Hoplisoma tenebrosum, Tencatt & Ohara & Carvalho & Grant & Britto, 2025

Tencatt, Luiz Fernando Caserta, Ohara, Willian Massaharu, Carvalho, Vandergleison de, Grant, Steven & Britto, Marcelo, 2025, Two exquisite new species of Hoplisoma (Siluriformes: Callichthyidae) from the rio Tapajós basin, Brazil, with a discussion on the morphology of the mesethmoid within Corydoradinae, Neotropical Ichthyology (e 240100) 23 (1) : -

Hoplisoma tenebrosum , new species urn:lsid:zoobank.org:act:

( Fig. 19; Tab. 2)

Corydoras sp. CW168. ―Tencatt et al., 2022a:2 (listed as a putative undescribed species). ―Tencatt et al., 2024a:3 (listed as a putative undescribed species).

Holotype. MNRJ 55757 , 52.5 mm SL, Brazil, Amazonas , Maués , igarapé Água-branca (= Ipixuna) at the confluence with the igarapé Palomita , rio Tapajós basin, rio Amazonas basin, approx. 06°36’S 58°37’W, 3–11 Jul 2023, collected by local fishermen.

Paratypes. All from Brazil, Amazonas , Maués , rio Tapajós basin, rio Amazonas basin, approx. 06°36’S 58°37’W, 3–11 Jul 2023, collected by local fishermen . CITL 1459 , 5 of 6, 43.2–49.2 mm SL, 1 c&s of 6, 47.4 mm SL ; INPA 61190 , 3 , 31.0– 42.8 mm SL ; MZUSP 130711 , 4 , 43.7–46.5 mm SL, igarapé do Roncador . CPUFMT 8620 , 6 , 29.3– 49.7 mm SL ; MNRJ 55758 , 5 of 6, 32.4–49.2 mm SL, 1 c&s of 6, 54.6 mm SL; NUP 25702, 5, 31.2–51.4 mm SL, collected with holotype .

Diagnosis. Hoplisoma tenebrosum can be distinguished from its congeners, except for H. araguaiaense , H. burgessi , H. colossus , H. concolor , H. esperanzae , H. evelynae , H. eversi , H. granti , H. julii , H. melanistium , H. oiapoquense , H. parallelum , H. pavanelliae , H. polystictum , H. schwartzi , H. surinamense , H. trilineatum , H. xinguense and H. noxium by having the ventral surface of trunk with small- to relatively large-sized coalescent platelets, forming a typical mosaic-like pattern ( vs. platelets on the ventral surface of trunk, when present, small-sized and not coalescent, not forming a mosaic-like pattern); from H. araguaiaense , H. burgessi , H. colossus , H. concolor , H. esperanzae , H. evelynae , H. eversi , H. granti , H. julii , H. melanistium , H. oiapoquense , H. parallelum , H. pavanelliae , H. polystictum , H. schwartzi , H. surinamense , H. trilineatum , and H. xinguense by having small, whitish to brownish yellow or pale yellow/beige blotches on the predorsal portion of body ( vs. blotches on predorsal portion of body, when present, dark brown or black); from H. noxium by having two or three longitudinal dark brown or black stripes on flanks ( vs. flanks covered by small dark brown or black blotches, which can be roughly longitudinally aligned and variably fused with each other, forming narrow irregular lines, but not solid stripes), and by typically having fewer pale blotches on the body ( vs. typically more numerous pale blotches). The new species can be further distinguished from H. araguaiaense , H. eversi , H. granti , H. julii , H. pavanelliae , H. polystictum , H. trilineatum , and H. xinguense by the presence of a large, smoothly arched dark brown or black patch extending from the anterior portion of parieto-supraoccipital towards the region of interopercle, transversally crossing the orbit and forming a typical mask-like blotch ( vs. mask-like blotch absent); from H. burgessi , H. melanistium , H. oiapoquense , H. parallelum , H. pavanelliae , H. polystictum and H. surinamense by having a mosaic-like pattern of plates entirely or almost entirely covering the ventral surface of trunk ( vs. mosaic-like pattern of plates restricted to some portions of the ventral surface of trunk, representing up to about 50% of its area).

Description. Morphometric data in Tab. 2. Head laterally compressed with convex dorsal profile, roughly triangular in dorsal view. Snout moderately to relatively well developed and somewhat straight. 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 anterior portion of mesethmoid and/or region of frontal fontanel typically slightly concave. Profile nearly straight to slightly convex along dorsal-fin base. Postdorsal-fin body profile slightly concave to adipose-fin spine, concave from this point to caudal-fin base; region between dorsal and preadipose platelets nearly straight to slightly convex in some specimens. 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 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 dorsal-fin origin.

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. 20). Anterior and posterior nares close to each other, separated only by flap of skin. Anterior naris tubular. Posterior naris close to anterodorsal margin of orbit, separated from it by distance similar to naris diameter. Mouth small, subterminal, width similar to bony orbit diameter. Maxillary barbel length ranging from moderate, not reaching anteroventral limit of gill opening, to relatively long, slightly surpassing anteroventral limit of gill opening. Outer mental barbel slightly longer than maxillary barbel. 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 ( Fig. 3). Small rounded papillae covering entire surface of all barbels, upper and lower lips, snout and isthmus.

Mesethmoid moderate in size, its length slightly smaller than frontal length; anterior tip roughly straight in lateral view, relatively short, slightly smaller than 50% of bone length; posterior margin relatively narrow, its width smaller than maximum width of posterior portion of bone; base relatively short, lateral posterodorsal expansion with relatively wide roughly triangular or trapezoid external projection, emerging relatively close to distal point of suture with frontal; external projection relatively long in size, its distal tip slightly surpassing outermost margin of nasal bone; posterodorsal portion partially exposed and bearing small odontodes; posterior ventrolateral expansion partially visible in dorsal view, emerging slightly anteriorly to external projection of lateral posterodorsal expansion ( Fig. 21). Upper and lower jaws edentulous; premaxilla overall funnel-like shaped, with anteroventral surface roughly square-shaped to slightly horizontally rectangular in frontal view; anteroventral margin irregular; posterodorsal portion with conspicuous pointed process, mesially set in frontal view ( Fig. 22); maxilla elongated, relatively slender and roughly hatchet shaped in frontal view, its proximal half with roughly triangular laminar process on posterolateral portion ( Fig. 22); dentary relatively slender, with roughly trapezoid to triangular expansion on its anteroventral portion, in ventrolateral perspective, smoothly bent anteriorly, with distal edge variably smoothly curved posteriorly; roughly triangular process on its posterodorsal portion, bent posteriorly; angulo-articular moderately deep in its middle portion, with roughly triangular to rounded dorsal laminar expansion, its deepest area just posterior to posterodorsal margin of dentary; posteroventral portion with roughly triangular process, in lateral view, bent posteriorly ( Fig. 22). Palatine longitudinally elongated, moderate-sized, with roughly triangular dorsolateral laminar expansion on its posterior portion, articulating with lateroventral expansion of posterior portion of mesethmoid; posterolateral process well developed, extending posteriorly in parallel to dorsal margin of anterior laminar expansion of metapterygoid ( Fig. 21).

Nasal capsule delimited posteriorly and dorsally by frontal, anteriorly by mesethmoid, and ventrally and posteriorly by lateral ethmoid. Nasal slender, laterally curved, inner margin typically with poorly-developed laminar expansion, contacting only frontal; outer margin with strongly reduced laminar expansion ( Figs. 20, 21). Lateral ethmoid moderately deep in lateral view, moderately expanded anteroventrally, with anterodorsal expansion slightly separated from nasal, and anterior margin contacting external projection of lateral posterodorsal expansion of mesethmoid ( Figs. 20, 21); partially exposed and bearing small odontodes in some specimens. Frontal elongated, narrow, width less than half of entire length; anterior projection short, size smaller than nasal length ( Fig. 23). Frontal fontanel large, slender, and somewhat ellipsoid; posterior tip extension slightly surpassing anterior margin of parieto-supraoccipital ( Figs. 23). Sphenotic somewhat trapezoid, contacting parieto-supraoccipital dorsally, pterotic-extrascapular posteriorly, second infraorbital posteroventrally and frontal anteriorly ( Figs. 20, 23). 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 swim bladder capsule, leaving slender area on its dorsal margin covered only by thick layer of skin ( Fig. 20). Parieto-supraoccipital wide, posterior process long, contacting nuchal plate; region of contact between posterior process and nuchal plate covered by thick layer of skin ( Fig. 23).

Two laminar infraorbitals with minute odontodes. Infraorbital 1 large, ventral laminar expansion typically ranging from well-developed to extremely well developed ( Fig. 20); juvenile specimens (up to about 40.0 mm SL; dorso- and ventrolateral body plates not contacting counterparts on region between dorsal and adipose fins and pelvic and anal fins, respectively) with ventral laminar expansion ranging from poorly to moderately developed; anterior portion with well-developed laminar expansion, surpassing middle of nasal capsule, nearly reaching anterior margin of nasal capsule in some specimens; inner laminar expansion poorly developed ( Fig. 20). Infraorbital 2 small, relatively slender, with posterior laminar expansion moderately to well developed ( Fig. 20); posteroventral margin contacting posterodorsal ridge of hyomandibula, posterior margin close but not in direct contact with opercle, and posterodorsal edge contacting sphenotic and pterotic-extrascapular ( Fig. 20); posterodorsal portion with roughly triangular to trapezoid expansion, and middle portion with poorly-developed and rounded to moderately-developed and roughly triangular expansion; when moderately developed, middle expansion smoothly bent downwards ( Fig. 20), with tip just above dorsal edge of posterodorsal ridge of hyomandibula ( Fig. 20); inner laminar expansion poorly developed ( Fig. 20). Posterodorsal ridge of hyomandibula close to its articulation with opercle slender, exposed, and bearing small odontodes ( Figs. 20, 22). Dorsal ridge of hyomandibula between pterotic-extrascapular and opercle covered by thick layer of skin, variably exposed and bearing small odontodes. Interopercle entirely or almost entirely covered by thick layer of skin; posterior portion variably exposed and bearing odontodes; subtriangular, anterior projection well developed ( Figs. 20, 22). Preopercle elongated, relatively slender; minute odontodes on external surface ( Figs. 20, 22). Opercle dorsoventrally elongated, with width slightly smaller than half of its entire length; free margin convex, without serrations and covered by small odontodes ( Figs. 20, 22).

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; process variably strongly reduced; ceratobranchial 3 notched on postero-lateral margin; variably with continuous laminar expansion on postero-lateral margin; ceratobranchial 5 toothed on posterodorsal surface, with 47 to 49 (2) 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 mesially-curved uncinate process on laminar expansion of posterior margin. Two wide pharyngobranchials (3 and 4); pharyngobranchial 3 with roughly triangular laminar expansion on posterior margin; laminar expansion variably notched. Upper tooth plate roughly oval, 50 to 58 (2) 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, the 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 typically 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 dorsolateral body plate. Dorsal-fin rays II,7(1), II,8*(9), II,9(5), with first branched ray 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 dorsal-fin spine, remaining rays with similar size or shorter than spine; branched rays on middle portion of dorsal fin smaller than adjacent rays in some specimens, forming notch on dorsal-fin posterior margin; first and second branched rays clearly more elongated than spine in some specimens; posterior margin of dorsal-fin spine with 18 to 24 strongly reduced to poorly-developed serrations; most serrations antrorse; some serrations variably perpendicularly directed (relative to main axis of spine), especially on distal portion of spine; serrations absent close to origin of spine; small odontodes on anterior and lateral surfaces of spine ( Fig. 24). Nuchal plate moderately developed, almost entirely exposed, with minute odontodes. Spinelet short; spine typically well developed, with adpressed distal tip clearly surpassing posterior origin of dorsal-fin base; variably moderately developed, with adpressed distal tip slightly surpassing posterior origin of dorsal-fin base. Pectoral fin roughly triangular, its origin just posterior to gill opening. Pectoral-fin I,8(4), I,9(8), I,9,i(1), I,10*(2), with first branched ray as longest fin element; branched rays decreasing in size posteriorly, with first and typically second branched rays slightly longer than ossified portion of pectoral-fin spine, remaining rays with similar size or shorter than spine; some specimens with second branched ray nearly equal in size to spine; posterior margin of pectoral spine with 26 to 33 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 (relative to main axis of spine) and/or bifid; 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 ( Fig. 24). Anteroventral portion of cleithrum and anterolateral portion of scapulocoracoid exposed; posterolateral portion of scapulocoracoid moderately developed, exposed, with anterior portion slightly 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, located just below first or second ventrolateral body plate, and at vertical through first or second dorsal-fin branched ray. Pelvic-fin rays i,5*(15); second branched ray typically as longest fin element, with rays decreasing in size towards both anterior and posterior margins of fin; third branched ray variably as long as second branched ray; unbranched ray typically as shorter fin element. Anterior internal process of basipterygium well developed and ranging from slightly to moderately laterally expanded, with obliquely placed dorsal lamina, converging mesially towards anterior edge of process; ventral laminar expansion obliquely placed, smoothly converging mesially towards anterior edge of process, clearly less developed than dorsal lamina; anterior external process laminar, moderately developed and slightly to moderately expanded posteriorly; dorsal ischiac process well developed, with anterior laminar expansion moderately expanded anteriorly, and posterior laminar expansion slightly to moderately expanded posteriorly; anterior and posterior laminar expansions of ischiac process roughly triangular or rounded; ventral ischiac process clearly smaller than dorsal process, roughly triangular, bent anteriorly ( Fig. 25). Adipose fin roughly triangular, separated from base of last dorsal-fin ray by six or seven 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*(10), i,6(1), i,6,i(1), ii,6(2), single specimen with ii,2,i, possibly due to malformation; third or fourth ray as the longest fin element, with rays decreasing in size towards both anterior and posterior margins of fin; first or last ray as shorter fin element. 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,12,i*(13), i,13,i(1), single specimen with i,5,i,1,i,7,i (possibly due to malformation), with generally four or five dorsal and ventral procurrent rays increasing in size posteriorly; small cartilage between upper principal and procurrent caudal-fin rays not observed ( Fig. 26).

Two to four laterosensory canals on trunk; first ossicle tubular, second ossicle laminar, both bearing small odontodes; third and fourth, if 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 22(3), 23*(9), 24(3); ventrolateral body plates 19(1), 20*(8), 21(6). Dorsolateral body plates along dorsal-fin base 6 (4), 7* (11); dorsolateral body plates between adipose- and caudal-fin 5(1), 6*(10), 7(4). Preadipose platelets 2(9), 3*(5). Ventral surface of trunk between posteroventral margin of cleithrum and pelvic-fin origin typically laterally delimited only by first ventrolateral body plate; 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- to moderate-sized platelets bearing odontodes. Ventral surface of trunk mostly covered by small- to relatively large-sized coalescent platelets, forming typical mosaic-like pattern; platelets irregular in shape and bearing odontodes ( Fig. 27).

Vertebral count 21(1), 22(1); ribs 6(1), 7(1); first pair conspicuously large, its middle portion closely connected to first ventrolateral body plate; its tip connected to anterior external process of basipterygium. Parapophysis of complex vertebra well developed.

Color in alcohol. Ground color of body brownish yellow to pale yellow or beige, with dorsal surface of head dark brown or black ( Fig. 19). Dorsal and lateral surface of head, and lateral surface of cleithrum densely covered by dark brown or black chromatophores, not forming small dark blotches; lateral and dorsal surface of body anterior to dorsal fin with rounded, elliptical or vermiculated/irregular whitish to brownish yellow or pale yellow/beige blotches, especially on snout and posterodorsally to orbit; pale blotches small to moderate in size, variably scarce and/or diffuse; large, smoothly arched dark brown or black patch extending from anterior portion of parieto-supraoccipital towards region of interopercle, transversally crossing orbit and forming typical mask-like blotch; mask-like blotch typically not reaching interopercle, ending on middle portion of preopercle; some specimens with less evident mask-like blotch; anterior portion of opercle with more concentrated dark brown or black chromatophores, especially on its ventral half, forming irregular dark patch, typically fused with mask-like blotch; opercular patch variably diffuse; lips and barbels with dark brown or black chromatophores, conspicuously more concentrated on upper lip and maxillary barbel; ventrolateral portion of head typically with dark brown or black chromatophores, with isthmus devoid of chromatophores; isthmus with scarce dark brown or black chromatophores in some specimens. Region of first dorsolateral body plate surrounding parieto-supraoccipital with conspicuous concentration of dark brown or black chromatophores, variably forming V-shaped pattern in dorsal view; some specimens with diffuse dark patch. Border of pores of laterosensory canals variably with conspicuous concentration of dark brown or black chromatophores. Anterodorsal portion of trunk, around anterior portion of dorsal-fin base, with relatively large, dark brown or black blotch, clearly darker than ground color of body. Dorsal portion of dorsolateral body plates with conspicuous concentration of dark brown or black chromatophores, variably forming dorsal longitudinal stripe or roughly longitudinally aligned dark blotches; dorsal stripe fragmented in some specimens; region of dorsolateral body plates just above midline of flank with conspicuous concentration of dark brown or black chromatophores, forming dark longitudinal stripe extending from region at vertical through anterior portion of dorsal-fin base to caudal-fin base; mid-dorsolateral stripe not reaching caudal-fin base and/or fragmented posteriorly in some specimens; dorsal and mid-dorsolateral stripe variably fused with dark blotch on anterior portion of dorsal-fin base; remaining portions of dorsolateral body plates clearly paler, with conspicuously less concentrated dark brown or black chromatophores. Region of ventrolateral body plates just below midline of flank with conspicuous concentration of dark brown or black chromatophores, forming dark longitudinal stripe extending from region at vertical through anterior portion of dorsal-fin base to caudal-fin base; mid-ventrolateral stripe not reaching caudal-fin base and/or fragmented posteriorly in some specimens; remaining portions of ventrolateral body plates paler, with conspicuously less concentrated dark brown or black chromatophores, especially on region around pelvic-fin base; mid-dorsolateral and mid-ventrolateral stripes ranging from slender to wide; some specimens with ventral half of ventrolateral body plates on region between pelvic and anal fins with conspicuous concentration of dark brown or black chromatophores, forming rounded, irregular or vertically elongated blotches, roughly aligned longitudinally and variably diffuse. Middle portion of flank, especially on anterior half of trunk, with small dark brown or black blotches in some specimens. Posterior margin of body plates variably with conspicuous concentration of dark brown or black chromatophores, forming thin dark lines. Dorsal fin covered by dark brown or black chromatophores, conspicuously more concentrated on its anterior portion, especially between spine and third branched ray, forming dark patch, which is variably diffuse; remaining portion of fin with clearly less concentrated dark brown or black chromatophores, variably forming small- to moderate-sized blotches; dorsal-fin blotches typically elongated in parallel to main axis of rays (“dash-like”), variably roughly rounded and/or diffuse. Pectoral, pelvic, and anal fins with dark brown or black chromatophores, typically more concentrated on rays and not forming blotches; anal fin with small- to moderate-sized dark markings in some specimens, typically elongated in parallel to main axis of rays (“dash-like”) and diffuse. Adipose fin covered by dark brown or black chromatophores, typically more concentrated on spine and posterior portion of membrane, forming one or more irregular dark patches, with moderate to relatively large size; patches variably diffuse. Caudal fin with dark brown or black chromatophores, conspicuously more concentrated on rays, variably forming small- to moderate-sized blotches, typically longitudinally elongated (“dash-like”) and diffuse; blotches roughly transversally aligned in some specimens, forming transversal dark stripes; middle portion of caudal-fin base with small irregular dark brown or black dot, variably diffuse or indistinct from mid-dorsolateral stripe; small platelets covering base of caudal-fin rays with conspicuous concentration of dark brown or black chromatophores, variably more concentrated on posterior portion of plates.

Color in life. Similar to color pattern of preserved specimens, but with brighter ground color of body. Wild-caught specimens typically with light brownish orange coloration ( Fig. 28); aquarium specimens often with pale brownish yellow or beige coloration ( Figs. 29, 30). Additionally, adult aquarium specimens variably with transversal dark stripe on posterior portion of caudal peduncle, just anterior to dark dot on middle portion of caudal-fin base, typical of specimens in early development (see Remarks section below) ( Figs. 29, 30); similar pattern on caudal peduncle in one paratype (MNRJ 55758, 46.9 mm SL; Fig. 28). Body covered by greenish yellow iridescent coloration, especially on region of opercle and cleithrum.

Sexual dimorphism. Male specimens of H. tenebrosum present a genital papilla, which is somewhat tubular, a condition well-documented in Corydoradinae (see Britto, 2003; Nijssen, Isbrücker, 1980b; Spadella et al., 2017). The presence of elongated dorsal-fin spine plus two first dorsal-fin branched rays have been associated with dimorphic males of Hoplisoma (see Britto, Lima, 2003; Knaack, 2007; Tencatt et al., 2014a), a feature observed in some specimens of H. tenebrosum . Nonetheless, it was not possible to undoubtedly assign it as a sexually dimorphic feature as most large-sized specimens (fully-developed adults with up to about 50.0 mm SL) with well-developed tubular genital papilla (allegedly males) do not have clearly elongated first branched rays, similar to the observed in specimens lacking genital papilla (allegedly females). Among the paratypes, only one specimen (MNRJ 55758, 46.9 mm SL; Fig. 28) has clearly elongated first and second dorsal-fin branched rays, which also presents a tubular genital papilla.

Geographical distribution. Hoplisoma tenebrosum is so far known from its type-locality, the igarapé Água-branca (= igarapé Ipixuna), and one of its tributaries, the igarapé Palomita, and also from the igarapé do Roncador, all tributaries of the rio Tapajós, rio Amazonas basin, Amazonas State, Brazil ( Fig. 15).

Ecological notes. According to local fishermen from Jacareacanga, Pará State (M. Pinheiro, 2023, pers. comm.), H. tenebrosum presents similar behavior and habitat to the described for H. noxium (see Ecological notes section). At the type-locality, the igarapé Água-branca, H. tenebrosum cooccurs with two other Corydoradinae species,

Brochis bifasciatus and Hoplisoma sp. CW127, and with B. ornata (Nijssen & Isbrücker 1976) and Hoplisoma sp. C 133 in the igarapé do Roncador, forming mimetic trios in these two sites.

Etymology. The specific epithet “ tenebrosum ” derives from the Latin noun “ tenebrae ”,

which means “darkness, gloom”, plus “ -ōsum ”, a Latin suffix used to form adjectives from nouns. In Brazil, the adjective tenebrous is often used to describe something that is frightening, gloomy, or malevolent. The name alludes to the powerful toxin released by the new species under stress, which kills any fish kept in the same bag/container during transport. An adjective.

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

the igarapé Água-branca, and a single additional record in the igarapé do Roncador,

both tributaries of the rio Tapajós in Amazonas State, Brazil. Despite the relatively restricted geographical distribution, the region where both streams flow is protected by

Conservation Units, such as the Floresta Nacional de Urupadi and Parque Nacional do

Acari, as well as by indigenous territories. 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, 2022), Hoplisoma tenebrosum would be classified as Least Concern (LC).

Remarks. Currently, the new species is known to occur in three localities, the igarapé

Água-branca (or Ipixuna) and its smaller tributary, the igarapé Palomita, and the igarapé do Roncador, both draining to the rio Tapajós, presenting subtle differences in color pattern in each of them. Specimens from the igarapé Água-branca and igarapé Palomita typically present wider mid-dorsolateral and mid-ventrolateral ( Fig. 28), while the specimens from the igarapé do Roncador typically display slender stripes ( Fig. 28). Similar to the case involving H. noxium , the syntopic (regarding water body, not mesohabitat)

Corydoradinae species with convergent color pattern ( i.e., mask-like blotch, anterior portion of dorsal fin and the region surrounding anterior portion of its base dark, plus dark longitudinal stripes on flanks) in each of these sites are taxonomically different. As aforementioned, in the Água-branca/Palomita system, the new species cooccurs with B. bifasciata (CW135) and a putatively undescribed species of Hoplisoma , coded as CW127, both with wide longitudinal dark stripes on flanks, whereas the population of the igarapé do Roncador was found in syntopy with B. ornata (CW134) and a putatively undescribed congener, Hoplisoma sp. C 133, both with slender stripes. The snout shape of H. tenebrosum presents a range of variation similar to that described for H. noxium (see Remarks section). Likewise, the snout becomes more pronounced with growth and no differences among different populations were observed ( Figs. 28–30).

Hoplisoma tenebrosum has been bred under aquarium conditions by Hans-Georg Evers ( Fig. 30), who documented its ontogenetic development from 12.0 to 28.0 mm LT, showing general changes in external morphology and color pattern ( Fig. 30). Specimen with 12.0 mm TL in early post-flexion stage ( Fig. 30); head slightly depressed, becoming gradually deeper along individual’s growth; snout relatively short and rounded; barbels with relatively well-developed papillae, which will gradually become less developed along individual’s growth; eye moderate in size; median fin fold present, slightly absorbed, extending from posterior margin of dorsal fin to genital opening; all rayed fins with distinct rays, including dorsal- and pectoral-fin spines; dorsal fin almost completely detached from median fold; pelvic fin distinct; anal and caudal fins partially detached from fin fold, with caudal-fin asymmetrical, its dorsal portion slightly longer than ventral; hypural plates visible by transparency; adipose fin indistinct; body plates apparently absent.

Specimen in early post-flexion stage (12.0 mm TL; Fig. 30) with body covered by dark brown or black chromatophores, clearly more evident on head, forming large, smoothly arched dark patch extending from anterior portion of parieto-supraoccipital towards region of interopercle, transversally crossing orbit and forming typical mask-like blotch; oblique dark stripe from anteroventral margin of orbit to upper lip lateral area, which gradually fades along individual’s growth. Flanks with diffuse dark markings; middle portion of caudal-fin base with small irregular dark brown or black dot; diffuse transversal dark stripe on posterior portion of caudal peduncle, just anterior to dark dot on middle portion of caudal-fin base; body covered by greenish yellow iridescent coloration.

Specimen with 18.0 mm TL in early juvenile stage ( Fig. 30), with snout slightly more pronounced; median fold almost completely absorbed, with all fins more developed, especially adipose and anal fins; adipose fin almost completely detached from fin fold, with distinct spine; caudal fin smoothly bilobed, connected to adipose fin by small portion of fin fold; body plates visible. Body slightly more pigmented, with conspicuous concentration of dark-brown or black chromatophores, forming longitudinal series of diffuse moderate-sized blotches on flanks, more evident on flank midline; transversal dark stripe on caudal peduncle slightly more evident; caudal fin with somewhat transversal dark stripes. Specimen with 28.0 mm TL in juvenile stage, with fin fold completely absorbed, all fins clearly more developed; caudal-fin ventral lobe more evident; body plates conspicuously more developed but not fully covering flanks; color pattern overall similar to adults, except for mid-ventrolateral dark stripe, which is still not well defined, and the presence of conspicuous transversal dark stripe on caudal peduncle, which is typically absent in adults.