anterior to rib of fifth vertebra.

Snout round in lateral view. Eye about one-third of head length. Infraorbitals 2 and 3 present but not fully developed in three specimens, absent in the others. Infraorbitals 1, 4 to 6 and supraorbital absent in all specimens. Antorbital present but poorly developed in five specimens ( Fig. 2 ), absent in one specimen. Mouth terminal with lower jaw slightly shorter than upper jaw. Tip of maxilla elongate, posterior border reaching vertical through posterior border of pupil. Premaxillary teeth in single series, premaxilla with 20(3), 22(1), or 23(2) teeth. Maxilla with 21(1), 22(1), 23(1), 25(1), 26(1), or 30(1) teeth. Dentary with 26(1), 27(3), 28(1), or 29(1) teeth. Dentary teeth in single series, with few anterior teeth slightly displaced anteriorly. A conspicuous elongate foramen at the anterior portion of the dentary. All jaw teeth small, conical and lingually curved to a moderate extent ( Fig. 3 ).

Dorsal-fin rays ii,9*(43). Endoskeletal part of pectoral fin and some thin exoskeletal bones showing larval structure ( Fig. 4 ). Cartilaginous pectoral-radial plate with incomplete longitudinal middle fissure leaving upper and lower halves connected at base and tip; base articulating with vertically elongated scapulocoracoid cartilage and round distal margin with larval-like pectoral-fin fold supported solely by actinotrichia.

Pectoral-fin rays absent. All bones of endoskeletal pectoral girdle absent, exoskeletal part with posttemporal, supracleithrum, cleithrum and two postcleithra. Postcleithrum

3 poorly developed, slender and sinuous. Cleithrum with posteriorly directed, curved process immediately below ventral tip of supracleithrum. Pelvic-fin rays i,5*(47).

Posterior tip of pelvic fin falling short of origin of anal fin but extending slightly beyond vent. Anal-fin rays ii, 22(2), 23(8), 24(7), 25*(13), 26(13), or 27(2). Anal-fin margin concave with anterior elongate lobe formed by elongated fin rays and posterior section of short rays. Caudal-fin rays i,9,7,i (1), or i,9,8,i*(37), dorsal procurrent rays 8(2) or

9(4), ventral procurrent rays 6(3), 7(2), or 8(1). Caudal fin forked. Adipose fin absent.

Squamation present in almost all specimens, but scales highly deciduous and easily lost during handling. Scales cycloid, very thin, with no obvious circuli or radii. Scales in midlateral row 23(2), 24(6), 25(12), 26*(17), 27(4), or 28(3); no canal bearing lateral-

line scales on body. Scale rows between dorsal-fin origin and pelvic-fin origin 8(1),

9(7), or 10*(2). Scale rows around caudal peduncle 8(7), 9(1), or 10*(1). Predorsal scales typically absent with one or two scales just anterior to dorsal fin in few specimens. Scales restricted to base of caudal-fin rays, not covering caudal-fin lobes.

Total vertebrae 32(1) or 33(5); abdominal vertebrae 14(6); caudal vertebrae 18(1)

or 19(5). Total number of gill-rakers on first branchial arch 9(1) or 11(5), upper limb gill-rakers 1(1) or 3(5), lower limb gill-rakers 8(6). Weberian apparatus well-developed,

all components ossified except for claustrum (Fig. 5). Large gap between neural arches

3 and 4, with gap partially filled by dorsally projecting pointed process from vertebral centrum 3. Inner arm of suspensorium large, projecting forward to vertical through middle of second centrum. Supraneurals 5(5) or 6(1).

Coloration in alcohol. Overall ground coloration pale yellow ( Fig. 1A ). Scattered melanophores on dorsal portion of head in a few specimens. Guanine present in eye of most specimens, with melanophores on dorsal surface of eye. Scattered melanophores on bases of posteriormost anal-fin rays forming irregular line along posterior half of fin. Another irregular dark line extending along ventral margin of hypaxial myomeres posteriorly from vertical through seventh branched anal-fin ray. Both lines separated anteriorly but approaching each other posteriorly. A few scattered melanophores on bases of dorsalmost and ventralmost caudal-fin rays. All fins hyaline.

Coloration in life. Body mostly translucent ( Fig. 1B ). A few melanophores and xanthophores scattered along dorsal surface of vertebral column and on dorsal surface of swim bladder. Xanthophores on dorsal surface of head from vertical through middle of eye to posterior margin of skull. A thin line of chromatophores along anal-fin base, from vertical through 6 th branched ray to terminus of anal-fin base. Two faint blotches of xanthophores on bases of caudal-fin lobes, remaining caudal fin and other fins hyaline.

Eye silvery, dorsal half predominantly dark.

Sexual dimorphism. Two of 48 analysed specimens with hooks on anal-fin rays (12.4–13.7 mm SL), the larger specimen also with hooks on pelvic-fin rays ( Fig. 6 ). Hooks on anal fin located on posterior margins of posterior unbranched and three anterior most branched rays, typically one hook per segment. Five, seven, seven, and four hooks respectively on each anal-fin ray of larger specimen, two, four, two, one hooks on anal-fin rays of smaller one. Hooks of larger specimen well developed, especially along middle portion of each ray. Hooks of smaller specimen less developed. Hooks on pelvic fin of larger specimen not as developed and resembling bumps along branched rays 1–3 of contralateral fins, more developed on more lateral branched ray. Hooks on pelvic fin always along medial edge of rays. Smaller specimen without hooks on pelvic fin.

Molecular analysis. The final matrix comprised 26 terminals with 642 bp and 213 variable sites (33.2%). The nucleotide composition was 23.4% adenine, 17.8% guanine, 32.4% thymine, and 26.2% cytosine. The Iss values were lower than Iss.c values, indicating the absence of saturation. The maximum likelihood (ML) tree showed high bootstrap values supporting each of the analysed species ( Fig. 7 ). The convergence of initial and recursive partitions of the ABGD and ML solution of the PTP analysis delimited five species of Priocharax : P. varii , P. ariel , P. pygmaeus , P. nanus , and P. britzi ( Fig. 7 ; S 1 and S 2 ). The overall mean of genetic distances (K2P) among Priocharax species was 0.194±0.018. Intraspecific genetic distances ranged from 0.000 within P. pygmaeus to 0.002±0.001 within P. ariel and P. varii . The values of interspecific distances ranged from 0.179±0.021 between P. varii and P. nanus to 0.249±0.025 between P. pygmaeus and P. ariel ( Tab. 3).

Geographical distribution. Priocharax britzi is known only from its type locality in a marginal lake approximately 400 m from the rio Ipixuna , a tributary of the rio Purus ( Fig. 8 ). Specimens from a close locality in the main channel of the rio Ipixuna ( UFRO 15521 ) were examined morphologically and did not match the diagnostic characters of P. britzi (see discussion below). The Fig. 8 also shows the known distribution of all congeners .

Ecological notes. Specimens of Priocharax britzi were collected between 3 pm and 5 pm in a small lake approximately 15 m wide and 35 m long, apparently isolated from the main channel of the rio Ipixuna ( Fig. 9 ). The water level was low as sampling occurred during the dry season (September), so it is possible that this lake connects to the main channel in the wet season. The specimens were collected near the shore at depths varying from 0.5–1.0 m. Vegetation around the lake was composed mainly of dead logs and branches. At the time of sampling, the bottom was muddy with patches of leaf litter. Priocharax britzi was collected with the characiforms Carnegiella marthae Myers, 1927 , Hemigrammus sp. , Hemigrammus cf. gracilis (Lütken, 1875) , Hyphessobrycon rosaceus Durbin, 1909 , Iguanodectes spilurus (Günther, 1864) , Microcharacidium sp. , Nannostomus digrammus (Fowler, 1913) , Nannostomus eques Steindachner, 1867 , the catfish Amblydoras affinis (Kner, 1855) , the knifefishes Brachyhypopomus beebei (Schultz, 1944) , Microsternarchus bilineatus Fernández-Yépez, 1968 , the cichlids Apistogramma cf. pulchra Kullander, 1980 , Biotoecus opercularis (Steindachner, 1875) , and Crenicichla cyanotus Cope, 1870 .

Etymology. Priocharax britzi is named after Dr. Ralf Britz, noteworthy ichthyologist and a dear friend. Dr. Britz has mastered the world of small fishes and has described more than 20 miniature species, including two species of Priocharax . A noun in the genitive case.

Conservation status. Priocharax britzi was found in a single location, an isolated small lake near the road ( Fig. 8 ). The environmental conditions of the lake seem degraded, with depauperate riparian vegetation and silted substrate. Efforts to find specimens of the new species in the nearby rio Ipixuna and other close locations were not successful. However, the type locality is near a protected area (Floresta Nacional de Balata-Tufari), which was not sampled. Future studies should better investigate the possibility that P. britzi occuring in other localities within the protected area and along portions of the rio Ipixuna that may connect to the lake during the wet season. Furthermore, there are no known imminent threats that would put the species at risk of extinction. Hence, we suggest that P. britzi should be classified as Least Concern (LC) according to International Union for Conservation of Nature (IUCN) categories and criteria (IUCN Standards and Petitions Subcommittee, 2019) pending further information on its real distribution.

Figures

FIGURE 1 | Priocharax britzi, A. Holotype, male, MZUSP 125822, 13.7 mm SL; Brazil, Amazonas, Canutama, small lake approximately 400 m from the rio Ipixuna, after bridge crossing this river, on the right side of the Transamazônica Road (BR–230) going from Humaitá to Lábrea, rio Ipixuna drainage. B. Live paratype photographed immediately after capture.

FIGURE 2 | Priocharax britzi, paratypes, MZUSP 125823, c&s; anterior region of head in lateral view detailing the orbital region. A. 12.5 mm SL. B. 12.9 mm SL, image flipped. Antorbital and infraorbitals delimited by black line for clarity. Ant = antorbital; IO2 – 3 = infraorbitals 2 – 3. Scale bar = 0.2 mm.

FIGURE 3 | Priocharax britzi, paratype, MZUSP 125823, 12.9 mm SL, c&s; jaws in lateral view. Ana = anguloarticular; Cm = coronomeckelian; De = dentary; MC = Meckel’s cartilage; Mx = maxilla; Pmx = premaxilla; Ra = retroarticular. Scale bar = 0.2 mm.

FIGURE 4 | Shoulder girdle in lateral view of A. Priocharax britzi, paratype, MZUSP 125823, 12.9 mm SL, c&s. Image flipped. B. Priocharax cf. ariel, UFRO 15521, 11.8 mm SL, c&s. Cl = cleithrum; PecRdC = pectoral-fin radial cartilage; FIGURE 5 | Priocharax britzi, paratype, MZUSP 125823, 12.9 Pcl = unidentified postcleithrum; Pcl3 = postcleithrum 3; Pt = posttemporal; mm SL, c&s; Weberian apparatus in lateral view. Exoc = ScCoC = scapulocoracoid cartilage; Sucl = supracleithrum. Arrows point exoccipital; Int = intercalarium; NA3 – 4 = neural arches to middle fissure on pectoral-fin radial cartilage. Pcl3 slightly displaced 3 – 4; NS4 = neural spine 4; OsS = os suspensorium; Sca = ventrally from original position in A. Scale bars = 0.5 mm. scaphium; Sn3 = supraneural 3; Soc = supraoccipital; T = tripus. Scale bar = 0.2 mm.

FIGURE 6 | Priocharax britzi, holotype, MZUSP 125822, 13.7 mm SL. A. Left lateral view of anal fin showing bony hooks. B. Ventral view of pelvic fin showing bony hooks. Scale bars = 0.5 mm.

FIGURE 7 | Maximum likelihood tree of five species of Priocharax, based on the COI gene (642 bp). Bars represent the number of species obtained by the ABGD and bPTP analyses. Numbers near nodes represent bootstrap values.

FIGURE 8 | Map of South America with a close-up of the upper and central portions of the Amazon basin, Brazil, illustrating the distribution of the five species of Priocharax: P. britzi described herein from the rio Purus basin (green losangle), P. varii (red dot), P. ariel (black dots), P. nanus (blue dot), and P. pygmaeus (yellow dots). Colors are the same as in the Fig. 7. Some dots may represent more than one lot.

FIGURE 9 | Small lake approximately 400 m from the rio Ipixuna, Canutama, Amazonas, Brazil, the type locality of Priocharax britzi.