Pimelodella nuchalis, Cortés-Hernández & Conde-Saldaña & Bermúdez-Casas & Villa-Navarro & DoNascimiento, 2025

Cortés-Hernández, Miguel Ángel, Conde-Saldaña, Cristhian Camilo, Bermúdez-Casas, Karol Vanessa, Villa-Navarro, Francisco Antonio & DoNascimiento, Carlos, 2025, A long-hidden species of Pimelodella (Siluriformes: Heptapteridae) from the upper and middle Orinoco, and western Amazon tributaries, Neotropical Ichthyology (e 240112) 23 (2) : -

Pimelodella nuchalis , new species urn:lsid:zoobank.org:act:F19135AD-C93A-455E-8A2F-87D5122197C3

( Figs. 1–5; Tab. 1)

Pimelodella megalops Mesa-Salazar et al., 2019:30 (taxonomic list, IAvH-P 16640). —Cortés-Hernández et al., 2020:504 (taxonomic treatment). —DoNascimiento et al., 2023 (taxonomic list). —Cortés-Hernández, Ramírez-Gil, 2024:94–97 (morphometric analysis).

Pimelodella sp. Acosta-Santos et al., 2019:74 (visual guide). —Villa-Navarro et al., 2022:160 (taxonomic list).

Holotype. CZUT-IC 27776 , 67.3 mm SL; Colombia, Guainía, río Atabapo, isla Chamochina , border between Colombia and Venezuela, 03°46’57.11”N 67°37’59.1”W, 24 Aug 2008, C. A. Lasso, M. Sierra, M. Patiño & F. A. Villa-Navarro.

Paratypes. Colombia: CZUT-IC 16674 , 2 , 45.1–55.6 mm SL, collected with the holotype . CZUT-IC 16675 , 30 , 34.5–67.3 mm SL (3 c&s, 44.8–56.0 mm SL), collected with the holotype . CIACOL 3238 , 3 , 49.1–69.1 mm SL, Vaupés, cachivera Morroco, río Pacoa, tributary of río Apaporis , adjacent to community El Morroco , 00°08’37.4”N 70°57’30.8”W, 25 Feb 2018, E. Agudelo-Córdoba & A. Acosta-Santos . CIACOL 3239 , 2 , 60.4 –70.0 mm SL, Vaupés, cachivera Morroco, río Pacoa, tributary of río Apaporis , adjacent to community El Morroco , 00°08’38.9”N 70°57’25.3”W, 26 Feb 2018, E. Agudelo-Córdoba & A. Acosta-Santos . CZUT-IC 16521 , 3 , 39.7–41.1 mm SL (2 c&s, 40.0–41.0 mm SL), Guainía, río Inírida, upstream of community Remanso , 03°27’39’’N 67°58’23.5’’W, 18 Feb 2008, C. A. Lasso, M. Sierra, M. Patiño, F. A. Villa-Navarro, A. Ortega-Lara & J. S. Usma-Oviedo . CZUT-IC 24773 , 1 , 56.1 mm SL, Guainía, vereda San Felipe, río Guainía, indigenous community Frito , 02°15’08.0”N 67°13’32.0”W, 86 m a.s.l., 10 Oct 2021, F. A. Villa-Navarro & A. Méndez-López . CZUT-IC 24774 , 2 , 53.3–57.9 mm SL, Guainía, vereda San Felipe, río Guainía, dock at community Punta Barbosa , 01°58’37.0”N 67°07’12.0”W, 80 m a.s.l., 14 Oct 2021, F. A. Villa-Navarro & A. Méndez-López . IAvH-P 16640 , 8 , 42.7–54.3 mm SL (1 c&s, 53.3 mm SL), Vichada, Puerto Carreño, caño Terecay, tributary of río Tomo , 05°34’57.9’’N 68°29’55.4’’W, 3 Mar 2017, L. M. Mesa-Salazar, C. DoNascimiento, C. A. Lasso & Z. Rivas .

Diagnosis. Pimelodella nuchalis differs from all congeners by having a dark saddle on the predorsal region ( Figs. 1–2, 5) (vs. absent). Additionally, P. nuchalis is distinguished from most congeners (except P. bockmanni , P. buckleyi , P. geryi , P. grisea , P. hasemani , P. howesi , P. leptosoma , P. macturki , P. megalops , P. metae , P. notomelas , and P. procera ) by having a dark band on the dorsal fin (vs. absent). Pimelodella nuchalis differs from P. bockmanni , P. leptosoma , and P. metae by having the supraoccipital process reaching the anterior nuchal plate (vs. disconnected, leaving a free space between the supraoccipital process and anterior nuchal plate). Pimelodella nuchalis differs from P. buckleyi , P. grisea , P. hasemani , P. howesi , P. macturki , and P. megalops by having fewer total vertebrae (38 vs. 39–40 in P. grisea , 39–42 in P. megalops , 40–41 in P. macturki , 42–43 in P. hasemani , 43–44 in P. buckleyi and P. howesi ). Pimelodella nuchalis differs from P. geryi , P. notomelas , and P. procera by having a shorter maxillary barbel (not exceeding the anterior fourth of the adipose-fin base vs. reaching the caudal-fin base in P. geryi ; reaching the tip of middle caudal-fin rays or a little shorter in P. notomelas ; reaching from mid-length of adipose-fin base to its posterior end in P. procera ); anterior margin of pectoral-fin spine with minute straight dentations covering 60% of basal portion, and serrae restricted to its distal third ( Fig. 3) (vs. serrae covering two-thirds in P. geryi and P. procera ; basal two thirds smooth in P. notomelas ); and posterior margin with 8–10 retrorse dentations ( Fig. 3) (vs. 4–6 dentations in P. notomelas ). Pimelodella nuchalis is furtherly distinguished from P. hasemani by having a shorter maxillary barbel, never exceeding the anterior fourth of the adipose-fin base (vs. reaching between middle of adipose fin base to its distal posterior end), epiphyseal branches fused medially or giving origin to two or three unfused pores, aligned anteroposteriorly along sagittal plane (vs. unfused paired pores, symmetrically placed to both sides of sagittal plane), anterior margin of pectoral-fin spine with minute straight dentations covering approximately 60% of basal portion (vs. covering almost entire margin), and posterior margin with 8–10 retrorse dentations (vs. 12–14).

Description. Morphometric data summarized in Tab. 1. Body elongate, compressed towards caudal fin. Greater depth of body at dorsal-fin insertion. Dorsal profile of body convex from snout to base of supraoccipital process, then straight to dorsal-fin insertion, and descending along dorsal-fin base, slightly concave from posterior end of dorsal-fin base to adipose-fin insertion, straight and descending along adipose-fin base, and slightly concave along caudal peduncle. Ventral profile of body straight to slightly convex from jaw tip to pelvic-fin insertion, almost straight between pelvic-fin insertion to anal-fin insertion, straight and ascending along anal-fin base, and slightly concave along caudal peduncle ( Figs. 1–2).

Head conical. Mouth subterminal, upper jaw more pronounced than lower jaw. Premaxilla with 4–5 rows and dentary with four rows of villiform teeth. Anterior naris tubular. Posterior naris ovoid, closer (half of internarial distance) to anterior ocular margin than to anterior naris, bordered by a fleshy margin leaving a narrow notch on posterior margin. Nares disposed in rectangular arrangement. Barbels thin and slightly depressed. Maxillary barbel reaching or slightly exceeding insertion of adipose fin, but never exceeding anterior fourth of adipose-fin base. Outer mental barbel reaching vertical through spinelet base. Inner mental barbel reaching half of length of pectoral-fin spine when adpressed. Eye large and rounded, occupying most of dorsolateral surface of head. Orbital rim well defined. Supraoccipital process subrectangular in shape and long, reaching anterior nuchal plate. Branchiostegal membranes almost entirely free, united to isthmus only at medial apex and not joined to each other anteriorly. Branchiostegal rays six (6). Gill rakers on first gill arch 12–16; 9–12 associated to ceratobranchial, one to cartilage between ceratobranchial and epibranchial, and 2–3 to epibranchial (6).

Dorsal fin with spinelet, spine, and six branched rays; originating at vertical through posterior end of pseudotympanum. Distal margin of fin slightly convex. Spinelet large with wide base and rounded distal tip. Spine straight, slender, pungent, shorter than first branched ray (14.4–21.0% of SL). Anterior margin of spine with feeble, almost imperceptible serrae along distal third, remaining two thirds completely smooth, posterior margin entirely smooth. Anteriormost pterygiophore inserted above complex vertebra; posteriormost pterygiophore anterior to neural spine of vertebrae 10(2) or 11(4).

Pectoral fin with spine and seven (4), eight (12) or nine* (3) branched rays (c&s specimen of 41.0 mm SL with six branched rays on right side). Distal margin of fin slightly convex. Spine almost straight, entirely ossified, and pungent. Anterior margin of spine with minute straight dentations covering approximately 60% of basal portion, and serrae extending along its distal third. Posterior margin with 8–10 retrorse dentations covering 40–45% of margin, with proximal-most dentation closer to base of spine than distal-most dentation to distal tip of spine ( Fig. 3A).

Pelvic fin with one unbranched and five branched rays, originating at vertical through last dorsal-fin ray. First ray distinctly shorter than second and third rays (first and second branched rays, respectively). Distal margin of fin convex. Tip of fin not reaching adipose-fin insertion.

Anal fin with two (2) or three (4) procurrent, two (2) or three (4) unbranched, and six (1), seven (4) or eight (1) branched rays. Base of anteriormost two or three rays embedded in thick skin. Distal margin of fin convex. Insertion of fin approximately at level of first third of adipose-fin base. Terminus of adpressed fin slightly surpassing adpressed terminus of adipose fin. Anteriormost pterygiophore inserted posterior to hemal spine of vertebrae 22(1) or 23(5); posteriormost pterygiophore inserted anterior to hemal spine of vertebrae 27(2) or 28(4).

Adipose fin short (23.5–27.2% of SL), with rounded margin and deepest point approximately at its mid-length. Distance from dorsal fin to adipose fin (13.6–16.5% of SL) equal than length of dorsal-fin base (14.4–16.2% of SL). Insertion of fin posterior to mid-length of trunk, approximately at vertical through vertebral centrum 20(1), 21(4) or 22(1). Terminus of fin at vertical through vertebral centrum 32(1) or 33(5).

Caudal fin deeply forked with lobes subequal, and margin of tips rounded. Lower lobe with 17–21 procurrent rays, one unbranched, and eight branched principal rays. Upper lobe with 19–21 procurrent rays, one unbranched, and seven branched principal rays. Ventral caudal-fin plate with eight rays articulated (two to parhypural and six to hypurals 1+2) and dorsal caudal-fin plate with seven rays articulated (five rays to hypurals 3+4 and two rays to hypural 5). Upper innermost ray not articulated to caudal skeleton. Parhypural not fused to hypural 1. Hypurals 1 and 2 completely fused. Hypurals 3 and 4 completely fused. Hypural 5 free. Total vertebrae 38(6). Ribs 7(4) or 8(2).

Lateral line canal complete, extending to basal portion of interradial membrane of middle caudal-fin rays. Head laterosensory canals with non-dendritic branches, ending in single pores. Supraorbital pore s1 medially adjacent to anterior naris, s2 + i2 between anterior and posterior nares, slightly closer to anterior naris. Sensory pore s3 just posterior to rim of posterior naris and s4 (one specimen of 56 mm SL with two pores on left side) located anteromedial to anterior ocular margin. Branches of sensory pores s6 fused to each other medially (13) or failing to fuse in most specimens (26), giving origin to two or three pores, aligned anteroposteriorly along sagittal plane ( Fig. 4). Postorbital sensory pore (s7) located anterior to the posterior orbital edge. Parietal sensory pore (s8) posterior to eye and aligned with its dorsal margin. Infraorbital canal with six sensory pores: i1, s2+i2, i3, i4, i5, and i6. Preoperculomandibular canal with 11 sensory pores with relative positions similar to those described for other congeneric species. Postotic canal with three sensory pores, po1 + pm11, po2 slightly posterior to dorsal corner of gill opening, emerging from a posteroventrally directed branch (pterotic branch), and po3 between first two lateral line sensory pores (slightly closer to vertical through second sensory pore), emerging dorsal to lateral line canal.

Coloration in alcohol. Background coloration of body yellow. Sides of body with uniformly spaced chromatophores. Ventral region of head and body pale, with few sparse chromatophores around chin region. Dorsal surface of head with dense concentration of dark brown chromatophores, framing a pair of slightly lighter elliptical areas corresponding to nasal capsules. Occipital surface around fontanel darker, forming a conspicuously v-shaped spot, continuous posteriorly with dark area on pseudotympanum, connected to its counterpart through a transverse predorsal band, forming a conspicuous dark saddle on predorsal region. Dorsal surface of maxillary barbel darkly pigmented. Mental barbels pale. Subdistal region of dorsal fin with black melanophores on interradial membrane, forming a conspicuous band between spine and third ray (inconspicuous in specimens less than 40 mm SL). Pectoral, pelvic, adipose, anal, and caudal fins hyaline with few sparse chromatophores along fin rays, but not forming any evident macroscopic dark pattern ( Figs. 1–2, 5).

Etymology. From the Latin nuchalis (neck), alluding to the dark saddle on the predorsal region ( Fig. 5).

Geographical distribution. Pimelodella nuchalis is known from the Atabapo and Inírida rivers of the upper basin of the Orinoco River, and the Tomo River, a direct tributary of the middle basin of the Orinoco River. Additionally, the new species occurs in the Amazon River basin, in the Guainía River, tributary of the upper basin of the Negro River, and in the Pacoa stream, a tributary of the Apaporis River from the Caquetá River drainage ( Fig. 6).

Ecological notes. The diet of Pimelodella species consists mainly of crustaceans and aquatic insects (e.g., copepods, ostracods, and chironomid and ephemeropteran larvae) (Mazzoni, Costa, 2007; Mazzoni et al., 2010; García et al., 2017). However, the digestive tract of a c&s specimen of P. nuchalis (46.7 mm SL) contained an Acestrorhamphidae tetra of approximately 18.1 mm SL ( Fig. 7), probably a specimen of Thayeria Eigenmann, 1908 ; Hemigrammus analis Durbin, 1909 ; or H. hyanuary Durbin, 1918 (J. M. Mirande and F. C. T. Lima , 2024, pers. comm.). The size ratio between the P. nuchalis specimen and its fish prey highlights the great capacity to ingest large food items, which is especially true when considering the actual volume capacity of the abdominal cavity.

Morphometric analysis. We found a clear morphometric differentiation between Pimelodella megalops and P. nuchalis along PC 1 that accounts for 74.1% of total variability of our data ( Fig. 8). The variables with highest values on the positive axis of PC 1 were anterior nostril width, mouth width, postorbital distance, internostril distance, and dorsal fin to adipose fin; and for the negative axis were anal-fin length, maxillary-barbel length, outer mental-barbel length, inner mental-barbel length, adipose-fin length, lower caudal-fin lobe length and interorbital width ( Tab. S1). Simple linear regressions of lower caudal-fin lobe length for these species show that P. megalops has a significatively longer lobe than P. nuchalis through its growth (ANCOVA F = 26.85, p = 5.31e- 05) ( Fig. 9).