Trichomycterus ruficaudatus, Costa & Mattos & Amorim & Gago & Katz, 2025

Trichomycterus ruficaudatus sp. nov.

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( Figs. 1–3 View FIGURE 1 View FIGURE 2 View FIGURE 3 , Table 1 View TABLE 1 )

Trichomycterus cf. caipora View in CoL (non Trichomycterus caipora Lima, Lazzaroto & Costa, 2008 View in CoL ): Costa et al. 2020b: 78.

Holotype. UFRJ 14437, 61.4 mm SL; Brazil: Estado do Rio de Janeiro: Município de Campos dos Goytacazes: Rio Mocotó , a tributary of Rio Imbé , Sistema Lagoa Feia , 21˚50'26"S 41˚44'22"W, about 20 m asl; W.J.E.M. Costa et al., 3 October 2024.

Paratypes. All collected in Brazil: Estado do Rio de Janeiro: Município de Campos dos Goytacazes: Rio Mocotó drainage, Rio Imbé basin, Lagoa Feia system. UFRJ 14347, 15 ex., 34.8–58.4 mm SL; CICCAA 08273, 4 ex., 35.3–51.7 mm SL; UFRJ 14438, 4 ex. (C&S), 47.2–51.3 mm SL; collected with holotype. UFRJ 14363, 3 ex., 37.0– 45.9 mm SL; same locality as holotype; A.M. Katz & P.F. Amorim, 14 Nov. 2024 . UFRJ 10059; 2 ex. (DNA), 35.2–75.0 mm SL; Rio Sabiá near confluence with Rio Mocotó , 21˚50'28"S 41˚44'27"W, about 20 m asl; P. F. Amorim and F. R. Pereira, 15 Jul. 2014 .

Diagnosis. Trichomycterus ruficaudatus is distinguished from all other species of the subgenus Trichomycterus , except T. nigricans and T. santaeritae , by having a relatively long maxilla, longer than the premaxilla ( Fig. 3A View FIGURE 3 ; vs. shorter, Costa 2021: fig. 2C, D). Trichomycterus ruficaudatus differs from T. nigricans and T. santaeritae by having a different colour pattern of flank, which is brownish yellow, slightly darker dorsally, with great concentration of melanophores, forming diffuse spots of different sizes ( Figs. 1 View FIGURE 1 , 2 View FIGURE 2 ; vs. blackened in T. nigricans, Costa et al. 2020a : fig. 1A, and light yellow with dark brown blotches in T. santaeritae, Costa et al. 2020a : fig. 1C) and fewer interopercular odontodes (44–49 vs. 58–66 in T. nigricans and 62–70 in T. santaeritae ). Trichomycterus ruficaudatus also differs from T. nigricans by having fewer premaxillary (44–52 vs. 75–82) and dentary teeth (45–47 vs. 62–75), and larger eye (eye diameter 15.5–16.1 % of the head length, vs. 9.2–12.7 of that length), and from T. santaeritae by having the dorsal-fin origin at a vertical through the centrum of the 18th or 19th vertebra (vs. 15th or 16th vertebra) and a shorter snout (snout length 37.3–42.8 % SL vs. 43.8–46.5 % SL). Trichomycterus ruficaudatus is distinguished from the distant related T. caipora , that occurs in the same hydrographic system, by having pelvic-fin bases separated by a small interspace, about one third of the base length (vs. broad interspace, about two thirds of that length), fewer opercular (18–20 vs. 26–28) and interopercular odontodes (44–49 vs. 65–70), and longer barbels, nasal barbel reaching area between eye and opercular patch of odontodes (vs. reaching orbit), maxillary and rictal barbels reaching between middle and posterior portions of the interopercular patch of odontodes (vs. reaching the anterior-most portion of that bone).

Description. Morphometric data in Table 1 View TABLE 1 . Body slender, greatest body depth at vertical through area midway distance between pectoral and pelvic-fin bases. Trunk subcylindrical on its anterior portion, compressed posteriorly. Dorsal and ventral profiles slightly convex between snout and dorsal-fin origin, about straight posteriorly. Anus and urogenital papilla opening located at vertical through middle of dorsal-fin base. Lateral line canal of trunk short, with two pores just posterior to opercular patch of odontodes.

Head sub-trapezoidal in dorsal view, anterior profile of snout slightly convex. Eye relatively large, dorsally positioned on head, nearer snout than posterior margin of opercle. Distance between anterior and posterior nostrils about three fourths of distance between posterior nostril and orbital rim. Tip of nasal barbel reaching area between orbit and opercular patch of odontodes, tip of maxillary barbel reaching posterior portion of interopercular patch of odontodes, rictal barbel reaching middle portion of interopercular patch of odontodes. Mouth subterminal. Lateral fleshy lobe of mouth large, its largest length approximately three fourths of lower jaw excluding lobes. Jaw teeth pointed, arranged in four irregular rows, larger specimens with more teeth, ranging from 44–52 on premaxilla and 45–47 on dentary. Whole head surface covered by minute skin papillae. Branchial membrane thickened, attached to isthmus only at its anterior-most point, in ventral midline. Odontodes conical, straight to slightly curved, 18–20 on opercle, 44–52 on interopercle.

Supraorbital sensory canal continuous, with three paired pores: s1, s3, and s6. Pores s6 medially separated by about half distance between pore s6 and orbit. Infraorbital canal represented by two separate segments, anterior segment delimited by pores i1 and i3, posterior segment with pores i10 and i11. Posterior infraorbital, supraorbital and postorbital canals continuous. Postorbital canal with two pores: po1 and po2.

Fins well-developed, dorsal, anal and pectoral fins with thickened bases. Dorsal and anal fins subtriangular, margins about straight to slightly convex, dorsal-fin origin nearer caudal-fin base than nape, anal-fin origin at vertical immediately posterior to dorsal-fin base; total of 12 or 13 (iii–iv + II + 7) dorsal-fin rays, total of 10 or 11 (iii–iv + II + 5) anal-fin rays. Pectoral fin broad, subtriangular in dorsal view, first ray distally terminating in filament about 30–50 % of pectoral-fin length; nine (I + 8) pectoral-fin rays. Pelvic fin with rounded posterior margin, reaching vertical through middle of dorsal-fin base, its extremity reaching vertical through anus; pelvic-fin bases medially separated by short interspace, about one third of pelvic-fin base width; five (I + 4) pelvic-fin rays. Caudal fin truncate to slightly emarginate; thirteen (I + 11 + I) principal caudal-fin rays, 15–17 (xiv-xvi + I) dorsal procurrent rays, and 13 or 14 (xii–xiii + I) ventral procurrent rays.

Osteology ( Figs. 3A–C View FIGURE 3 ). Mesethmoid T-shaped, anterior margin slightly concave in dorsal view, cornu narrow with rounded tip. Lateral ethmoid without distinctive lateral projections. Lacrimal thin, elliptical. Sesamoid supraorbital narrow and long, narrower than lacrimal, without lateral expansions, its longitudinal length about three or four times lacrimal longitudinal length. Premaxilla subrectangular in dorsal view, tapering laterally, maxilla slender, longer than premaxilla, with prominent posterior process, slightly curved. Autopalatine excluding posterolateral process subrectangular in dorsal view, medial and lateral margins weakly concave. Autopalatine posterolateral process triangular.

Metapterygoid trapezoid, longer than deep, small, its surface about equal quadrate lateral surface. Quadrate L-shaped, with short antero-dorsal expansion. Hyomandibula with shallow anterior outgrowth. Opercle relatively short, opercular odontode patch robust, its depth about three fourths of hyomandibula articular facet length. Dorsal process of opercle short, with rounded extremity. Opercular articular facet for hyomandibula with short rounded laminar projection. Articular facet for preopercle rounded, short. Interopercle relatively long, its length slightly shorter than total hyomandibula longitudinal length. Preopercle slender, with moderate ventral projection.

Parurohyal thin, lateral process narrow, about straight, with weakly pointed extremity. Parurohyal head with prominent anterolateral paired process. Parurohyal middle foramen large, oval. Parurohyal posterior process moderate in length, slightly shorter than distance between anterior margin of parurohyal and anterior insertion of posterior process. Branchiostegal rays 6 or 7. Vertebrae 36. Ribs 10–12. Dorsal-fin origin at vertical through centrum of 18th vertebra; anal-fin origin at vertical through centrum of 23rd vertebra. Two dorsal and single ventral hypural plate.

Colouration in life ( Fig. 1 View FIGURE 1 ). Dorsal and lateral surface of head and trunk brownish yellow, slightly darker dorsally, with great concentration of melanophores, forming diffuse spots of different sizes, more concentrated on and above lateral longitudinal midline. Reddish brown pigmentation dispersed on lateral midline of caudal peduncle and caudal-fin base, more visible in specimens just after collection. Nasal barbel dark brown, maxillary and rictal barbels pale yellow with dark brown chromatophores. Iris dark purplish grey, with narrow yellow line around pupil. Ventral surface of head and venter yellowish white, with dark pigment scattered over anterior part of head and pre-pelvic region. Dorsal and lateral regions, including barbels brown, to dark brown on interorbital region, nape and area above opercle. Fins brownish yellow, with great concentration of dark chromatophores forming diffuse marks.

Colouration in alcohol ( Fig. 2 View FIGURE 2 ). Similar to colouration in life, but with paler colours.

Etymology. From the Latin, the name ruficaudatus (with rufous tail) refers to the reddish-brown pigmentation present on the caudal peduncle and caudal-fin base that is more conspicuous in live specimens just after collection.

Distribution and habitat. Trichomycterus ruficaudatus is only known from the type locality area, in the Rio Mocotó drainage, Rio Imbé basin, Lagoa Feia system ( Fig. 4 View FIGURE 4 ). In the 2024 collections, the holotype and all other specimens of the type series were found in a small area representing a segment of the Rio Mocotó approximately 20 m long ( Figs. 5A, B View FIGURE 5 ), with the following characteristics: depth approximately 30–60 cm; river width at this point 15 m, with relatively strong water flow; crystal clear water; varied bottom, including gravel, sand and pebbles; banks devoid of natural vegetation. The specimens were found amidst the gravel and small pebbles ( Figs. 5C, D View FIGURE 5 ), actively swimming in sunlight. Specimens were easily found, showing a large concentration of individuals in those environmental conditions. At other adjacent points of the Rio Mocotó and in the nearby tributary, the Rio Sabiá, no specimens were found in 2024, but the only two specimens collected in 2014 were found in this latter stream, which is approximately 6 m wide and has a predominantly sandy bottom, with gravel in some stretches.

Conservation notes: The area of the type locality is located in a pasture, where natural vegetation has been removed. However, no signs of water pollution were observed. Most of the Rio Mocotó course about 1 km upstream from the area where T. ruficaudatus was collected is within the boundaries of the Desengano State Park, where the native forest is well preserved, but no samples were taken in this region upstream. There is no evidence of threats to T. ruficaudatus .