Priocharax nanus, Toledo-Piza & Mattox & Britz, 2014

Toledo-Piza, Mônica, Mattox, George M. T. & Britz, Ralf, 2014, Priocharax nanus, a new miniature characid from the rio Negro, Amazon basin (Ostariophysi: Characiformes), with an updated list of miniature Neotropical freshwater fishes, Neotropical Ichthyology 12 (2), pp. 229-246 : 231-233

Priocharax nanus , new species

Figs. 1-4

Holotype. MZUSP 114014 , 13.8 mm SL, Brazil, Amazonas , Santa Isabel do Rio Negro , rio Negro basin, lake at right bank of rio Urubaxi, near igarapé Tapage, 0º33’44.2”S 64º49’40.8”W, 26 Oct 2011, M. Toledo-Piza, G. Mattox, M. Marinho & R. Britz.

Paratypes. MZUSP 114015 , 9 , 12.1-15.3 mm SL (3 c&s, 14.1-15.3 mm SL), same data as holotype . MZUSP 114016 , 5 , 12.6-14.6 mm SL (2 c&s, 13.4-13.8 mm SL), Brazil, Amazonas , Santa Isabel do Rio Negro , rio Negro basin, igarapé Tapage at left bank of rio Urubaxi, 0º30’5.3”S 64º49’11.7”W, 26 Oct 2011, M. Toledo-Piza, G. Mattox, M. Marinho & R. Britz. MZUSP 114017 , 3 , 13.5-14.6 mm SL (1 c&s, 14.6 mm SL), Brazil, Amazonas , Santa Isabel do Rio Negro , rio Negro basin, first tributary of rio Negro above rio Daraá, 0º27’24.2”S 64º46’54.1”W, 27 Oct 2011, M. Toledo-Piza, G. Mattox, M. Marinho & R. Britz. INPA 39891 , 4 , 12.5-13.9 mm SL ; MZUSP 114018 , 11 , 11.1-15.4 mm SL (5 c&s, 12.0-14.0 mm SL) ; USNM 427007 , 4 , 12.1-13.3 mm SL; Brazil, Amazonas , Santa Isabel do Rio Negro , rio Negro basin, rio Negro and tributaries near Santa Isabel do Rio Negro, 23-30 Oct 2011, M. Toledo-Piza, G. Mattox, M. Marinho & R. Britz.

Diagnosis. Priocharax nanus is distinguished from P. ariel and P. pygmaeus by the presence of i,6 pelvic-fin rays (vs. i,5), the presence of the claustrum (vs. claustrum absent) and the presence of two postcleithra (versus postcleithra absent). Priocharax nanus can be further distinguished from P. ariel by the lower number of gill rakers on the lower limb of the first branchial arch (9-10, n=11 vs. 11-13) and by the relatively shorter caudal peduncle (13.5-16.8 % SL vs. 18.1- 23.7 % SL). Although there is some overlap between the species, Priocharax nanus has a higher number of branched anal-fin rays compared to P. ariel (21-26, mean = 22.5, n = 36 vs. 16-21, mean = 18.5, n = 96 respectively).

Description. For overall appearance see Figure 1 . Morphometric data provided in Table 1 .

Body laterally compressed. Greatest body depth at vertical through dorsal-fin origin. Dorsal-fin origin approximately at midbody, at vertical through anal-fin origin. Pelvic-fin origin approximately midway between posterior margin of opercle and anal-fin origin. Dorsal profile of head and body gently convex from tip of snout to dorsal-fin origin. Dorsal profile of body along dorsal-fin base nearly straight, gently sloping posteroventrally; straight and posteroventrally inclined from latter point to caudal peduncle. Dorsal profile of caudal peduncle gently concave to base of dorsal procurrent rays. Ventral profile of head and body gently 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 inclined from pelvic-fin to anal-fin origin; straight and posterodorsally slanted along anterior one-half of anal-fin base, gently concave from latter point to base of ventral procurrent rays. Caudal peduncle elongate. Pseudotympanum present, located anterior to rib of fifth vertebra.

Snout blunt in lateral view. Eye about one-third of head length. Infraorbitals 1 to 6 and supraorbital absent, antorbital present. Mouth terminal with lower jaw slightly included. Tip of maxilla elongate, posterior border reaching vertical through posterior border of pupil. Premaxillary teeth in single series with 23(2), 24(4), 25(2), 27(1), or 29(2) teeth. Maxilla with 32(1), 33(1), 34(1), 35(2), 36(2), 37(1), 38(1), 39(1), or 41(1) teeth. Dentary with 33(2), 34(1), 35(2), 36(1), 38(2), 39(2), or 40(1) teeth. Dentary teeth in single series, with few anterior teeth slightly displaced anteriorly.All jaw teeth small, conical and lingually curved to a moderate extent ( Fig. 2 ).

Dorsal-fin rays ii.8(2) or 9*(35). Pectoral fin with larval structure ( Fig. 3 ). Cartilaginous pectoral radial plate incompletely divided longitudinally, articulating anteriorly with vertically elongated scapulocoracoid cartilage and posteriorly with larval-like pectoral-fin fold supported only by actinotrichia. Pectoral-fin rays absent. Endoskeletal bones of pectoral girdle absent, exoskeletal part with posttemporal, supracleithrum, cleithrum and two postcleithra. Cleithrum with posteriorly directed process at region immediately below ventral tip of supracleithrum. Pelvic-fin rays i,6* in all specimens (n=37). Posterior tip of pelvic fin falling short of origin of anal fin but extending slightly beyond anus. Analfin rays iv-v, 21(7), 22*(13), 23(12), 24(3), 25(1), or 26(1). Anal-fin margin concave with anterior elongate lobe and posterior section of short rays. Caudal-fin rays i,9,8,i (16), dorsal procurrent rays 8 (8) or 9 (3), ventral procurrent rays 6 (4) or 7 (7). 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 28(1), 29(2), 30(2), 31(1), or 32(1); no canal bearing lateral-line scales on body. Scale rows between dorsal-fin origin and pelvic-fin origin 7(1) or 8(7). Scale rows around caudal peduncle 9(4) or 10(2). 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(2), 33(7), or 34(2); abdominal vertebrae 14(10) or 15(1); caudal vertebrae 18(3), 19(6), or 20(2). Upper limb gill-rakers 3(7) or 4(4), lower limb gill-rakers 9(6) or 10(5). Weberian apparatus well developed, all components ossified. Claustrum present as tiny, circular bone ( Fig. 4 ). Large gap present between neural arches 3 and 4, with gap partially filled by dorsally projecting pointed process from vertebral centrum 3. Inner arm of os suspensorium large, projecting forward to vertical through middle of second centrum.

Color in alcohol. Overall ground color pale yellow ( Fig. 1a ). Patch of dark chromatophores present on dorsal portion of head and scattered dark chromatophores on opercle. Head with two dark stripes radiating from eye, one anteriorly to tip of snout and another ventrally. Line of dark chromatophores extends along dentary and on anterior tip of lower jaw. Iridophores present in orbit of some specimens. Longitudinal line of dark chromatophores along midlateral side of body. Triangular patch of dark chromatophores at base of caudal fin forming inconspicuous spot. Scattered dark chromatophores on posterior half of body, probably remnants of chevron-shaped dark thin lines present in live specimens. Dark chromatophores along predorsal midline forming two incomplete separate lines. Bases of anal-fin rays dark and forming irregular line along fin. Line of dark, more deeply located chromatophores slightly dorsal to base of anal-fin rays and also extending along fin base. Another dark line, dorsal and more superficial than latter, extending posteriorly from vertical through third to fourth branched anal-fin ray. These three lines more evident and better separated anteriorly and merging posteriorly. Three patches of dark chromatophores ventrally on body anterior to pelvic fin. Posteriormost patch elongated and located anterior to basipterygium, middle one more rounded and located at point of contact of contralateral pectoral girdles, anteriormost in form of a small spot on isthmus. Few dark chromatophores present in region around anus. Dark chromatophores at origins of dorsal, pelvic, and anal fins.All fins except pectoral with scattered dark chromatophores along borders of fin rays.

Color in life. Body mostly transparent ( Fig. 1b ). Pattern of distribution of dark chromatophores on head, along lateral sides of body, on caudal peduncle, and on all fins except pectoral as described above for color in alcohol. Dark chromatophores also scattered on dorsal surface of swim bladder and along anterior half of vertebral column. Approximately 10 vertical dark narrow bars along body from vertical through posterior margin of opercle to vertical through tip of posteriormost anal-fin ray, more or less evenly spaced and in a chevron-shaped pattern. Most narrow bars extend from dorsal to ventral margins of body, occasionally incomplete. Each bar W-shaped, following course of myoseptum. Numerous, tiny bright orange spots scattered over entire head and body, frequently forming longitudinal lines along anterior predorsal line and dorsal-fin base, anal-fin base and vertebral column. Patch of similar orange spots on dorsal surface of swim bladder and base of caudal fin. Orange spots forming approximately five vertical lines along caudal-fin rays, anterior lines better defined than more diffuse posterior lines. Orange spots scattered mainly along anterior four or five dorsal-fin rays. Iridophores covering swim bladder dorsally. Eye silvery, dorsal margin with dark and orange chromatophores.

Sexual dimorphism. Gonads not checked. Hooks absent in dorsal-, pectoral-, pelvic-, and anal-fins of all examined specimens (n=37).

Geographic distribution. Priocharax nanus is presently known from the rio Negro basin, Amazonas , Brazil ( Fig. 5 ), in the surroundings of Santa Isabel do Rio Negro. The type locality near igarapé Tapaje is located in the rio Urubaxi basin, near its confluence with the rio Negro ( Fig. 6 ). The new species was also collected from two other localities: one near the type locality in the rio Urubaxi , a right bank tributary of rio Negro and the other in a tributary of the left bank of the rio Negro. Specimens from a fourth locality also located in a tributary of the left margin of the rio Negro were only recorded from photographs. This locality ( Igarapé Tibarrá , approximately 300 m above confluence with rio Negro, 0º26’28.4”S 64º56’57.5”W) the western most point in the map on Fig. 5 , is the nearest to Santa Isabel do Rio Negro .

Ecological notes. All specimens were collected between 9:00 and 17:00h, during the dry season (October), in the black acidic waters of the rio Negro basin. Three of the four localities were in shaded areas, close to the shore line where there was emergent and marginal vegetation. In the latter case trunks and branches were partially submerged ( Fig. 6 ). Specimens were caught with dip nets around the submerged vegetation, at depths of approximately 1 m or less. In the other locality, located in the first tributary of the rio Negro above rio Daraá (0º27’24.2”S 64º46’54), the vegetation on the river bank had been recently burnt and some newly grown submerged and emergent vegetation was present along with scattered tree trunks. The specimens were collected from an area exposed to the sun in warm, shallow water, approximately 50 cm deep.

Etymology. The species name is derived from the Latin, nanus , meaning a dwarf and alludes to the tiny size of adult specimens of the species. A noun in apposition.

Figures

Fig. 1. Priocharax nanus, (a) holotype, MZUSP 114014, 13.8 mm SL; Brazil, Amazonas, Santa Isabel do Rio Negro, rio Negro basin, lake at right bank of rio Urubaxi, near igarapé Tapage; (b) live specimen, one of the paratypes, photographed right after capture. Photograph by Ralf Britz.

Fig. 2. Priocharax nanus, paratype, MZUSP 114018, 13.2 mm SL, c&s; jaws in lateral view illustrating dentition. Ana = anguloarticular; Cm = coronomeckelian; De = dentary; MC = Meckel’s cartilage; Mx = maxilla; Pmx = premaxilla; Ra = retroarticular. Scale bar = 0.5 mm.

Fig. 3. Priocharax nanus, paratype, MZUSP 114015, 14.9 mm SL, c&s; shoulder girdle in lateral view. Cl = cleithrum; PecRdC = pectoral-fin radial cartilage; Pcl = unidentified postcleithrum; Pcl3 = postcleithrum 3; Pt = posttemporal; ScCoC = scapulocoracoid cartilage; Sucl = supracleithrum. Arrow points to middle incision on pectoral-fin radial cartilage. Scale bar = 0.5 mm.

Fig. 4. Priocharax nanus, paratype, MZUSP 114015, 14.9 mm SL, c&s; Weberian apparatus in lateral view. Cla = claustrum; Exoc = Exoccipital; Int = intercalarium; NA3-4 = neural archs3-4; NS4 = neural spine 4; OsS = os suspensorium; Sca = scaphium; Sn3 = supraneural 3; Soc = supraoccipital; T = tripus. Scale bar = 0.2 mm.

Fig. 5. Map of northern South America and surroundings of Santa Isabel do Rio Negro, rio Negro basin (detail), showing the distribution of Priocharax nanus. Type locality (red dot), other localities (yellow dots).

Fig. 6. Lake at right bank of rio Urubaxi, near igarapé Tapage, rio Negro, the type locality of Priocharax nanus. Photograph by Manoela M. F. Marinho.