Chrysobrycon ximango, Silva-Oliveira & Vanegas-Ríos & Canto & Ribeiro, 2025

Chrysobrycon ximango , new species urn:lsid:zoobank.org:act:

( Figs. 1–4; Tab. 1)

Holotype. UFOPA-I 1375 , 41.2 mm SL, male, Brazil, Pará State, Alenquer, igarapé Visão 2, a tributary of rio Mamiá , rio Curuá basin, 01°32’36.73”S 55°11’38.49”W, 16 Jan 2020, A. L. C. Canto, F. Ribeiro, S. Silva, M. Lima & E. Barbosa. GoogleMaps

Paratypes. All from Brazil, Pará State, Alenquer, rio Mamiá , rio Curuá basin: UFOPA-I 1376 , 6 (3 alc., 3 males c&s), 40.5–45.7 mm SL; collected with the holotype. INPA 61047 View Materials , 4 View Materials , 31.9–42.7 mm SL, unnamed stream, 01°33’51.17”S 55°5’19.38”W, 15 Jan 2020, A. L. C. Canto, M. Lima & M. Barbosa. MLP-Ict 11736, 4, 31.9–44.5 mm SL, igarapé Mandioca, 01°33’51.17”S 55°5’19.38”W, 10 Jul 2021, A. L. C. Canto, M. Lima & M. Barbosa. UFOPA-I 1377 , 20 , 21.7 –35.0 mm SL, igarapé Visão 2, 10 Jul 2021, A. L. C. Canto, F. Ribeiro, S. Silva, M. Lima & M. Barbosa GoogleMaps .

Diagnosis. Chrysobrycon ximango is differentiated from most congeners, except C. calamar , C. eliasi , and C. guahibo , by having a reduced or absent fourth infraorbital ( vs. well-developed infraorbital series, with fourth infraorbital extending completely between third and fourth infraorbitals; Fig. 2). It differs from C. calamar in the total number of the gill rakers on the first branchial arch (14–16 vs. 17–20). From C. eliasi , as well as C. myersi and C. yoliae , by the number of circumpeduncular scales (13–14 vs. 15–16 in C. eliasi and C. yoliae , and 17–19 in C. myersi ).

In addition, C. ximango is distinguished from C. guahibo , C. mojicai and C. yoliae by having 15–19 dentary teeth ( vs. 8–14 in C. guahibo and 20–27 in C. mojicai and C. yoliae ) and 4–7 maxillary teeth ( vs. 1–3 in C. guahibo , 9–17 in C. mojicai , and 9–16 in C. yoliae ). Chrysobrycon ximango is distinguished from C. hesperus and C. myersi by having a terminal lateral-line tube between the caudal-fin rays 10 and 11 ( vs. absence of this tube) and by possessing a greater number of neural spines between the posteriormost supraneural and the anteriormost dorsal-fin pterygiophore (4–5 vs. 2–3). Chrysobrycon ximango can be also separated from C. hesperus by the maximum number of cusps in the maxillary teeth (tricuspid vs. pentacuspid teeth) and from C. myersi by the fewer branched anal-fin rays (26–28 vs. 33–39). From C. eliasi , C. guahibo , C. mojicai and C. yoliae by the presence of a clear or fully depigmented area located posterior to the humeral blotch, which vertically separates it from the longitudinal band of chromatophores along the body flanks ( vs. humeral blotch bordered posteriorly by dark chromatophores, often forming a continuous set with longitudinal band).

Description. Morphometric data in Tab. 1. Body laterally compressed, maximum depth at vertical through area closer to anal-fin origins ( Fig. 1). Dorsal profile of head slightly convex from upper jaw to anterior nostril and straight from that point to supraoccipital spine. Dorsal profile of body slightly convex from supraoccipital to dorsal-fin origin; straight to slightly concave from first dorsal-fin ray to adipose-fin origin and slightly concave from that point to anteriormost dorsal procurrent caudal-fin rays. Ventral profile of body convex from tip of lower lip to pelvic-fin origin, straight or slightly convex between pelvic and anal-fin origins, straight or slightly concave and slanting dorsally from this point to caudal peduncle. Dorsal and ventral profile of caudal peduncle slightly concave.

Mouth superior, lower jaw more anterior than upper jaw. Posterior extension of maxilla not extending beyond anterior margin of orbit. Third infraorbital well-developed, reaching preopercle ventrally; fourth infraorbital reduced or absent ( Fig. 2). Premaxillary teeth in two rows; outer row with 3(20) or 4*(5), usually tricuspid, rarely conical; central cusp when present longest; inner row with 4*(22) or 5(3) pentacuspid teeth, central cusp longest (rarely posteriormost tooth tricuspid). Maxilla with 4(7), 5*(12), or 6(3), tricuspid teeth. Dentary with 4*(25) large penta to tricuspid teeth, followed by 11(1), 12*(5), 13(11), 14(7) or 15(2) smaller, conical teeth ( Fig. 3).

Dorsal-fin rays ii,8*(30); first unbranched ray about one-half length of second unbranched ray. Dorsal-fin origin posterior to midbody, at vertical between 7 th and 9 th anal-fin branched rays. Dorsal-fin posterior margin slightly convex. Adipose-fin origin at vertical crossing posteriormost anal-fin ray. Pectoral-fin rays i,9*(11), i,10(10) or i,11(3). Anal-fin origin at posterior half of body, anterior to vertical through dorsal-fin origin. Anal-fin distal margin straight to slightly concave, with last unbranched ray and first three branched rays slightly longer than remaining rays. Pectoral-fin distal tip reaching one-quarter to one-half of pelvic-fin length ( Fig. 1). Pelvic-fin rays i,6,i*(30). Pelvic-fin origin anterior to half of body. Anal-fin rays iv(3), 27(3), 28(14), 29*(10) or 30(2). Principal caudal-fin rays 10+9*(30). Caudal fin forked, with lobes of equal size, slightly pointed. Dorsal procurrent caudal-fin rays 8(1) or 9(2), ventral procurrent caudal-fin rays 9(3).

Scales cycloid, moderately large, with many well-marked radii (8–12); circuli only present anteriorly. Lateral line complete with 43*(8), 44(17) or 45(5) scales. Terminal lateral-line tube present on caudal-fin interradial membrane. Predorsal scales 21(5), 22(12), 23*(11) or 24(1), forming continuous row ( three specimens with anteriormost scales misaligned). Scale rows between dorsal fin and lateral line 5(5) or 6*(25). Scale rows between lateral line and pelvic fin 5*(26) or 6(4). Circumpeduncular scales 13(18) or 14(12). Single series of scales (15–16*) covering proximal third of anal-fin base up to 17 th branched ray. Total number of vertebrae 41(3 c&s), 17 precaudal and 24 caudal. Supraneurals 11(2) or 12(1). Gill rakers on first gill arch 14(6), 15(5) or 16(2): 2(8) or 3(5) on hypobranchial, 9(9) or 10(4) on ceratobranchial, and 4(6) or 5(7) on epibranchial.

Coloration in alcohol. Overall color of body pale yellow to pale brown. Infraorbitals, maxillary, ventral half of opercle, and gular region silvery. Lower jaw, snout, anterior portion of maxilla, dorsal region of head, and middorsal region of body with high concentration of chromatophores. Scales above midlateral line, at dorsolateral portion of body bordered by dense concentration of small dark chromatophores, forming reticulated pattern. Humeral blotch present, vertically elongated, extending horizontally over three rows of scales and vertically over four rows of scales, surpassing the lateral line ventrally; formed by relatively large, densely concentrated dark chromatophores; frequently less developed vertically or somewhat rounded in female specimens. Margins of humeral blotch slightly blurred, delimited anteriorly and posteriorly by a clear area separating it from chromatophores on body flanks. Thin,

dark midlateral line, extending from near vertical through pelvic-fin origin to caudal peduncle. Mid-ventral region, from 6 th or 7 th pored scale to caudal peduncle, with large, scattered black chromatophores, especially over midline and caudal peduncle. Peritoneal region without melanophores. Dorsal fin with chromatophores more concentrated along interradial membranes. Adipose fin with few scattered chromatophores. Hyaline pectoral and pelvic fins, with few scattered chromatophores. Anal fin slightly dusky, with chromatophores on interradial membranes. Caudal fin with melanophores concentrated in the basal portion of the central rays.

Coloration in life. Based on freshly captured specimens. Overall body coloration bluish to silvery. Opercle, infraorbital bones, eye, posteromedial region of maxilla and ventral region of head silvery. Adipose-fin yellow. Dorsal, pectoral, pelvic and anal fins similar to preserved specimens.

Sexual dimorphism. Adult males ( 40.6–42.2 mm SL) possess bony hooks on the pelvic-, anal-, and caudal-fin rays. All pelvic-fin rays bear short slender hooks, positioned anteroventrally along almost the entire length of rays; 5–7 hooks located from the first segment of the first unbranched ray to the second branched ray; remaining rays have one or two pairs per segment. Anal-fin hooks are placed from the 3 rd unbranched ray to 12 th branched ray and gradually decreasing in size and number anteroposteriorly; these hooks are dorsolaterally paired in each ray. Caudal fin has 5–23 tiny hooks usually paired (typically more than one pair per segment), anterodorsally oriented, and placed on the dorsal margin of the lower rays 13–17.

The lower caudal-fin lobe of adult males ( 31.4–42.2 mm SL) has some modified scales that form a broadly open pocket ( Fig. 4). The lateral wall of the pocket is formed mainly by the laterodorsal accessory scale (ldas) and by the lateroventral accessory scale (lvas), which are attached to the fin only basally (and dorsally, in the case of ldas), so that the pocket is the space formed between those scales and the fin rays. The laterodorsal accessory scale is curved and elongated, with 15–18 radii. The lateroventral accessory scale has 16–19 radii almost parallel to the body axis ( Fig. 4). Two other scales, viz. the pocket scale (ps) and the medial accessory scale (mas), are almost completely adnate to the fin, only their posteroventral margin being bent outwards, and form the dorsomedial pocket wall.

The pouch scale is small, noticeably developed dorsoventrally, slightly elongated, curved, and somewhat folded laterally on its posterior region; with hypertrophied radii on its ventral portion. This scale is also expanded ventrally, forming a convex ventral border. The anterodorsal portion of the pouch scale is hidden, in lateral view, by the laterodorsal and lateroventral accessory scales, while its posteroventral portion is exposed. The very similar medial is layered between the pouch scale and the caudal-fin rays, and is thus almost imperceptible in lateral view ( Fig. 4).

Body chromatophores are more concentrated in males than in females, but both are similarly organized (see Color in alcohol). Adult males ( 40.6–42.2 mm SL) have a dark blotch, horizontally extending from urogenital pore to the sixth or seventh branched anal-fin ray. Anal fin of adult males has chromatophores more concentrated on the interradial membranes, forming a dark band at its basal portion. In adult males, the ventral region between the pelvic- and anal-fin origins possesses a series of one or two rows of scales forming a sharp border that slightly protrudes from the ventral profile, being weakly pigmented by melanophores lateroventrally. Furthermore, males differ from females by the ventral profile of body along the anal-fin base (straight or slightly convex vs. concave). In adult males, a relatively long gill gland was observed, formed by junction of the anteriormost 10–15 filaments of the ventral branch of the first branchial arch.

Etymology. The specific epithet is a homage to the people from the city of Alenquer,

Pará State, which is known in the region as “Ximango”. A noun in apposition.

Geographical distribution. Chrysobrycon ximango is known from two small tributaries in the rio Mamiá, lower rio Curuá basin, Alenquer, Pará State, Brazil ( Fig. 5).

Conservation status. Chrysobrycon ximango is only known from two streams flowing into the rio Curuá drainage, left bank of the lower rio Amazon. Fish samplings in drainages of the “Calha Norte Paraense” (Trombetas to Paru) were carried out twice a year, in the flood and dry periods over three years (2020–2023), but individuals of C. ximango were not recorded in any other drainage. Additionally, we did not identify specific threats affecting its population. Thus, following the IUCN categories and criteria ( IUCN, 2022), C. ximango can be categorized as Least Concern (LC).