Osteogaster oharai, Tencatt & Carvalho & Silva & Britto, 2025

Tencatt, Luiz Fernando Caserta, Carvalho, Vandergleison de, Silva, Hugmar Pains da & Britto, Marcelo Ribeiro de, 2025, A new striped Osteogaster (Siluriformes: Callichthyidae) from the Madeira and Tapajós river basins, Brazil, with a new generic diagnosis and an identification key to the species of Osteogaster, Neotropical Ichthyology (e 240134) 23 (2) : -

Osteogaster oharai , new species urn:lsid:zoobank.org:act:ED4C6156-E673-4ACA-986C-964760E6BF04

( Fig. 1)

Corydoras aff. rabauti . —Silva et al., 2019:615 (tab. 3; list of species).

Holotype. MNRJ 55884 , 43.4 mm SL, Brazil, Mato Grosso, Aripuanã, stream tributary of the rio Aripuanã, upstream Salto das Andorinhas and Dardanelos waterfalls, rio Madeira basin, 10°16’46”S 59°33’08”W, 23 Jul 2014, H. P. Silva.

Paratypes. All from Brazil, Mato Grosso, Aripuanã (except when noted), rio Aripuanã basin upstream Salto das Andorinhas and Dardanelos waterfalls, rio Madeira basin. CPUFMT 4889, 2 c&s, 35.8–44.7 mm SL, stream tributary of the rio Aripuanã , 10°20’29”S 59°28’23”W, 20 Jul 2014, H. P. Silva. CPUFMT 4934, 5, 29.9–39.9 mm SL, stream tributary of the rio Aripuanã , 10°23’35”S 59°28’01”W, 24 Jul 2014, H. P. Silva. CPUFMT 5015, 2, 34.4-35.4 mm SL, stream tributary of the rio Aripuanã , 10°21’32”S 59°33’06”W, 24 Jul 2014, H. P. Silva. CPUFMT 5032, 1, 26.2 mm SL, stream tributary of the rio Aripuanã , 10°13’49”S 59°24’08”W, 18 Jul 2014, H. P. Silva. CPUFMT 5106, 2, 25.0– 36.7 mm SL, stream tributary of the rio Aripuanã , 10°16’36”S 59°26’53”W, 20 Jul 2014, H. P. Silva . INPA 11590 , 1 , 33.1 mm SL ; INPA 11721 , 1 , 34.4 mm SL ; INPA 12130 , 1 , 33.3 mm SL ; INPA 12131 , 3 , 29.7 - 30.1 mm SL, stream tributary of the rio Aripuanã , 10°10’00”S 59°27’34”W, 9 Nov 1976 , INPA staff. UFRO-ICT 21340 , 5 of 11, 34.1–42.8 mm SL, stream tributary of the rio Aripuanã , 10°10’42”S 59°27’26”W, 20 Jul 2013, D. Hungria . UFRO-ICT 21931 , 5 of 10, 32.1–42.5 mm SL, stream tributary of the rio Aripuanã , 10°11’43”S 59°26’58”W, 19 Jul 2013, W. M. Ohara . UFRO-ICT 22240 , 3 , 24.5–31.3 mm SL, igarapé do Areião , 10°27’26”S 59°22’08”W, 21 Jul 2013, W. M. Ohara . UFRO-ICT 22456 , 1 , 41.7 mm SL, Juína, stream tributary of the rio Aripuanã , 10°53’12”S 59°11’37”W, 23 Jul 2013, B. Barros. CPUFMT 5077, 1, 40.9 mm SL, collected with the holotype .

Non-type specimens. All from Brazil, Mato Grosso, Cotriguaçu (except when noted), rio Juruena basin, rio Tapajós basin. UFRO-ICT 21258 , 3 , 26.4–37.2 mm SL, Juruena , 10°21’02”S 58 ° 30’08”W, 22 Jul 2013, D. Hungria . UFRO-ICT 22449 , 4 of 7, 23.6–27.7 mm SL, Juína , igarapé do Boi Preto, 11 ° 13’06”S 58 ° 54’53”W, 11 Sep 2013, W. M. Ohara . CITL 1504 , 16 of 19, 25.8–37.7 mm SL, 3 c&s of 19, 38.3–41.1 mm SL, stream with unknown name, 09°51’36”S 58°28’34”W, 28 Oct 2010, I. M. Fernandes . CITL 1505 , 6 , 26.0– 38.9 mm SL, stream with unknown name, 09°49’14”S 58°27’50”W, 28 Oct 2010, I. M. Fernandes . CITL 1506 , 12 , 24.4–37.3 mm SL, stream with unknown name, 09°52’09”S 58°28’39”W, 28 Oct 2010, I. M. Fernandes . CITL 1507 , 2 , 28.3 –37.0 mm SL, stream with unknown name, 09°51’36”S 58°28’34”W, 29 Oct 2010, I. M. Fernandes . CITL 1508 , 6 , 26.7–37.8 mm SL, stream with unknown name, 09°52’09”S 58°28’39”W, 29 Oct 2010, I. M. Fernandes . CPUFMT 8621 , 633 , 19.7–43.2 mm SL ; MNRJ 55728 , 57 of 60, 21.6–39.7 mm SL, 3 c&s of 60, 35.6–41.1 mm SL, stream with unknown name, 09°48’43”S 58°15’50”W, Oct 2010 , T. Izzo . MZUSP 130834 , 11 , 21.9–37.3 mm SL, stream with unknown name, 09°51’36”S 58°28’34”W, 27 Oct 2010, I. M. Fernandes . NUP 25764 , 27 , 22.4–39.7 mm SL, stream with unknown name, 09°52’09”S 58°28’39”W, 27 Oct 2010, I. M. Fernandes .

Diagnosis. Osteogaster oharai can be distinguished from its congeners, except O. eques , O. rabauti , and O. zygata by having an oblique dorsolateral longitudinal dark stripe on flanks, starting at first dorsolateral body plate and gradually descending towards middle portion of caudal-fin base, variably extending diffuse on ventral lobe of caudal fin (vs. anterior portion of flank with large, conspicuous roughly rounded to longitudinally elliptical dark brown or black blotch, variably extending towards caudal-fin base, forming a somewhat mace-shaped longitudinal patch running parallel to longitudinal axis of the body or slightly inclined upwards anteriorly, with wider portion on anterior half of trunk, gradually becoming narrower posteriorly in O. aenea ; almost entirely dark brown or black in O. hephaestus ; a large, conspicuous dark brown or black blotch on anterodorsal portion of flank; blotch somewhat rounded to roughly diamond shaped in O. maclurei ; a longitudinal dark stripe almost entirely covering dorsal and middle portion of flank, running in parallel to longitudinal axis of the body or slightly inclined upwards anteriorly, with anterior portion slightly wider, smoothly becoming narrower posteriorly in O. melanotaenia ). The new species can be distinguished from O. eques , O. rabauti and O. zygata , by having moderately-developed posterolateral portion of scapulocoracoid, not expanded mesially (vs. strongly well-developed posterolateral portion of scapulocoracoid, conspicuously expanded mesially, contacting its counterpart and almost entirely covering ventral surface of trunk), and by having dorsolateral stripe not extending anteriorly and meeting its counterpart on head (vs. dorsolateral stripe extending anteriorly on head, converging mesially and meeting its counterpart on parieto-supraoccipital, forming a V-shaped pattern in dorsal view, with anteriormost portion variably reaching the frontals); it can be further distinguished from O. eques by having conspicuous dorsolateral dark stripe on flanks (vs. dorsolateral stripe indistinct from dark patch almost entirely covering flanks, conspicuous only on predorsal region of body).

Description. Morphometric data in Tab. 1. Head laterally compressed with convex dorsal profile, roughly triangular in dorsal view. Snout short to intermediate long, ranging from smoothly rounded to somewhat straight, variably with smoothly concave area on region of mesethmoid. Head profile convex from tip of snout to anterior nares, ascending nearly straight or slightly convex from this point to dorsal-fin origin; region of frontal fontanel variably slightly concave. Profile nearly straight to slightly convex along dorsal-fin base. Postdorsal-fin body profile slightly concave to adipose-fin spine, slightly concave from this point to caudal-fin base; region between dorsal and preadipose platelets typically ranging from smoothly concave to roughly straight. Ventral profile of body nearly straight or slightly convex from isthmus to pectoral girdle, and slightly convex from this point until pelvic girdle. Profile nearly straight to slightly convex from pelvic girdle to base of first anal-fin ray, ascending slightly concave until caudal-fin base. Body roughly elliptical in cross section at pectoral girdle, gradually becoming more compressed toward caudal fin. Highest body depth at vertical through anterior origin of dorsal fin.

Eye rounded, located dorsolaterally on head. Orbit delimited anteriorly by lateral ethmoid, anterodorsally by frontal, posterodorsally by sphenotic, posteroventrally by infraorbital 2, and anteroventrally by infraorbital 1 ( Fig. 2). Anterior and posterior nares close to each other, only separated by flap of skin. Anterior naris tubular. Posterior naris close to anterodorsal margin of orbit, separated from it by distance typically similar to naris diameter; slightly larger than naris diameter in some specimens. Mouth small, subterminal, width similar to bony orbit diameter. Maxillary barbel ranging from relatively short, not reaching posteroventral edge of opercle, to relatively long in size, slightly surpassing anteroventral limit of gill opening. Outer mental barbel typically slightly longer than maxillary barbel; outer mental barbel variably with similar size, or slightly shorter than maxillary barbel, apparently due to barbel damage in some specimens. Inner mental barbel fleshy, base of each counterpart slightly separated from each other. Area at mouth corner, ventral to maxillary barbel, typically with small wrinkle of skin. Small rounded papillae covering entire surface of all barbels, upper and lower lips, snout and isthmus.

Mesethmoid moderate-sized, its length slightly smaller than frontal length; anterior tip roughly straight in lateral view, relatively long, with size similar to or slightly larger than 50% of bone length; lateral cornua variably present; posterior margin relatively narrow, its width smaller than maximum width of posterior portion of bone; base typically relatively long, lateral posterodorsal expansion with relatively wide roughly triangular or trapezoid external projection, emerging relatively distant to distal point of suture with frontal; relatively short base, emerging relatively close to distal point of suture with frontal, or relatively narrow external projection in some specimens; external projection typically relatively long in size, distal tip slightly surpassing outermost margin of nasal bone, or variably short, not surpassing outermost margin of nasal bone; posterodorsal portion typically partially exposed and bearing small odontodes, variably almost entirely covered by thick skin layer, with tiny exposed portion; posterior ventrolateral expansion partially visible in dorsal view, emerging slightly anteriorly to external projection of lateral posterodorsal expansion ( Fig. 3). Upper and lower jaws edentulous; premaxilla overall funnel-like shaped, with anteroventral surface roughly square-, triangular- or rectangular-shaped in frontal view; anteroventral margin irregular; posterodorsal portion with conspicuous pointed process, mesially set in frontal view ( Fig. 4A); maxilla elongated, relatively slender and roughly hatchet shaped in frontal view, proximal half with roughly trapezoid to rounded laminar process on posterolateral portion ( Fig. 4A); dentary relatively slender, with roughly trapezoid to triangular expansion on anteroventral portion, in ventrolateral perspective, smoothly bent anteriorly, with distal edge variably smoothly curved posteriorly; roughly triangular process on its posterodorsal portion, bent posteriorly; anguloarticular moderately deep in middle portion, with roughly triangular to rounded dorsal laminar expansion, deepest area slightly posterior to posterodorsal margin of dentary; posteroventral portion with roughly triangular process, in lateral view, bent posteriorly ( Fig. 4B). Palatine longitudinally elongated, moderate-sized, with roughly triangular to rounded or trapezoid dorsolateral laminar expansion on its posterior portion, articulating with lateroventral expansion of posterior portion of mesethmoid; dorsolateral laminar expansion roughly falciform, curved anteriorly, in right side of specimen CPUFMT 4889, 35.8 mm SL, apparently due to malformation; posterolateral process well developed, extending posteriorly in parallel to dorsal margin of anterior laminar expansion of metapterygoid ( Fig. 4C).

Nasal capsule delimited posteriorly and dorsally by frontal, anteriorly by mesethmoid, and ventrally and posteriorly by lateral ethmoid ( Figs. 2‒3). Nasal slender, laterally curved, inner margin typically with poorly-developed laminar expansion, contacting only frontal or frontal and mesethmoid; outer margin with strongly reduced laminar expansion, nearly imperceptible in some specimens ( Figs. 2B, 3C, 5A). Lateral ethmoid ranging from slender to moderately deep in lateral view, moderately expanded anteroventrally, with anterodorsal expansion clearly separated from nasal, and anterior margin contacting external projection of lateral posterodorsal expansion of mesethmoid ( Fig. 3). Frontal elongated, typically narrow, width less than half of entire length, variably relatively wide, width equal to or slightly larger than half of entire length; anterior projection ranging from short, size smaller than nasal length, to moderate, size equal to nasal length ( Fig. 6); left side of specimen CITL 1504, 41.1 mm SL, not considered due to malformation of nasal ( Fig. 6B). Frontal fontanel large, slender, and somewhat ellipsoid; posterior tip extension slightly surpassing anterior margin of parieto-supraoccipital ( Figs. 6A, B, D). Sphenotic somewhat trapezoid,

contacting parieto-supraoccipital dorsally, pterotic-extrascapular posteriorly, second infraorbital posteroventrally and frontal anteriorly ( Fig. 2). Pterotic-extrascapular roughly pipe-shaped, with posteriormost portion contacting first lateral-line ossicle, posteroventral margin contacting cleithrum, and anteroventral margin contacting opercle and infraorbital 2; posterior expansion almost entirely covering lateral opening of swimbladder capsule, leaving slender area on its dorsal margin covered only by thick layer of skin ( Fig. 2). Parieto-supraoccipital wide, posterior process long, variably contacting nuchal plate; close but not in contact with nuchal plate in paratype CPUFMT 4889, 35.8 mm SL ( Fig. 6A); posterior process apparently not in contact with nuchal plate externally, but in contact under thick layer of skin in non-type specimen CITL 1504, 41.1 mm SL ( Fig. 6B); posterior process in direct contact with nuchal plate externally in paratype CPUFMT 4889, 44.7 mm SL ( Fig. 6C); posterior process relatively distant from nuchal plate externally, first dorsolateral body plate contacting its counterpart between both structures in a non-type specimen (MNRJ 55728, 35.6 mm SL; Fig. 6D; first dorsolateral body plate fragmented in one side of specimen, possibly due to malformation); region between posterior process and nuchal plate covered by thick layer of skin ( Fig. 6).

Two laminar infraorbitals with minute odontodes. Infraorbital 1 large, ventral laminar expansion typically ranging from well-developed to extremely well developed, covering most of lateral surface of snout in some specimens ( Figs. 2, 5B); some juvenile specimens (up to about 30.0 mm SL) with moderately-developed ventral expansion; anterior portion generally with conspicuously well-developed laminar expansion, reaching or surpassing anterior margin of nasal capsule; inner laminar expansion poorly developed ( Fig. 5C). Infraorbital 2 small, relatively wide, with posterior laminar expansion strongly well developed ( Figs. 2, 5B); posteroventral margin contacting posterodorsal ridge of hyomandibula, posterior margin contacting opercle, and posterodorsal edge contacting sphenotic and pterotic-extrascapular ( Fig. 2); posterodorsal portion with roughly triangular to trapezoid expansion, and middle portion with moderately- to well-developed roughly triangular expansion, smoothly bent downwards ( Figs. 2, 5B), with tip just above dorsal edge of posterodorsal ridge of hyomandibula ( Fig. 2); anterodorsal edge expanded anteriorly, reaching about middle portion of ventral margin of sphenotic ( Figs. 2, 5B); inner laminar expansion moderately developed ( Fig. 5C). Posterodorsal ridge of hyomandibula close to its articulation with opercle slender, exposed, and bearing small odontodes ( Figs. 2, 4B). Dorsal ridge of hyomandibula between pterotic-extrascapular and opercle typically covered by posterodorsal edge of infraorbital 2. Interopercle partially covered by thick layer of skin, with posterior portion exposed and bearing odontodes; subtriangular, anterior projection ranging from moderately to well developed ( Figs. 2, 4B). Preopercle elongated, relatively slender; minute odontodes on external surface ( Figs. 2, 4B). Opercle dorsoventrally elongated, relatively compact in shape, with width equal to or larger than half of entire length; free margin convex, without serrations and covered by small odontodes ( Figs. 2, 4B).

Four branchiostegal rays decreasing in size posteriorly. Hypobranchial 1 deep; hypobranchial 2 somewhat triangular, tip ossified and directed towards anterior portion, posterior margin cartilaginous; ossified portion well developed, its size about twice cartilaginous portion. Five ceratobranchials with expansions increasing posteriorly; ceratobranchial 1 with small process on anterior margin of mesial portion; ceratobranchial 3 notched on postero-lateral margin, variably with continuous laminar expansion on postero-lateral margin; ceratobranchial 5 toothed on posterodorsal surface, with 36 to 42 (8) teeth aligned in one row. Four epibranchials with similar size; epibranchial 2 slightly larger than others, with small pointed process on laminar expansion of posterior margin; epibranchial 3 with roughly triangular uncinate process on laminar expansion of posterior margin, uncinate process variably mesially curved. Two wide pharyngobranchials (3 and 4); pharyngobranchial 3 typically with roughly triangular laminar expansion on posterior margin, variably roughly rounded; laminar expansion typically notched. Upper tooth plate roughly oval, 39 to 52 (8) teeth aligned in two rows on posteroventral surface; rows closely aligned.

Lateral-line canal reaching cephalic laterosensory system through pterotic-extrascapular, branching twice before reaching sphenotic: pterotic branch, with single pore, preoperculomandibular branch conspicuously reduced, with single pore opening at postotic main canal; postotic main canal widens just posterior to pterotic branch. Sensory canal continuing through pterotic-extrascapular, reaching sphenotic as temporal canal, which splits into two branches: one branch giving rise to infraorbital canal, other branch connecting to frontal through supraorbital canal, both with single pore. Supraorbital canal branched, running through nasal bone. Epiphyseal branch conspicuously reduced; pore opening close to supraorbital main canal, directed towards frontal fontanel. Nasal canal with three openings, first on posterior edge, second on posterolateral portion and variably fused with first pore, and third on anterior edge. Infraorbital canal running through entire infraorbital 2, extending to infraorbital 1 and typically opening into two pores. Preoperculomandibular branch giving rise to preoperculo-mandibular canal, which runs through almost entire preopercle with three openings, leading to pores 3, 4, and 5, respectively.

Dorsal fin subtriangular, typically located just posterior to second or third dorsolateral body plate. Dorsal-fin rays II,8* (35), II,9 (1), with first or second branched rays as longest fin element, both typically with similar size, remaining branched rays typically decreasing in size posteriorly, with first, second and third branched rays slightly longer than ossified portion of dorsal-fin spine, remaining rays with similar size or shorter than spine; first branched rays (variably up to fifth) similar-sized, and just slightly longer than ossified portion of spine in some specimens; posterior margin of dorsal-fin spine with five to 13 strongly reduced serrations; most serrations antrorse; some serrations variably perpendicularly directed (in relation to main axis of spine), especially on distal portion of spine; bifid serrations variably present; serrations absent close to origin of spine; small odontodes on anterior and lateral surfaces of spine ( Fig. 7A). Nuchal plate moderately developed, almost entirely exposed, with minute odontodes. Spinelet short; spine ranging from poorly developed, with adpressed distal tip not reaching posterior origin of dorsal-fin base, to moderately developed, with adpressed distal tip reaching or slightly surpassing posterior origin of dorsal-fin base. Pectoral fin roughly triangular, its origin just posterior to gill opening. Pectoral-fin I,7 (2), I,7,i (3), I,8* (29), one specimen with I,6 on left pectoral fin, apparently due to malformation; first branched ray typically as longest fin element; branched rays decreasing in size posteriorly, with first and second, and variably third, branched rays slightly longer than ossified portion of pectoral-fin spine, remaining rays with similar size or shorter than spine; some specimens with first and second branched rays with similar size; posterior margin of pectoral-fin spine with 18 to 25 strongly reduced to poorly-developed serrations along almost its entire length, absent close to origin of spine; typically, most serrations antrorse, with some serrations perpendicularly directed (in relation to main axis of spine) and/or bifid or trifid; serrations fused at base variably present; some specimens with most serrations smoothly antrorse, nearly perpendicular in relation to main axis of spine; small odontodes on anterior, dorsal and ventral surfaces of spine ( Figs. 7B, C). Anteroventral portion of cleithrum and anterolateral portion of scapulocoracoid exposed; posterolateral portion of scapulocoracoid moderately developed, exposed, with anterior portion slightly to moderately expanded anteriorly, not in contact with anteroventral portion of cleithrum; exposed areas bearing small odontodes. Opening of axillary gland sensu Kiehl et al. (2006) located just posterior to pectoral-fin spine base.

Pelvic fin oblong, typically located just below second ventrolateral body plate, and at vertical through first or second dorsal-fin branched ray. Pelvic-fin rays i,5* (36); second branched ray typically as longest fin element, with rays decreasing in size towards both anterior and posterior margins of fin; unbranched ray as shorter fin element, or similar in size to last branched ray. Anterior internal process of basipterygium well developed and moderately laterally expanded, with obliquely placed dorsal lamina, smoothly converging mesially towards anterior edge of process; ventral laminar expansion obliquely placed, converging mesially towards anterior edge of process, clearly less developed than dorsal lamina; anterior external process laminar, somewhat sickleshaped, roughly rounded or quadrangular, or falciform (see Britto, 2003:141, fig. 22C), well developed and slightly to moderately expanded posteriorly; dorsal ischiac process well developed, with anterior laminar expansion irregular to roughly triangular, trapezoid or rounded, slightly to moderately expanded anteriorly, and posterior laminar expansion roughly triangular or rounded, moderately expanded posteriorly; ventral ischiac process clearly smaller than dorsal process, roughly triangular, bent anteriorly ( Fig. 8); specimen MNRJ 55728, 41.1 mm SL, with anterior internal process of basipterygium conspicuously narrow, anterior external process irregular, and right dorsal ischiac process just smoothly expanded anteriorly, apparently due to malformations ( Fig. 8D). Adipose fin roughly triangular, separated from base of last dorsal-fin ray by six to eight dorsolateral body plates. Anal fin subtriangular, located just posterior to 11 th, 12 th or 13 th ventrolateral body plates, and at vertical through adipose-fin spine base or region of preadipose platelets. Anal-fin rays ii,5,i (3), ii,6* (31); third ray typically as longest fin element, with rays decreasing in size towards both anterior and posterior margins of fin; fourth ray variably similar in size to third ray; first or last ray as shorter fin element, both variably similar in size. Caudal fin bilobed, with dorsal and ventral lobes similar in size or dorsal lobe slightly larger than ventral lobe; some specimens undergoing caudal regeneration with dorsal lobe smaller than ventral lobe. Caudal-fin rays i,11,i (1), i,12,i* (35), with generally five dorsal and ventral procurrent rays increasing in size posteriorly; small cartilage between upper principal and procurrent caudal-fin rays not observed ( Fig. 9).

Two to four laterosensory canals on trunk; first ossicle tubular, second ossicle laminar, both bearing small odontodes; third and fourth, when present, encased in third and fourth dorsolateral body plates, respectively. Body plates with minute odontodes scattered over exposed area, with conspicuous line of odontodes confined to posterior margins. Dorsolateral body plates 23 (12), 24* (24); ventrolateral body plates 20 (6), 21* (26), 22 (4). Dorsolateral body plates along dorsal-fin base 6 (24), 7* (12); dorsolateral body plates between adipose- and caudal-fin 6 (2), 7* (23), 8 (10), 9 (1). Preadipose platelets 2 (10), 3 (20), 4 (4), 5* (2). Ventral surface of trunk between posteroventral margin of cleithrum and pelvic-fin origin typically laterally delimited only by first ventrolateral body plate, variably by first and second ventrolateral body plates; ventral portion of first ventrolateral body plate ranging from slightly to moderately expanded anteriorly. Small platelets covering base of caudal-fin rays; small platelets disposed dorsally and ventrally between junctions of lateral plates on posterior portion of caudal peduncle. Anterior margin of orbit, above region of junction between frontal and lateral ethmoid, variably with small-sized platelets bearing odontodes. Ventral surface of trunk devoid of platelets, or covered by sparse and conspicuously small platelets in some specimens; platelets irregular in shape and bearing odontodes.

Vertebral count 21 (2), 22 (6); ribs 6 (5), 7 (3); first pair conspicuously large, middle portion closely connected to first ventrolateral body plate; tip close but not directly connected to anterior external process of basipterygium. Parapophysis of complex vertebra ranging from moderately to well developed.

Coloration in alcohol. Overall color pattern variations in Figs. 1 and 10. Ground color of body pale- to brownish yellow or beige, with top of head dark brown. Dorsal and lateral surface of head, and lateral surface of cleithrum covered by dark brown or black chromatophores, not forming blotches; posterior margin of cleithrum typically with conspicuous concentration of dark brown or black chromatophores, forming thin dark line, more evident on dorsal half of cleithrum. Border of pores of laterosensory canals typically with conspicuous concentration of dark brown or black chromatophores. Flank with oblique dorsolateral longitudinal dark brown or black stripe, typically starting at first dorsolateral body plate and gradually descending towards midventral portion of caudal-fin base, variably extending diffuse on ventral lobe of caudal fin; stripe slightly less inclined downwards, ending at middorsal portion of caudal-fin base in some specimens; stripe variably less evident on predorsal portion of flank, fragmented and/or not reaching caudal-fin base; stripe typically interrupted on dorsolateral body plate just ventral to dorsal-fin spine base; remaining portions of dorso- and ventrolateral body plates covered by dark brown or black chromatophores, not forming blotches. Ventrolateral body plates with conspicuous concentration of dark brown or black chromatophores on their anterior or middle portion, forming diffuse, transversally elongated patches, typically more evident on anterior half of flank; ventral portion of ventrolateral plates of body, especially on region around pelvic fin, typically devoid of or with sparse chromatophores. Posterior margin of body plates typically with conspicuous concentration of dark brown or black chromatophores, forming thin dark lines; such pattern variably absent or less evident in some body plates. Fins covered by dark brown or black chromatophores, not forming blotches; dorsal, pectoral, pelvic and caudal fins with chromatophores typically more concentrated on rays; dorsal-fin membranes variably with concentrations of dark brown or black chromatophores, especially close to dorsal-fin spine and/or on proximal portion of fin; ventral caudal-fin lobe slightly darker than dorsal lobe, especially on its proximal portion in some specimens.

Coloration in life. Similar to color pattern of preserved specimens, but with lighter ground color of body, which is typically greyish orange; specimens in larval stage with typical dark blotched pattern, and juveniles with large, ill-defined roughly elliptical dark patch on anterodorsal portion of trunk ( Fig. 11). Some specimens alternatively with greyish yellow ground color of body, which tends to be brighter on pectoral and pelvic fins ( Fig. 11E). Body covered by greenish yellow iridescent coloration; some specimens with yellow to green bright patches on opercle ( Figs. 11B, C, D, F).

Sexual dimorphism. As well-documented in Corydoradinae (see Nijssen, Isbrücker, 1980a; Britto, 2003; Spadella et al., 2017), male specimens of Osteogaster oharai present a genital papilla, which is somewhat tubular or lanceolate in shape.

Geographical distribution. Osteogaster oharai is currently known from its type-locality, the rio Aripuanã basin, rio Madeira drainage, and from the rio Juruena basin, rio Tapajós drainage, both in Mato Grosso State, Brazil ( Fig. 12).

Ecological notes. In the rio Aripuanã basin, the new species was captured in small streams tributaries of the rio Aripuanã upstream the Dardanelos-Andorinhas waterfalls complex (Silva et al., 2019:610, figs. 2A, B). The average width was estimated in 2.18 m ± 0.80, depth in 17.0 cm ± 7.6, water current in 3.83 m /s ± 7.4, dissolved oxygen in 1.18 mg /l ± 0.99, and temperature in 25.1 °C ± 2.0 (Silva et al., 2019:611, tab. 2). Upstream the Dardanelos-Andorinhas waterfalls complex, the only additional species of Corydoradinae found is a putatively undescribed species of Hoplisoma , referred as Corydoras sp. 2 in Silva et al. (2019:617, fig. 4). In the rio Juruena basin, most specimens were captured in small and shallow isolated pools of intermittent streams, directly flowing into the rio Juruena ( Fig. 13A), also occurring in larger/deeper streams, as the igarapé do Boi Preto ( Fig. 13B). In these sites (rio Juruena basin), the streams present dense marginal vegetation, muddy or dark tea-colored water, and substrate composed by sand and/or clay, leaf litter, submerged logs and branches, and large rocks ( Fig. 13). In the rio Juruena basin, no additional Corydoradinae species was found in syntopy.

Etymology. Osteogaster oharai is named in honor of Dr. Willian Massaharu Ohara,

dear friend and distinctive ichthyologist, not only for his extensive contributions to knowledge of South American freshwater fishes, but for having played a fundamental role in the professional history of LFCT, having generously donated dozens of species of Corydoradinae collected by him over the last decade, some of which were described in collaboration. A noun in a genitive case.

Conservation status. Currently, the new species is known from its type-locality,

the rio Aripuanã basin, with an additional record in the rio Juruena basin, both Mato

Grosso State, Brazil. Additionally, the species seems to occur in high abundance considering the available material, especially in the rio Juruena basin. It is also important to emphasize that the species is relatively frequent in the aquarium trade. Therefore,

considering the currently available data and according to the International Union for

Conservation of Nature (IUCN) categories and criteria (IUCN Standards and Petitions

Subcommittee, 2024), Osteogaster oharai can be classified as Least Concern (LC).

Remarks. As described for the new species (see “Coloration in alcohol” section), some congeners, especially O. zygata , and other species of Corydoradinae , such as Brochis arcuatus (Elwin, 1938) , B. bethanae (Bentley, Grant & Tencatt, 2021) , Corydoras narcissus

Nijssen & Isbrücker, 1980, Hoplisoma granti (Tencatt, Lima & Britto, 2019), and H. urucu

(Britto, Wosiacki & Montag, 2009), commonly present a small, vertical “pigmentation gap” (variably more or less intense depending on specimen) just ventral to dorsal-fin spine base, which can be extended further ventrally towards first ventrolateral body plate as a bright whitish vertically elongated patch, the latter typically more evident in living specimens ( Figs. 1, 10, 11B–F, 14; see Bentley et al., 2021). In some specimens,

only the pigmentation gap or the bright whitish vertically elongated patch are present,

while in others both are simultaneously present. Interestingly, such feature seems to be typically present (or at least more evident) in the Corydoradinae displaying dorsolateral stripe. This pigmentation gap and the vertically elongated whitish patch are placed just above the transverse process of the second pterygiophore (likely formed by the fusion of pterygiophores 2 and 3 according to Reis (1998)) and the strong ligament connecting this process to the first rib, respectively. Interestingly, there is a precise alignment between these structures and such distinct coloration (i.e., reduced dark brown or black pigmentation and bright whitish vertically elongated patch) (Bentley et al., 2021), but a possible relationship between them requires further investigation.