Priocharax rex Mattox, Acosta-Santos, Gregory, Agudelo & Lima, 2025, new species
Creators: name: Mattox, George Mendes Taliaferro
name: Acosta-Santos, Astrid
name: Bogotá- Gregory, Juan D.
name: Agudelo, Edwin
name: Lima, Flávio C. T.
Publisher: name: Zenodo
Year: 2025
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Characiformes
Characidae
Priocharax
Priocharax rex

description: Priocharax rex, new species
(Figs. 1–4)
Holotype: CIACOL 7935 (19.8 mm SL), Colombia, Departamento Amazonas, Corregimiento San Rafael, forest stream (caño) tributary of río Caraparaná, río Putumayo drainage, 1º41’3’’S 73°14’15’’W; April 2022, A. Acosta-Santos & E. Agudelo.
Paratypes: All from Colombia, Departamento Amazonas. CIACOL 5845 (3, 16.1–17.1 mm SL), Corregimiento El Encanto mouth of forest stream (caño) El Arador, río Putumayo drainage, 1°44’44.5’’S 73°12’29.70’’W; April 2022, A. Acosta-Santos & E. Agudelo; CIACOL 5896 (1, 15.9 mm SL), Corregimiento El Encanto, forest stream (“caño”) Ziringa-Jizɨraye, río Putumayo drainage, 1°44’38.8’’S 73°11’10.10’’W; April 2022, A. Acosta-Santos & E. Agudelo; CIACOL 7936 (6, 19.1–20.7 mm SL, 2 c&s, 19.2–19.7 mm SL); CZUT-IC 27943 (2, 18.3–19.7 mm SL, 1 c&s, 19.0 mm SL); ZUEC 18315 (2, 17.8–19.9 mm SL, 1 c&s, 19.9 mm SL), collected with holotype.
Non-types: CIACOL 5880 (4, 17.0– 19.7 mm SL), Colombia, Departamento Amazonas, El Encanto village, forest stream (“caño”) near airfield, 1º45’20’’S 73º11’29’’W; 4 Dec 2022, A. Acosta-Santos & E. Agudelo.
Diagnosis. Priocharax rex is distinguished from all congeners by the presence of a round flap of skin between contralateral pelvic-fins bases (vs. absence). Priocharax rex is further distinguished from all congeners except P. nanus, P. pygmaeus, and P. varii by the absence of infraorbitals (vs. presence of infraorbitals, Ios 1–3 in P. ariel and P. conwayi; presence of a single infraorbital, Io 2, in P. britzi and P. marupiara; presence of two infraorbitals, Ios 1+2, in P. marupiara, P. phasma and P. toledopizae). Priocharax rex can be distinguished from P. britzi, P. nanus, P. pygmaeus and P. varii by having fewer premaxillary teeth (14–19 vs. 20–29), from P. conwayi, P. marupiara and P. phasma by more maxillary teeth (30–38 vs. 13–29). The presence of two postcleithra distinguishes P. rex from P. ariel, P. phasma, and P. conwayi (vs. one), and from P. pygmaeus (vs. absence). Priocharax rex can be further distinguished from P. nanus and P. varii by having five branched pelvic-fin rays (vs. six), and from P. varii by the absence of adipose fin (vs. presence).
Description. For overall appearance, see Fig. 1. Morphometric data presented in Table 1. Body laterally compressed and elongated, greatest depth at vertical through pelvic-fin in small specimens, at vertical through dorsal-fin origin in larger specimens. Dorsal-fin origin approximately at midbody, at vertical through vent and slightly anterior to anal-fin origin. Pectoral-fin bud at vertical through anterior portion of pseudotympanum. Pelvic-fin origin slightly posterior to vertical at 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; slope less conspicuous from latter point to caudal peduncle. Dorsal profile of caudal peduncle straight 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 anal-fin origin. Ventral profile of body slightly concave and posterodorsally rising along anal-fin base, slightly concave from end of anal-fin base to origin of ventral procurrent rays. Round skin flap between contralateral pelvic-fin bases (Fig. 2). Caudal peduncle short and relatively robust. Pseudotympanum large, located anterior to rib of fifth vertebra.
Snout round in lateral view. Eye diameter about one-third of head length. Antorbital present (n=4) (Fig. 3A). Infraorbitals 1 to 6 and supraorbital absent (n=4). Mouth terminal. Tip of maxilla elongate, posterior border reaching vertical through midway between middle and posterior border of eye, but closer to middle of eye in largest males. Premaxillary teeth in single series, premaxilla with 14 (1), 16(1), or 19(2) teeth. Maxilla with 30(3), or 38(1) teeth, in single series. Dentary with 26(1), 28(1), 35(1), or 39(1) teeth. Dentary teeth in single series, with few anterior teeth slightly displaced from series anteriorly. A conspicuous, elongate foramen at anterior portion of the dentary. All jaw teeth small, conical and curved lingually to a moderate extent (Fig. 3B).
Dorsal-fin rays ii,9*(15). Endoskeletal part of pectoral fin and some thin exoskeletal bones of the pectoral girdle showing larval structure (Fig. 3C). 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, with postcleithrum 3 slender and forming a sinuous arch. Cleithrum with posteriorly directed, curved process immediately below ventral tip of supracleithrum. Pelvic-fin rays i,5*(15). Posterior tip of pelvic fin on vertical through vent in females and extending further posterior slightly beyond anal-fin origin in males. Anal-fin rays ii,18(3), ii,19(2), ii,20(4), ii,21*(3), ii,22(2) or ii,23(1). Anal-fin margin concave with anterior lobe formed by elongated fin rays and posterior section of short rays. Principal caudal-fin rays i,8,8(1) or i.,9.8.i*(14), dorsal procurrent rays 8(2) or 9(2), ventral procurrent rays 6(1) or 7(3). 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 20(1), 21(2), 22*(2), 23(4), 25(2) or 27(1); no canal-bearing lateral-line scales. Scale rows between dorsal-fin origin and pelvic-fin origin 8(7) or 10*(3). Scale rows around caudal peduncle 10*(10). Predorsal scales typically absent but occasionally one or two scales present immediately anterior to dorsal fin. Caudal-fin squamation restricted to base of caudal-fin rays, no scales on caudal-fin lobes.
Total vertebrae 32(1) or 33(3), with 14(4) abdominal vertebrae and 18(1) or 19(3) caudal vertebrae. Total number of gill-rakers on first branchial arch 12(2) or 13(2), with 2(1), 3(2) or 4(1) gill-rakers on upper limb, and 9(3) or 11(1) gill-rakers on lower limb. Weberian apparatus well-developed, all components ossified including claustrum (n=4) (Fig. 3D). Large gap between neural arches 3 and 4, with gap partially covered by dorsally projecting pointed process from vertebra 3. Inner arm of os suspensorium large, projecting forward to vertical through middle of second centrum. Supraneurals 5(4).
Colour in alcohol. Overall body colour pale beige (Fig. 1). Melanophores scattered on dorsal surface of head in region of brain. Horizontal blotch composed by a few melanophores from tip of snout to anterior margin of orbital cavity in some specimens. A conspicuous group of melanophores around nostrils. Thin line of three to four melanophores along anterolateral margin of dentary, approximately on region of large dentary foramen. Horizontal line of 3–4 superficial melanophores along dorsal portion of lateral surface of opercle. Superficial melanophores on lateral surfaces of isthmus inside branchial chamber, and a single melanophore on ventral surface of body just posterior to isthmus. Thin line of melanophores along posteroventral margin of orbital cavity. Eye black around pupil. Overall coloration of eye black with faint silver of guanine. Line of superficial melanophores along pelvic-fin base. Deep inconspicuous blotches of melanophores on lateral surface of abdominal cavity. Irregular thin line of large melanophores along dorsal-fin base. A few specimens with irregular line of large melanophores along dorsal profile of body, scarcer on predorsal area. Lines of melanophores paired posterior to dorsal fin, with transversal lines of small melanophores connecting contralateral lines in some specimens. Thin line of small both deep and superficial melanophores along lateral surface of body, irregular anterior to dorsal fin origin, more regular from that point towards caudal peduncle. Thin line of scattered melanophores along entire anal-fin base, and another thicker curved line of melanophores along ventral margin of hypaxial myomeres starting at vertical through fourth to fifth anal-fin ray, approaching thin line of anal-fin base posteriorly. Both lines continuing posteriorly to caudal-fin anteriormost ventral procurrent rays. Two or three isolated melanophores along midlateral surface of caudal peduncle. Scattered melanophores forming irregular triangle on lateral surface of hypural plate. Fins mostly hyaline, with small dark patch on first rays of dorsal- and anal-fin bases, and on dorsal and ventral procurrent caudal-fin rays. Dorsal, anal and caudal fins with scattered melanophores along rays.
Sexual dimorphism. Hooks present on anal- and pelvic-fin rays of mature males as small as 15.9 mm SL (Fig. 4). Hooks better developed on pelvic fin. Hooks on anal fin present in longest unbranched and subsequent branched rays up to third ray, one pair of hooks on posterior edge of six to nine distal segments of rays.Approximately six to ten small hooks on medial margin of the three lateralmost branched pelvic-fin rays in all mature males (n=4). All mature male specimens (15.9–18.3 mm SL) with a horizontal slit along the dorsal portion of the pelvic-fin musculature. Overall size of pelvic girdle in mature males larger than that of females, pelvic bone restricted posteriorly to level of rib of sixth vertebra in females, but reaching further anteriorly (e.g., halfway between ribs of fifth and sixth vertebra, or rib of fifth vertebra) in mature males. Basipterygium of males better ossified, more robust, and closer to contralateral part than in females. General colour pattern more pronounced in males than in females.
Distribution. Priocharax rex is only known from small tributaries in the middle río Putumayo system, Departamento Amazonas, Colombia, near the border with Peru (Fig. 5).
Ecological notes. Priocharax rex is recorded from small, shallow (less than 1 meter deep), slow-flowing terrafirme dark water streams, with channel width less than 15 meters at its widest point (Fig. 6). At the time of sampling the water temperature was 25.6°C, conductivity 34 µS/cm, pH 5.3, and dissolved oxygen 2.03 mg /l. The substrate of the sampled section was composed mainly by silt and clay, with some leaf litter, and logs. All specimens were collected at the marginal vegetation.
Etymology. The specific name rex refers to the Latin word for king, in allusion to the large size of this species when compared with congeners. A noun in apposition.
Discussion. With the description of Priocharax rex, Priocharax now has ten valid species. Priocharax rex is so far only known from a few localities in the middle portion of the río Putumayo basin in Colombia. Consequently, it follows a similar pattern to most species of Priocharax, which are geographically restricted to a single or few locations (Mattox et al., 2020; 2021; 2023; 2024), making them potentially vulnerable to threats caused by anthropogenic impacts.
Achieving a size of 20.7 mm SL (CIACOL 7936), Priocharax rex is the largest known species of the genus so far. The species presenting the larger specimens reported previously were P. conwayi (19.0 mm SL), P. toledopizae (18.8 mm SL), P. marupiara (17.2 mm SL), and P. ariel (17.1 mm SL) (Mattox et al., 2023, 2024; Weitzman & Vari, 1987). However, P. rex is not noticeably distinct from its congeners regarding paedomorphisms.
The round skin flap, the main diagnostic feature of Priocharax rex, is so far unique to this species among valid species of Priocharax and as far as we know, no other characiform has such a structure. It is a loose skin flap between contralateral pelvic fins, attached only by its base and covering the urogenital pore (Fig. 2), present in all specimens (n=15), males and females, which indicates that it probably plays no role in sexual dimorphism. Apparently, there is no musculature that would move the skin flap, but further dissection and histological sections are necessary to elucidate the nature of this flap. Its function is beyond the scope of the present paper.
Another interesting feature noted herein concerns the sexual dimorphism.All mature males of P. rex (n=4) possess a similar unusual sexual dimorphism as mature males of P. toledopizae (Mattox et al., 2023) and of P. conwayi and P. phasma (Mattox et al., 2024), consisting of a horizontal slit along the pelvic-fin musculature separating it from the main hypaxial muscle mass of the body. Cases of conspicuous sexual dimorphism are common in heterochronic miniaturized lineages (e.g., Conway & Britz, 2007; Britz & Conway, 2016). The functional role of this character is unknown and its phylogenetic significance will be explored in the future.
description: Published as part of Mattox, George Mendes Taliaferro, Acosta-Santos, Astrid, Bogotá- Gregory, Juan D., Agudelo, Edwin & Lima, Flávio C. T., 2025, A new miniature Priocharax from the río Putumayo drainage, Amazonas State, Colombia (Teleostei: Characiformes: Characidae) with an unusual skin flap between pelvic fins, pp. 545-554 in Zootaxa 5575 (4) on pages 546-553, DOI: 10.11646/zootaxa.5575.4.4, http://zenodo.org/record/14762751