Trichomycterus nigricans Valenciennes 1832
Creators: name: Costa, Wilson J. E. M.
name: Katz, Axel M.
name: Mattos, José Leonardo O.
name: Amorim, Pedro F.
name: Mesquita, Beatrizz O.
name: Vilardo, Paulo J.
name: Barbosa, Maria Anais
Publisher: name: Zenodo
Year: 2020
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Actinopterygii
Siluriformes
Trichomycteridae
Trichomycterus
Trichomycterus nigricans

description: Trichomycterus nigricans Valenciennes, 1832   
(Figures 1a, 2a–c)  
 Trichomycterus nigricans Valenciennes in Humboldt and Bonpland, 1832: 348. Holotype: MNHN B. 251, 125 mm SL (photographs available at http://acsi.acnatsci.org/ base/image_list.html?search=true&orderby=genus&genus= Trichomycterus &trivial=nigri cans; type locality: environs de Rio-Janeiro [vicinity of Rio de Janeiro]).   
 Material examined   
All from Brazil: Estado do Rio de Janeiro: MNHN B-0251, holotype, about 127 mm in total length; vicinity of Rio de Janeiro. Município de Cachoeira de Macacu, Rio Guapiaçu basin: – UFRJ 5322, 2; Rio Faraó, tributary of Rio Macacu, - 22.47833S, - 42.62916W, about 110 m asl; W.J.E.M. Costa et al., 25 May 2011. – UFRJ 10,996, 18; UFRJ 10,989, 5 (DNA); UFRJ 11,897, 3 (C&S); small tributary stream of the Rio Manuel Alexandre, - 22.41916S, - 42.73861W, about 165 m above sea level (asl); A. Katz et al., 16 June 2016. – UFRJ 11898, 1 (C&S); UFRJ 11899, 2 (C&S); Riacho Mineiro Branco, tributary of Rio Macacu, - 22.42277S, - 42.74194W, about 145 m asl; T. Barros et al., 11 July 2015. – UFRJ 8433, 3; Rio do Gato, - 22.43416S, - 42.76000W, about 40 m asl; B. Terra, 24 August 2011. – UFRJ 8498, 4; Rio Macacu; B. Terra, 28 September 2011. – UFRJ 8499, 1; Rio Macacu; B. Terra, 4 August 2011. – UFRJ 8500, 2; Rio Macacu; B. Terra, 4 August 2011.  
 Diagnosis   
 Trichomycterus nigricans differs from all other congeners, except T. caipora, T. giganteus, T. immaculatus, T. pradensis and T. santaeritae, by having 9 pectoral-fin rays (vs 6–8). Trichomycterus nigricans differs from all other species here osteologically examined, except T. caipora, T. immaculatus and T. santaeritae, by the presence of a pronounced posterior process on the maxilla (Figure 2a) (vs a rudimentary process). It is also distinguished from all congeners, except T. caipora, T. immaculatus and T. santaeritae, by having the caudal fin emarginate at least in larger specimens (vs truncate or subtruncate, rarely bilobed). Trichomycterus nigricans is distinguished from T. immaculatus by having a long maxilla, about as long as the premaxilla or longer (Figure 2a) (vs distinctively shorter); a slender autopalatine, its width about two-fifths the autopalatine length without the posterolateral process (Figure 2a) (vs about half autopalatine length); a robust opercular odontode patch, its depth about three-quarters of the opercle length, with about 25 odontodes (Figure 2b) (vs slender, its depth about two-fifths of the opercle length, with about 15–20 odontodes; Figure 2e); and the anterior extremity of hypobranchial 3 branched (Figure 2c) (vs unbranched). Trichomycterus nigricans is distinguished from T. immaculatus and T. santaeritae by having an anterior expansion in the interopercle (Figure 2b) (vs interopercle without anterior expansion; Figure 2e, h). Trichomycterus nigricans differs from T. caipora and T. santaeritae by having a homogeneous black colouration on the body side, dorsum and fins (vs head, dorsum and flank spotted).   
 Description   
Morphometric data are given in Table 1. Largest specimen examined 98.0 mm SL. Body moderately slender, subcylindrical and slightly depressed anteriorly, compressed posteriorly. Greatest body depth at vertical just in front pelvic-fin base. Dorsal profile of head and trunk slightly convex, approximately straight on caudal peduncle; ventral profile straight to slightly convex between lower jaw and end of anal-fin base, straight on caudal peduncle. Skin papillae minute. Anus and urogenital papilla in vertical through anterior third of dorsal-fin base. Head trapezoidal in dorsal view. Anterior profile of snout convex in dorsal view. Eye small, dorsally positioned in head. Posterior nostril nearer to anterior nostril than to orbit. Tip of maxillary barbel reaching between posterior edge of interopercular patch of odontodes and middle portion of opercular patch of odontodes; tip of rictal barbel reaching between middle and posterior part of interopercular patch of odontodes; tip of nasal barbel reaching anterior margin of opercular patch of odontodes. Mouth subterminal. Jaw teeth arranged in irregular rows, sharply pointed in internal rows, gradually becoming incisiform on outermost rows; premaxillary teeth nearly straight, dentary teeth slightly curved inside mouth. Branchial membrane attached to isthmus only at its anterior point. Branchiostegal rays 7 or 8.  
Dorsal and anal fins subtriangular; total dorsal-fin rays 11 or 12 (ii–iv + I–II + 6–7), total anal-fin rays 9–11 (ii–iii + II–III + 5); anal-fin origin in vertical just posterior to dorsal-fin base. Dorsal-fin origin in vertical through centrum of 18th or 19th vertebrae; anal-fin origin in vertical between centrum of 23rd and 25th vertebrae. Pectoral fin subtriangular in dorsal view, posterior margin slightly convex, first pectoral-fin ray terminating in short filament reaching about 25% of pectoral-fin length without filament; total pectoral-fin rays 9 (I + 8). Pelvic fin truncate, its posterior extremity reaching urogenital papilla; pelvic-fin bases medially separated by minute interspace; total pelvic-fin rays 5 (I + 4). Caudal fin truncate to slightly emarginate in large specimens above about 80 mm SL, posterior margin straight to gently concave; total principal caudal-fin rays 13 (I + 11 + I), total dorsal procurrent rays 12–15 (xii–xv), total ventral procurrent rays 10–14 (x–xiv). Vertebrae 35 or 36. Ribs 11 or 12. Two dorsal hypural plates, corresponding to hypurals 4 + 5 and 3, respectively; single ventral hypural plate corresponding to hypurals 1 and 2 and parhypural.  
Laterosensory system. Supraorbital sensory canal continuous, connected to infraorbital sensory canal posteriorly. Supraorbital sensory canal with 3 pores: s1, adjacent to medial margin of anterior nostril; s3, adjacent to medial margin of posterior nostril; and s6, on middle part of dorsal surface of head, in transverse line just posterior to orbit; pores s6 medially in close proximity. Infraorbital sensory canal arranged in 2 segments, each with 2 pores; anterior segment with pore i1, in transverse line through anterior nostril, and pore i3, in transverse line just anterior to posterior nostril; posterior segment with pore i10, adjacent to ventral margin of orbit, and pore i11, posterior to orbit. Postorbital canal with 2 pores: po1, in vertical line above posterior portion of interopercular patch of odontodes, and po2, in vertical line above posterior portion of opercular patch of odontodes. Lateral line of body short, with 2 pores, posterior-most pore in vertical just posterior to pectoral-fin base.  
Mesethmoidal region and adjacent structures (Figure 2a). Anterior margin of mesethmoid slightly concave, mesethmoid cornu robust, subcylindrical, tip rounded. Antorbital and sesamoid supraorbital narrow, rod-like, sesamoid supraorbital long, its length about 2.5 times antorbital length. Premaxilla sub-retangular in dorsal view, without processes. Maxilla boomerang-shaped, slender, about equal to premaxilla in length, slightly curved, with prominent posterior process. Autopalatine subrectangular in dorsal view when excluding posterolateral process, narrow, its shortest width about two-fifths autopalatine length without posterolateral process, lateral and medial margins slightly concave; no concavity on posterior margin of autopalatine between posteromedial corner and posterior articulatory process; latero-posterior process of autopalatine triangular, long, its length about three-fifths autopalatine length. Posterior articulatory process well developed, shell-shaped.  
Suspensorium and opercular apparatus (Figure 2b). Metapterygoid subtriangular, longer than deep, anterior face curved, posterior and ventral face nearly straight, posterior margin separated from anterior hyomandibula flap by small interspace. Quadrate robust, dorsoposterior flap continuous to hyomandibular flap. Hyomandibula broad, with well-developed flap; middle portion of dorsal margin of hyomandibular flap with sharp concavity; posterodorsal process of hyomandibula moderate, pointed. Opercle robust, its smallest depth about half opercle length; dorsal process of opercle short and blunt, about one-third opercle length; ventral process of opercle short, slightly shorter than half opercle length; odontode patch relatively deep, its depth about three-quarters opercle length, with about 25 odontodes; odontodes pointed, arranged in irregular transverse rows. Interopercle long, slightly shorter than hyomandibula; dorsal interopercular process with deep anterior concavity; interopercular odontode patch long, its length about 2.5 times depth of opercular odontodes patch, with about 65–70 odontodes; odontodes pointed, arranged in irregular longitudinal rows. Preopercle compact, with pronounced ventral flap.  
Branchial arches (Figure 2c). Basibranchial 2 and 3 subcylindrical, basibranchial 2 slightly longer and wider, basibranchial 3 slightly constricted in its medial portion; basibranchial 4 cartilage pentagonal, longer than wide. Hypobranchial 1 subcylindrical; hypobranchial 2 and 3 subtriangular, anterior extremity of hypobranchial 3 branched. Ceratobranchial 1 broad in its proximal tip, narrowing to its distal tip; ceratobranchials 2 and 3 widened in their middle portion, with shallow concavity on posterior margin of basal portion; ceratobranchial 4 sub-rectangular; ceratobranchial 5 sub-rectangular, slightly curved, postero-proximal portion bearing patch of small, slightly curved, conical teeth. Epibranchial 1 slender, with well-developed sharp anterior uncinate process and minute posterior process; epibranchial 2 slender, with rudimentary anterior uncinate process; epibranchial 3 slender, with well-developed, curved posterior uncinate process; epibranchial 4 broad, sub-retangular, proximal portion broader than distal portion; epibranchial 5 cartilage (= accessory element of ceratobranchial 4) minute. Pharyngobranchial 3 short, subcylindrical; pharyngobranchial 4 long, bearing broad dentigerous tooth-plate with curved conical teeth.  
Colouration in life (Figure 1a). Just after collection, whole body black, except ventral portion of head and trunk grey, distal extremity of fins hyaline, and maxillary and rictal barbels pale brown; after collection, black colouration becoming yellowish dark brown. Iris dark brown, with narrow pale yellow line around pupil. No colouration polymorphism recorded.  
 Colouration in alcohol. Dorsal and lateral surface of head and trunk homogeneously dark brown. Venter pale yellowish grey. Nasal barbels pale brown, with dark brown edges. Maxillary and rictal barbels pale grey. Fins dark brown with greyish extremities.   
 Distribution and habitat notes   
 Historical data and recent collections indicate that T. nigricans is only known from coastal river basins near the city of Rio de Janeiro. Specimens recently collected were found in the Rio Macacu basin (Figure 3), but not in other nearby basins. According to field studies, T. nigricans is found in clearwater streams with intense water flow, at altitudes between about 40 and 165 m asl. All collections were made during daylight, and specimens were always collected hidden under large rocks, in the middle of the streams, suggesting nocturnal habits.
description: Published as part of Costa, Wilson J. E. M., Katz, Axel M., Mattos, José Leonardo O., Amorim, Pedro F., Mesquita, Beatrizz O., Vilardo, Paulo J. & Barbosa, Maria Anais, 2020, Historical review and redescription of three poorly known species of the catfish genus Trichomycterus from south-eastern Brazil (Siluriformes: Trichomycteridae), pp. 2905-2928 in Journal of Natural History 53 (47 - 48) on pages 2911-2915, DOI: 10.1080/00222933.2020.1752406, http://zenodo.org/record/10831395