A new rheophilic species of Knodus Eigenmann (Characiformes: Characidae Stevardiinae) from the upper rio Juruena, rio Tapajós basin, Chapada dos Parecis Mato Grosso, Brazil Author Ferreira, Katiane M. 0000-0002-8793-7343 Instituto de Biociências, Departamento de Biologia e Zoologia, Universidade Federal de Mato Grosso-UFMT, Avenida Fernando Corrêa, 2367, Boa Esperança, 78060 - 900, Cuiabá, MT, Brazil. kmferreira @ gmail. com; https: // orcid. org / 0000 - 0002 - 8793 - 7343 Universidade Federal do Amazonas-UFAM, Departamento de Biologia, Programa de Pós-Graduação em Zoologia (PPGZOOL) 69080900, Manaus, AM, Brazil. ohara @ ufam. edu. br; https: // orcid. org / 0000 - 0003 - 1684 - 4198 kmferreira@gmail.com Author Ohara, Willian Massaharu 0000-0003-1684-4198 ohara@ufam.edu.br text Zootaxa 2023 2023-01-06 5227 3 365 377 journal article 222439 10.11646/zootaxa.5227.3.5 9d43ffff-c586-4711-b8f1-f4c8aa13207e 1175-5326 7518800 391BA629-7D97-4B93-B809-63FA24F95F89 Knodus ytuanama , new species ( Figs. 1 , 4–5 ) Knodus sp. Ohara & Loeb, 2016: 4–5 , fig. 3 ( Brazil , Mato Grosso , tributaries of rio Juruena basin: photograph of live specimen). Holotype . CPUFMT 7756 (1, 81.2 mm SL), Brazil , Mato Grosso , Comodoro , rio Mutum , near the road BR-174, 13º5′10.76″S , 59º53′33.76″W ; W.M. Ohara , 1 Oct 2021 . Paratypes . All from the same locality of the holotype. UFRO-I 22844 , 6 , 28.6–80.3 mm SL; W.M. Ohara et al ., 25 Jul 2013 . UFRO-I 12305 , 3 , 56.6–83.2 mm SL; W.M. Ohara et al ., 10 Dec 2011 . UFRO-I 22905 , 1 , 24.3 mm SL; W.M. Ohara et al , 25 Jul 2013 . INPA 59847 , 9 , 49.2–82.6 mm SL; W.M. Ohara, 1 Oct 2021 . CPUFMT 7647 (30, 48.1–90.9 mm SL, 1 C&S, 59.3 mm SL); MZUSP 126877 (2, 65.9–72.4 mm SL); ZUEC 17547 (2, 57.6–76.2 mm SL); all collected with the holotype . Diagnosis. Knodus ytuanama can be distinguished from all congeners by presenting the interradial membranes of the caudal fin thickened, forming folds between the branched rays of the upper and lower lobes (see Description for more details). Adult specimens of K. ytuanama can also be distinguished from most congeners, except K. borki Zarske , K . dorsomaculatus Ferreira & Netto-Ferreira , K. geryi Lima , Britski & Machado, K. savannensis Géry , and K. tanaothoros (Weitzman, Menezes, Evers & Burns) by typically lacking a humeral blotch (humeral blotch, when present, diffuse and ill-defined), and by the presence of a dark, wide midlateral stripe extending along the sides of body from the posterior region of opercle to the middle caudal-fin rays ( versus presence of a well-defined humeral blotch and sides of body either lacking a longitudinal stripe or, if present, starting approximately at vertical through the dorsal-fin origin). Knodus ytuanama can be diagnosed from Knodus borki by presenting the lateral line complete ( versus incomplete), from Knodus dorsomaculatus by the absence of a dark blotch on the basis of the first five branched dorsal-fin rays ( versus presence), and from Knodus geryi by the absence of two symmetric, large dark blotches basally on caudal fin lobes ( versus presence). Knodus ytuanama differs from K . savannensis by absence of lateral line interrupted many times ( versus lateral line complete), by absence of midlateral stripe extending to lower lobe of caudal fin ( versus midlateral stripe extending only to middle caudal-fin rays), by presenting a smaller eye, 28.3–35.0% HL ( versus 40.6% HL) and anal-fin length short, 21.4–24.9% SL ( versus 31.7% SL), and from Knodus tanaothoros by the lack of a modified glandular tissue on the anal-fin lobe in mature males ( versus glandular tissue present in mature males). Description. Morphometric and meristic data for holotype and paratypes presented in Table 1 . Largest specimen examined 90.9 mm SL. Body laterally compressed. Greatest body depth anterior to vertical through dorsal-fin origin. Dorsal profile of head convex from tip of upper jaw to vertical through anterior nostril; straight or slightly convex from that point to tip of supraoccipital spine. Dorsal body profile slightly convex from tip of supraoccipital spine to dorsal-fin origin; straight along dorsal-fin base; straight from terminus of dorsal-fin base to adipose-fin insertion, and slightly concave posteriorly from that point to anteriormost procurrent caudal-fin ray. Ventral profile of head and body convex from tip of lower jaw to pelvic-fin insertion, straight from that point to anal-fin origin, straight along anal-fin base and slightly concave along caudal peduncle. FIGURE 1. Knodus ytuanama , holotype, CPUFMT 7756, 81.2 mm SL: Brazil, Mato Grosso, Comodoro, rio Mutum. Mouth terminal with premaxilla protruding slightly over dentary. Posterior tip of maxilla reaching vertical through anterior margin of orbit. Premaxilla with two teeth rows ( Fig. 2 ). Outer tooth row aligned, with 5*(18) or 6(2) tri- to pentacuspid teeth, with median cusps slightly more developed. Inner row with four teeth larger than those of outer series; symphyseal tooth with four or five cusps, remaining teeth penta- to heptacuspid. Maxilla with 3*(14) or 4(6) tri- to pentacuspid teeth. Dentary with four large penta- to heptacuspid teeth, followed by seven to nine smaller tricuspid teeth. Scales cycloid. Lateral line complete, with 38*(10), 39(6), 40(3) or 41(1) perforated scales. Horizontal scales rows between dorsal-fin origin and lateral line 5*(18) or 6(2). Horizontal scales rows between lateral line and pelvic fin insertion 4(20). Predorsal scales 11(4), 12(12), 13*(3) or 14(1). Circumpeduncular scales 14*(20). Single row of 10–12 scales covering base of anal-fin rays. Presence of four rows of scales covering caudal lobes; squamation slightly more developed on lower caudal-fin lobe. Scales on caudal fin with irregular margins contrasting with caudal peduncle scales that have rounded margins. TABLE 1. Morphometric data of Knodus ytuanama . Range and mean includes the holotype. N = number of specimens; SD = standard deviation.

Holotype	N	Range	Mean	SD
Standard length	81.2	20	48.1–90.9	80.2	–
Percentages of standard length
Body depth	30.7	20	26.1–31.4	29.4	1.5
Snout to dorsal-fin origin	50.5	20	50.0–54.3	51.9	1.1
Snout to pectoral-fin origin	25.1	20	25.1–27.3	25.7	0.8
Snout to pelvic-fin origin	47.6	20	46.5–50.3	48.7	1.1
Snout to anal-fin origin	64.9	20	63.1–67.2	66.4	1.6
Pelvic-fin insertion to anal-fin origin	16.3	20	14.4–17.9	16.6	1.0
Caudal-peduncle depth	10.9	20	9.9–11.7	10.3	0.4
Caudal-peduncle length	16.2	20	13.5–16.2	15.3	0.9
Dorsal-fin origin to caudal-fin base	47.8	20	47.6–49.2	47.9	0.5
Dorsal-fin base length	12.1	20	11.8–18.6	12.3	1.5
Anal-fin base length	21.5	20	21.4–24.9	22.4	1.0
Pectoral-fin length	19.7	20	18.4–21.1	19.6	0.8
Pelvic-fin length	15.5	20	13.2–20.5	14.7	1.5
Dorsal-fin length	20.7	20	18.6–22.3	20.2	1.0
Anal-fin length	16.8	20	14.1–17.1	16.0	0.8
Bony head length	26.5	20	24.8–30.2	26.8	1.1
Head depth	22.8	20	21.4–25.2	23.2	1.1
Percentages of head length
Horizontal eye diameter	31.4	20	28.3–35.0	30.8	1.5
Snout length	25.2	20	25.2–30.1	26.8	1.1
Least interorbital distance	29.6	20	25.5–32.4	30.1	1.6
Upper jaw length	46.2	20	43.4–47.1	45.9	1.5

Dorsal-fin rays ii,8*(19) or iii,8(1). Dorsal-fin origin at midpoint of standard length.First dorsal-fin pterygiophore located after neural spine of 11 th (1) vertebra. First unbranched ray about half the length of second ray. Thickened membranes between dorsal rays forming discrete folds extending from base of dorsal fin to about halfway along its length ( Fig. 3A ). Adipose fin present, its origin at vertical through last anal-fin ray insertion. Pectoral-fin rays i,10(2), 11(10) or 12*(8). Tip of adpressed pectoral fin not reaching vertical through pelvic-fin insertion. Pelvicfin rays i,7(20). Tip of adpressed pelvic fin not reaching origin of anal fin. Anal-fin rays iv, 15(2), 16*(5), 17(10), 18(2), or 19(1). Anteriormost rays of anal fin (including two largest unbranched rays and anteriormost two to three branched rays) with thickened membranes forming pocket-like folds extending from base to slightly beyond middle portion of fin ray ( Fig. 3B ). First anal-fin pterygiophore inserted behind haemal spine of 20 th (1) vertebra. Caudal fin forked, lobes similar in size; principal caudal-fin rays i,9/8,i (20). Interradial membranes of caudal fin thickened, forming folds typically between third and ninth branched rays of upper caudal lobe and between first and fifth branched rays of lower lobe. Margin of membrane folds present in upper lobe oriented ventrally and margins of membrane folds present in lower lobe oriented dorsally ( Fig. 3C ). Membrane fin folds more developed in adult specimens. First gill arch with 8(1), 9(1), or 10(3) gill rakers on the hypobranchial and ceratobranchial, 6(5) on the epibranchial, and one on cartilage between ceratobranchial and epibranchial. Branchiostegal rays 4(1), 3(1) on anterior ceratohyal, and 1(1) on posterior ceratohyal. Vertebrae 37(1). Supraneurals 6(1) FIGURE 2. Knodus ytuanama , paratype, CPUFMT 7647, 59.3 mm SL. Upper and lower jaws, left side, lateral view. Color in alcohol. Ground color light beige ( Fig. 4A–C ). Silvery pigmentation present on opercle, infraorbitals, and sides of body. Dark chromatophores densely concentrated on dorsal surface of head, from tip of snout to end of supraoccipital spine, dark pigmentation extending posteriorly over predorsal scales. Small, dark chromatophores also present on maxilla, lower jaw, ventral margin of orbit, and first and second infraorbitals; third and fourth infraorbitals with scattered dark pigmentation and fifth and sixth infraorbitals densely pigmented with dark chromatophores. Upper half of opercle densely pigmented with large dark chromatophores. Dorsal half of body (above midlateral stripe) dark. Adults present a dark, wide midlateral stripe extending from posterior margin of opercle to middle caudal-fin rays, and lack a humeral blotch. Young specimens with diffuse, narrow, vertically elongated humeral blotch ( Fig. 4C ) and with midlateral stripe more evident, starting approximately at vertical through dorsal-fin origin. Abdominal region almost devoid of dark chromatophores, some scattered dark chromatophores at region of anal-fin base. Dorsal, pectoral, and anal fins hyaline, with scattered dark chromatophores on interradial membranes. Pelvic fins hyaline. Adipose fin pale, with small, dark chromatophores concentrated on center of fin. Caudal fin with dark chromatophores along interradial membranes, outlining the rays and forming horizontal, narrow lines on caudal-fin lobes. FIGURE 3. Knodus ytuanama , CPUFMT 7647, close-up of A) dorsal fin (83.7 mm SL), B) anal fin (83.7 mm SL), and C) caudal fin (71.3 mm SL). Arrows indicate the presence of folded membranes lateral view, anterior to left. FIGURE 4. Knodus ytuanama , A) INPA 59847, paratype, 71.8 mm SL); B) INPA 59847, paratype, 75.7 mm SL; C) UFRO-I 22844, paratype, 28.6 mm SL; all from Brazil, Mato Grosso, Comodoro, rio Mutum. Color in life. Based on photographs taken in the field of three specimens (see Fig. 5 ). Overall body coloration silvery with greenish tint on infraorbital 4–5, half upper of opercle and on body sides. Midlateral, longitudinal stripe silvery-green Upper portion of the eye dark. Lower jaw beige. Caudal, anal, and dorsal fins reddish and slightly darkened Adipose fin yellow. Pelvic fin whitish. Sexual dimorphism. Mature males of Knodus ytuanama present bilateral hooks on first unbranched and anteriormost six branched pelvic-fin rays, and on fourth up to ninth branched anal-fin rays. Distribution. Knodus ytuanama is only known from the rio Mutum, a tributary of the upper rio Juruena at Chapada dos Parecis, rio Tapajós basin, Mato Grosso State , Brazil ( Fig. 6 ). Ecological notes. The rio Mutum is a small highland river (altitude 501 meters a.s.l. at the type locality), about 5–9 meters wide, 0.5–2.5 meters deep, with preserved riparian vegetation, abundant aquatic macrophytes and a bottom composed by sand, pebbles and dead leaves ( Fig. 7 ). Knodus ytuanama is a rheophilic species that was observed singly or small groups of 3–8 individuals swimming near the surface in fast flowing waters. Knodus ytuanama was the unique nektonic species of Characidae found at the river stretches with stronger current, but other nektonic characids occur in slow to moderate flowing stretches in same the locality, such as Hasemania nambiquara Bertaco & Malabarba , Hemigrammus skolioplatus Bertaco & Carvalho Hyphessobrycon comodoro Dagosta, Seren, Ferreira & Marinho , and H . hexastichos Bertaco & Carvalho. In addition, three benthic rheophilic fishes co-occur with K. ytuanama : Melanocharacidium dispilomma Buckup , Phenacorhamdia sp. and Cetopsorhamdia sp. 1 (cf. Ohara & Loeb, 2016: 4–5 ; figs. 4–5). Stomach contents of two paratypes revealed only the presence of ants. FIGURE 5. Knodus ytuanama INPA 59847, paratypes, 83.2 mm SL (upper) and 75.7 mm SL (lower), immediately after capture. FIGURE 6. Map of central western Brazil showing hydrographic basins on the Chapada dos Parecis, with the type-locality of Knodus ytuanama indicated by a black star. FIGURE 7. Type locality of Knodus ytuanama , rio Mutum near road BR-174, affluent of upper rio Juruena, rio Tapajós basin, Comodoro, Mato Grosso, Brazil. Etymology. The specific epithet ytuanama derives from the Tupi language, from the words ytu , waterfall, and anama , friend, and it refers to the fast-flowing habitat of the new species. A noun in apposition.