Rhinotocinclus discolor, Reis & Lehmann A., 2022

Reis, Roberto E. & Lehmann A., Pablo, 2022, A new genus of armored catfish (Siluriformes: Loricariidae) from the Greater Amazon, with a review of the species and description of five new species, Neotropical Ichthyology (e 220002) 20 (2), pp. 1-100 : 43-47

Rhinotocinclus discolor , new species urn:lsid:zoobank.org:act:BDD5D811-D797-4190-97D6-B3479866A445

( Fig. 26 ; Tab. 5 )

Holotype. MHNLS 26183, 24.4 mm SL, Caño Pasa, tributary of Río Sipapo, 98 km from Puerto Ayacucho airport, Amazonas, Venezuela, 05°03’40.28”N 67°46’46.45”W, 20 Apr 2010, J. Birindelli, N. Lujan, V. Meza & O. Leon.

Paratypes. Amazonas, Venezuela: AUM 53425, 12, 22.1–24.1 mm SL (3 measured, 23.6–24.1 mm SL) ; MCP 54752, 3, 21.8–24.6 mm SL (3 measured) , MHNLS 26184, 2, 21.6–21.8 mm SL (2 measured), MZUSP 125868, 2, 21.8–23.3 mm SL (2 measured), ROM 94295, 4, 22.0– 25.1 mm SL (2 measured, 23.3–25.1 mm SL), 2 tissue vouchers, collected with holotype. ANSP 160731, 1, 25.2 mm SL (measured), creek entering Río Sipapo at raudal del Caldero, ca. 3 km upstream of confluence with Río Orinoco, Departamento Apure, approx. 05°04’N 67°46’W, 14 Nov 1985, B. Chernoff, W. Saul, R. Royero & O. Brull. AUM 40978, 2, 24.0– 24.2 mm SL, Río Ventuari, beach ca. 116 km NE of Macuruco, 169 km NE of San Fernando de Atabapo, 04°45’09.4”N 66°21’17.5”W, 7 Apr 2004, D. C. Werneke, N. K. Lujan, M. H. Sabaj, & L. S. Sousa. AUM 42102, 1, 23.6 mm SL, Río Orinoco at beach and bedrock outcrop ca. 50 km E of San Fernando de Atabapo, 03°58’13”N 67°15’18”W, 2 Mar 2005, N. K. Lujan, D.C. Werneke, M. H. Sabaj, M. Arce & R. Betancur. AUM 46313, 13, 17.8–22.9 mm SL, Caño Maraya, beach near mouth, 04°01’50”N 66°57’56”W, 12 Nov 2003, C. Montaña & S. Willis . MCNG 22280, 6, 21.8–24.6 mm SL (6 measured) + 1 cs, creek entering Río Sipapo at raudal Caldero, Departamento Apure, approx. 05°01’20”N 67°46’W, 16 May 1989, L. Nico and others .

Non-paratypes. Amazonas, Venezuela: MCNG 23757, 1, Río Matacuni, Departamento Atabapo, approx. 03°04’N 65°10’W, 22 Jan 1990, B. Stergios et al. MCNG 24047, 2, Río Putaco at confluence with Río Ocamo, Departamento Atabapo, approx. 2°57’N 64°36’W, 3 Feb 1990, L. Nico et al. MCNG 24270, 9 + 1 cs, Río Putaco near raudal Chicrita-Porã, Departamento Atabapo, approx. 02°56’N 64°33’W, 4 Feb 1990, L. Nico et al. MCNG 24302, 1, Río Ocamo, immediately above mouth of caño Jenita, Departamento Atabapo, 02°45’49.6”N 64°56’34.3”W, 7 Feb 1990, L. Nico et al.

Diagnosis. Rhinotocinclus discolor is distinguished from R. acuen , R. bockmanni , R. chromodontus , R. dani , R. dinizae , R. hera , R. jumaorum , R. loxochelis n. sp., R. pentakelis , and R. marginalis n. sp. and by possessing an adipose fin (vs. adipose fin absent), and by having a Y-shaped light mark from the snout tip to each nostril (Figs. 6A,B; vs. light mark V-shaped or present as two separate lines from snout tip diverging to each nostril). It is distinguished from R. collinsae , R. halbolthi , and R. hardmani by lacking accessory teeth on both premaxilla and dentary (Figs. 5B,C; vs. accessory teeth present, Fig. 5A); the odontodes on the ventral surface of first pelvic-fin ray bent and pointing mesially (Fig. 9A; vs. odontodes aligned with main ray axis, Fig. 9B); a Y-shaped light mark from snout tip to nostrils (vs. Y-shaped light mark absent); and a larger orbit, 29.8–36.2% snout length (vs. orbit 18.9–24.6% snout length). Rhinotocinclus discolor is distinguished from R. britskii , R. eppleyi , R. kwarup , R. isabelae n. sp., R. longirostris , R. pilosus n. sp., R. polyochrus , R. variola , and R. yaka by having a small, inconspicuous triangular spot at the dorsal-fin base (Fig. 8A; vs. spot large and conspicuous). It is further distinguished by having the dark transverse bar 2+3 fused together (vs. bars not fused in R. britskii , R. eppleyi , R. kwarup , and R. longirostris , or bars 1+2 fused in R. isabelae n. sp., R. pilosus n. sp., R. polyochrus , R. variola , and R. yaka ).

Description. Proportional measurements in Tab. 5 . Dorsal profile of head concave from snout tip to area between nares, convex from that point to parieto-supraoccipital tip and straight to slightly concave from that point to origin of dorsal fin. Dorsal profile of body mostly straight and descending from dorsal-fin origin to insertion of caudal fin. Trunk horizontally ovoid to roundly triangular and caudal peduncle vertically ovoid in cross section, vaguely flattened ventrally and compressed caudally. Body progressively narrowing posteriorly from cleithrum, more so behind dorsal fin.

Head flat to slightly convex between orbits; dorsal margin of orbit slightly elevated. Snout elongated, depressed, its anterior margin rounded in dorsal view, with small depression anterior to naris. Eye comparatively large, positioned dorsolaterally, with small to medium dorsal iris operculum. Posterior tip of parieto-supraoccipital without patch of enlarged odontodes. Slightly enlarged odontodes on snout border, especially on rostral and postrostral plates and on lower surface of pectoral and pelvic spines; enlarged odontodes curved and posteriorly oriented. Odontodes on head and trunk otherwise of uniform size and distribution. Canal cheek plate bent and elongated posteroventrally, contacting cleithum. Lips rounded, narrow, covered with minute papillae; papillae slightly decreasing in size towards lip margin. Lip margin with uniformly distributed papillae forming delicate fringe. Maxillary barbel completely adnate to lower lip without free distal portion. Teeth moderately slender, bifid. Larger, medial cusp blade-like and slightly rounded, not elongated. Smaller, lateral cusp minute and pointed. Premaxillary teeth 23–26 (24); dentary teeth 19–24 (24); accessory teeth absent.

Body entirely covered by dermal plates except for ventral surface of head around lips, area around pelvic-fin insertion, and area around anus. Lateral plates arranged in five longitudinal series on trunk. Dorsal plate series complete, beginning at origin of dorsal fin, with 19 plates; mid-dorsal series incomplete, with 6–7 plates; middle series complete, with two ossified tubes and 24 plates. Lateral line on middle plate series with two ossified tubes, six pored plates followed by one unperforated plate, then 14–15 plates bearing canal, and 2–3 terminal plates without canal. Mid-ventral series incomplete with 15 plates; series terminating below adipose fin. Ventral series complete and continuous from pelvic-fin origin to caudal-fin base, with 18 plates. Predorsal plates forming two transverse rows anterior to nuchal plate. Single preadipose azygous plate. Coracoid completely exposed ventrally, twice longer than cleithrum; cleithrum exposed laterally with medial area and arrector fossa covered by skin. Lateral abdominal plates 3–5 (4/3); plates transversely elongate, clearly arranged in line between coracoid and pelvic-fin origin. Middle abdominal plates 1–2 series, between the lateral abdominal plates. Preanal plate large, single or double, bordered anteriorly by two or three plates. First anal-fin pterygiophore exposed in front of anal-fin as small, plate-like bone supporting odontodes. Total vertebrae 27, in one dissected specimen.

Dorsal-fin rays I,7; spine slightly arched. Dorsal-fin origin slightly posterior to vertical through pelvic-fin origin. Dorsal-fin spinelet present, plate-like, roundly triangular dorsally and V-shaped anteriorly. Spinelet articulated to first dorsal-fin pterygiophore and dorsal-fin spine locking mechanism functional. Adipose fin present and well developed. Pectoral-fin rays I,6. Large spine slightly arched; tip of adpressed spine reaching between middle and distal third of pelvic fin. Pectoral-fin axillary slit present, with large slanted opening ventral to tip of posterior process of cleithrum. Pelvic-fin rays i,5, fin short, with tip of adpressed fin almost reaching or reaching to anal-fin origin in males, falling short of that point in females. Adult males with conspicuous fleshy flap along posterodorsal margin of thickened first pelvic-fin ray. Odontodes on ventral surface of thickened first pelvic-fin ray bent and oriented mesially. Anal-fin rays i,5. Caudal-fin rays i,14,i, with lower unbranched ray slightly longer than upper.

Color in alcohol. Dorsal portions of head and trunk light brown, lighter laterally, and cream to pale yellow ventrally. Dorsal and lateral surface of snout with patches of dark chromatophores forming darker lines. Light Y-shaped mark from snout tip diverging towards nostrils. Compound pterotic and most of parieto-supraoccipial behind eyes dark brown. Posterior portion of parieto-supraoccipital and predorsal region lighter than surrounding areas, but not forming inverted Y-shaped mark. Trunk with four conspicuous dark brown bars; bars 2 and 3 coalesced in wider bar. Bars extending transversely from dorsal midline and narrowing ventrally, barely reaching to ventral midline. Ventral surface mostly unpigmented, but small dark spots sometimes on cheeks, lateral abdominal plates, and caudal peduncle. Tooth cusps reddish brown. Fins with transverse, conspicuous brown bands formed by concentration of chromatophores on rays; bands more numerous on leading rays; membranes mostly hyaline. Dorsal fin with small dark triangular spot at anterior portion of membrane, spine with 3–4 dark brown spots, branched rays with 1–2 dark bands, especially in distal half. Pectoral-fin spine with 4–6 dark spots, branched rays with 2–3 irregular dark bands. Pelvic fin with 2 irregular dark bands. Anal fin with 1–2 dark bands. Adipose fin with 1–2 dark spot at anterior portion of spine. Caudal fin with dark transverse blotch at base and 2–3 irregular dark brown bands.

Sexual dimorphism. Males have a conspicuous urogenital papilla immediately behind the anus and a deep skin fold along the first, unbranched pelvic-fin ray, both characteristics being absent in females. Males also possess a larger nostril than females, and the snout profile somewhat elevated immediately in front of the eyes. Males also have slightly longer pelvic fins, which reach or almost reach to the anal-fin origin and fall short of that point in females.

Geographical distribution. Rhinotocinclus discolor is known from creeks of the Río Orinoco basin in the State of Amazonas, Venezuela ( Fig. 15 ).

Etymology. Rhinotocinclus discolor , from the Latin color, color, and the prefix dis -, meaning not of the same color, in allusion to the remarkable color pattern with the second and third dark bars coalesced. A noun in apposition.

Conservation status. The extinction risk of Rhinotocinclus discolor is assessed as low despite the limited knowledge of its geographic distribution. The species is so far known from ten localities in southern Venezuela, with an Extension of Occurrence (EOO) estimated by the convex polygon of those localities of approximately 33,000 square kilometers. The region is relatively well preserved but logging, gold mining and cattle ranching are common throughout the area. For this reason, R. discolor is preliminarily categorized as Least Concern (LC) according to the International Union for Conservation of Nature (IUCN) categories and criteria (IUCN Standards and Petitions Subcommittee, 2019).

Remarks. Four lots collected in 1990 from the Departamento Atabapo (MCNG 23757, MCNG 24047, MCNG 24270, MCNG 24302) are not well preserved, having the entire body softened and heavily darkened, not appropriate for a careful morphometric comparison, and for this reason were regarded as non-paratypes. Rhinotocinclus discolor is sympatric and probably syntopic with R. eppleyi , from which it is easily distinghished. The new species has one or two irregular series of middle abdominal plates (vs. four or five), a small, inconspicuous triangular spot at the dorsal-fin base (vs. spot large and conspicuous), and the dark transverse bars 2+3 fused together (vs. dark bars no fused in R. eppleyi ).

Figures

FIGURE 15 | Geographical distribution of Rhinotocinclus species of the R. britskii group in central and northwestern South America. Rhinotocinclus britskii (white dots); R. discolor n. sp. (purple dots); R. eppleyi (orange dots); R. isabelae n. sp. (pink dots); R. kwarup (red dots); R. longirostris (blue dots); R. pilosus n. sp. (yellow dots); R. polyochrus (cyan dot); R. variola (black dots); R. yaka (light green dots). T = Type-locality; T1 = type-locality of Parotocinclus britskii, T2 = type-locality of P. amazonensis, T3 = type-locality of P. aripuanensis.

FIGURE 26 | Rhinotocinclus discolor, holotype, MHNLS 26183, 24.4 mm SL, male, Caño Pasa, tributary of Río Sipapo, 98 km from Puerto Ayacucho airport, Amazonas, Venezuela.