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

( Fig. 1; Tab. 1)

Corydoras sp. CW004. ― Grant, 2019:8 (brief discussion on morphological variations; photo in life). ― Tencatt et al., 2022a:2 (listed as a putative undescribed species). ―Tencatt et al., 2024:3 (listed as a putative undescribed species).

Corydoras sp. ‘CW004 Ancestor I’. ― Grant, 2019:9 (brief discussion on morphological variations; photo in life).

Corydoras sp. ‘CW004 Ancestor II’. ― Grant, 2019:9 (brief discussion on morphological variations; photo in life).

Corydoras sp. ‘CW004 Ancestor III’. ― Grant, 2019:9 (brief discussion on morphological variations; photo in life).

Corydoras sp. CW155. ― Grant, 2020:25 (breeding report; photo in life). ―Tencatt et al., 2022a:2 (listed as a putative undescribed species). ―Tencatt et al., 2024a:3 (listed as a putative undescribed species).

Corydoras sp. CW156.― Grant, 2020:25 (photo in life). ―Tencatt et al., 2022a:2 (listed as a putative undescribed species). ―Tencatt et al., 2024a:3 (listed as a putative undescribed species).

Corydoras sp. CW167. ―Tencatt et al., 2022a:2 (listed as a putative undescribed species). ―Tencatt et al., 2024a:3 (listed as a putative undescribed species).

Holotype. MNRJ 55759 , 54.8 mm SL, Brazil, Pará , Jacareacanga , igarapé do Buriti (locally known as igarapé Sonrizal), rio Tapajós basin, approx. 06°13’S 57°55’W, 3–11 Jul 2023, W. M. Ohara, L. F. C. T. Tencatt & M. Pinheiro.

Paratypes. All from Brazil, Pará , Jacareacanga, rio Tapajós basin, 3–11 Jul 2023, except when noted . CPUFMT 8619 , 4 , 27.9–38.1 mm SL ; INPA 61189 , 4 , 30.7–41.4 mm SL, igarapé Miuçuzinho , approx. 06°13’S 57°55’W, W. M. Ohara, L. F. C . T. Tencatt & M. Pinheiro . MNRJ 55760 , 5 , 47.3–58.4 mm SL ; INPA 61188 , 4 , 39.5–56.4 mm SL, igarapé do Pinto , approx. 05°51’S 57°40’W, collected by local fishermen . INPA 61169 , 10 , 31.5–53.8 mm SL ; MNRJ 55905 , 15 , 36.2–56.1 mm SL, igarapé do Pinto , approx. 05°51’S 57°40’W, 4 May 2024, collected by local fishermen . CITL 1458 , 5 of 7, 49.8–51.2 mm SL, 2 c&s of 7, 39.7–53.0 mm SL ; MZUSP 130710 , 5 , 26.6–48.1 mm SL; NUP 25701, 4, 28.0– 48.5 mm SL ; INPA 61187 , 2 , 35.0– 41.1 mm SL, all collected with the holotype .

Diagnosis. Hoplisoma noxium can be distinguished from its congeners, except for H. araguaiaense (Sands, 1990) , H. burgessi (Axelrod, 1987) , H. colossus (Tencatt, Grant & Bentley, 2023) , H. concolor (Weitzman, 1961) , H. esperanzae (Castro, 1987) , H. evelynae (Rössel, 1963) , H. eversi (Tencatt & Britto, 2016) , H. granti (Tencatt, Lima & Britto, 2019), H. julii (Steindachner, 1906) , H. melanistium (Regan, 1912) , H. oiapoquense (Nijssen, 1972) , H. parallelum (Burgess, 1993) , H. pavanelliae (Tencatt & Ohara, 2016) , H. polystictum (Regan, 1912) , H. schwartzi (Rössel, 1963) , H. surinamense ( Nijssen, 1970) , H. trilineatum (Cope, 1872) , H. xinguense (Nijssen, 1972) and H. tenebrosum , new species, by having the ventral surface of trunk with small to relatively large 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 yellow or beige blotches at least on the predorsal portion of body ( vs. small blotches, when present, dark brown or black); from Hoplisoma n. sp. (described below) by having the flanks covered by small dark brown or black blotches, which can be roughly longitudinally aligned and variably fused with each other, forming narrow, disrupted irregular lines, but not solid stripes ( vs. two or three longitudinal dark brown or black stripes on flanks), and by typically having more numerous pale blotches on the body ( vs. typically fewer 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 anterior portion of parieto-supraoccipital towards region of interopercle, transversally crossing the orbit and forming the 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. 1. Head laterally compressed with convex dorsal profile, roughly triangular in dorsal view. Snout moderately to relatively well developed, typically somewhat straight; smoothly rounded or pointed in some specimens. 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 posterior process of parieto-supraoccipital slightly convex in some specimens; region of anterior portion of mesethmoid and/or 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, concave from this point to caudal-fin base; region between dorsal and preadipose platelets ranging from smoothly concave to smoothly convex. 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. 2). 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 typically slightly longer than maxillary barbel; outer mental barbel sometimes shorter than maxillary barbel, apparently due to barbel damage. 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. 4). Upper and lower jaws edentulous; premaxilla overall funnel-like shaped, with anteroventral surface roughly triangular- to square-shaped in frontal view; anteroventral margin irregular; posterodorsal portion with conspicuous pointed process, mesially set in frontal view ( Fig. 5); maxilla elongated, relatively slender and roughly hatchet shaped in frontal view, its proximal half with roughly trapezoid to rounded laminar process on posterolateral portion ( Fig. 5); dentary relatively slender, with roughly trapezoid to triangular expansion on its anteroventral portion, in ventrolateral perspective, smoothly bent anteriorly, with distal edge variably

Holotype Low–High Mean±SD Standard length (mm) 54.8 46.2–58.4 50.6–3.49 Percent of standard length Depth of body 41.6 41.1–46.9 43.7–1.6 Predorsal distance 51.5 51.5–57.4 54.5–1.6 Prepelvic distance 50.4 50.3–53.1 51.7–0.8 Preanal distance 82.7 82.7–86.9 84.6–1.2 Preadipose distance 86.7 86.7–90.8 89.2–1.0 Length of dorsal spine 26.1 20.9–30.8 27.2–2.9 Length of pectoral spine 30.7 26.2–31.9 29.8–1.6 Length of adipose-fin spine 7.3 5.5–8.4 7.4–0.8 Depth of caudal peduncle 15.0 15.0–17.0 16.3–0.6 Length of dorsal-fin base 20.4 19.2–22.9 21.2–0.9 Dorsal to adipose distance 18.4 15.5–20.7 18.0–1.2 Maximum cleithral width 28.1 28.1–32.7 30.4–1.2 Length of maxillary barbel 17.9 14.9–20.3 17.5–1.5 Head length 51.8 47.3–52.0 49.2–1.6 Percent of head length Head depth 77.1 77.1–87.7 85.0–2.6 Least interorbital distance 25.4 25.4–29.4 27.8–1.1 Horizontal orbit diameter 19.0 19.0–21.6 20.6–0.7 Snout length 34.9 34.9–43.8 40.8–2.2 Least internarial distance 14.1 13.7–17.4 15.6–1.0

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. 5). 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; posterolateral process well developed, extending posteriorly in parallel to dorsal margin of anterior laminar expansion of metapterygoid ( Fig. 4).

Nasal capsule delimited posteriorly and dorsally by frontal, anteriorly by mesethmoid,

and ventrally and posteriorly by lateral ethmoid ( Fig. 4). Nasal slender, laterally curved,

inner margin typically with moderately-developed laminar expansion, contacting frontal and mesethmoid; outer margin with strongly reduced laminar expansion ( Figs.

2, 4, 6); juvenile c&s specimen ( CITL 1458, 39.7 mm SL; dorso- and ventrolateral body plates not contacting counterparts on region between dorsal and adipose fins and pelvic and anal fins, respectively) with conspicuously smaller nasal laminar expansions,

inner margin contacting only frontal. 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. 2, 4); slender in lateral view, with anterodorsal expansion slightly more distant from nasal in juvenile specimen ( CITL 1458, 39.7 mm SL);

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. 6). Frontal fontanel large, slender, and somewhat ellipsoid; posterior tip extension slightly surpassing anterior margin of parieto-supraoccipital ( Figs. 2, 6).

Sphenotic somewhat trapezoid, contacting parieto-supraoccipital dorsally, pterotic-extrascapular posteriorly, second infraorbital posteroventrally and frontal anteriorly ( Figs. 2, 6). Pterotic-extrascapular roughly pipe-shaped, with posteriormost portion contacting first lateral-line ossicle, posteroventral margin contacting cleithrum, and anteroventral margin contacting opercle and typically 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, contacting nuchal plate; region of contact 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 ( Fig. 2); smaller specimens (up to about 40.0 mm SL) typically with moderately-developed ventral expansion; anterior portion with conspicuously well-developed laminar expansion, slightly surpassing anterior margin of nasal capsule; anterior laminar expansion well developed, surpassing middle of nasal capsule in juvenile specimen ( CITL 1458, 39.7 mm SL); inner laminar expansion poorly developed ( Fig. 2). Infraorbital 2 small, relatively slender, with posterior laminar expansion moderately to well developed ( Fig. 2); 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. 2); posterodorsal portion with roughly triangular to trapezoid expansion, and middle portion with moderately-developed roughly triangular expansion, smoothly bent downwards ( Fig. 2), with tip just above dorsal edge of posterodorsal ridge of hyomandibula ( Fig. 2); inner laminar expansion poorly developed ( Fig. 2); infraorbital 2 laminar expansions clearly reduced in juvenile specimen ( CITL 1458, 39.7 mm SL), especially posterior one, which lacks secondary expansions on posterodorsal and middle portions. Posterodorsal ridge of hyomandibula close to its articulation with opercle slender, exposed, and bearing small odontodes ( Figs. 2, 6). 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. 2, 6). Preopercle elongated, relatively slender; minute odontodes on external surface ( Figs. 2, 6). Opercle dorsoventrally elongated, with width slightly smaller than half of its entire length; free margin convex, without serrations and covered by small odontodes ( Figs. 2, 6).

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; ceratobranchial 5 toothed on posterodorsal surface, with 35 to 48 (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, 40 to 53 (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 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; variably into three 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*(14), with first branched ray as longest fin element; branched rays typically 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; first and second branched ray similar in size to ossified portion of spine in some specimens, with second branched ray variably 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 variably clearly more elongated than spine, especially in juvenile specimens (up to about 40.0 mm SL); posterior margin of dorsal-fin spine with six to 21 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; bifid serrations variably present; serrations absent close to origin of spine; small odontodes on anterior and lateral surfaces of spine ( Fig. 7). Nuchal plate moderately developed, almost entirely exposed, with minute odontodes. Spinelet short; spine typically ranging from moderately developed, with adpressed distal tip slightly surpassing posterior origin of dorsal-fin base, to well developed, with adpressed distal tip clearly surpassing posterior origin of dorsal-fin base; some specimens with poorly-developed spine, with adpressed distal tip not reaching posterior origin of dorsal-fin base. Pectoral fin roughly triangular, its origin just posterior to gill opening. Pectoral-fin I,8(4), I,9*(11), 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 13 to 28 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 or roughly perpendicular in relation to main axis of spine; small odontodes on anterior, dorsal and ventral surfaces of spine ( Fig. 7). 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 dorsal-fin spine or first 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; second branched ray variably as long as first or third branched rays; 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, 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 typically slightly 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. 8). Adipose fin roughly triangular, separated from base of last dorsal-fin ray by five to seven dorsolateral body plates. Anal fin subtriangular, located just posterior to 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(4), ii,6*(1); 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; last ray typically as shorter fin element, variably similar in size to first ray. 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(2), i,12,i*(13), 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. 9).

Three or four laterosensory canals on trunk; first ossicle tubular, second ossicle laminar, both bearing small odontodes; third and fourth 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*(10), 24(5); ventrolateral body plates 20(3), 21*(10), 22(2). Dorsolateral body plates along dorsal-fin base 6(7), 7*(7), 8(1); dorsolateral body plates between adipose- and caudal-fin 6*(8), 7(7). Preadipose platelets 1(1), 2(8), 3*(6). 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, and variably region around ventral margin of nasal capsule, with small- to relatively large-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. 10).

Vertebral count 21(2); ribs 6(2); 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. 1). 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, variably spreading posteriorly, especially on dorsal portion of dorsolateral body plates around dorsal-fin base region and flank midline on anteriormost portion of trunk; pale blotches small to moderate in size, variably 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, typically with relatively large, conspicuous dark brown or black blotch, clearly darker than ground color of body; blotch variably diffuse. Dorsal half of dorsolateral body plates with conspicuous concentration of dark brown or black chromatophores, forming small- to moderate-sized dark blotches, roughly longitudinally aligned. Portion of flank extending from ventral half of dorsolateral body plates to dorsal half of ventrolateral body plates with conspicuous concentrations of dark brown or black chromatophores, forming smallto moderate-sized dark blotches, typically roughly aligned in longitudinal rows, especially on region just above and just below flank midline; blotches longitudinally aligned around flank midline variably fused, forming narrow and irregular dark lines. Ventral half of ventrolateral body plates with less concentrated dark brown or black chromatophores, especially on region around pelvic-fin base, forming small-sized dark blotches in some specimens; blotches typically roughly aligned in longitudinal rows, variably diffuse. Dark blotches on flanks vertically fused, forming roughly transversally elongated 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, typically forming dark patch, which is variably diffuse or absent; in specimens with anterior dark patch, remaining portion of fin with clearly less concentrated dark brown or black chromatophores, typically forming small-sized blotches, which can be elongated in parallel to main axis of rays (“dash-like”) or roughly rounded; dark blotches homogeneously scattered all over dorsal fin in specimens lacking anterior dark patch; blotches variably diffuse. Pectoral, pelvic, and anal fins with dark brown or black chromatophores, typically not forming blotches; chromatophores more concentrated on rays in some specimens; anal fin with small-sized dark markings in some specimens, similar in shape to dorsal-fin blotches and typically 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, typically forming small- to moderate-sized blotches, ranging from elongated in parallel to main axis of rays (“dash-like”) to roughly rounded; blotches diffuse, nearly imperceptible in some specimens; blotches variably roughly aligned in vertical rows, forming transversal dark stripes; middle portion of caudal-fin base with small irregular dark brown or black dot, variably diffuse; 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 ( Figs. 11–14). Wild-caught specimens variably with light brownish orange coloration ( Figs. 11, 12); aquarium specimens often with greyish yellow coloration ( Figs. 13, 14). Additionally, both wild-caught and aquarium adult 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. 11, 13, 14). Body covered by greenish yellow iridescent coloration, especially on region of opercle and cleithrum.

Sexual dimorphism. Male specimens of Hoplisoma noxium present a genital papilla, which is somewhat tubular to conical, a condition well-documented in Corydoradinae (see Britto, 2003; Nijssen, Isbrücker, 1980b; Spadella et al., 2017). The presence of elongated 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. noxium . However, considering the available data, it was not possible to assign it as a sexually dimorphic feature as it seems to be more related to age/size, being mostly observed in juvenile specimens (up to about 40.0 mm SL).

Geographical distribution. Hoplisoma noxium is so far known from its type-locality, the igarapé do Buriti (= igarapé Sonrizal), igarapé Miuçuzinho and igarapé do Pinto, all tributaries of the rio Tapajós, rio Amazonas basin, Pará State, Brazil ( Fig. 15).

Ecological notes. The igarapé do Buriti (= igarapé Sonrizal) ( Fig. 16), type-locality of the new species, is a medium-sized tributary of the rio Tapajós characterized by having relatively clear tea-colored water, width ranging from about 10.0 m to about 30.0 m, depth typically not surpassing 1.5 m, and slow to moderate water current in the sampled sites. The substrate is mostly composed of sand and fine gravel, with stretches accumulating leaf litter and submerged logs/branches. Additionally, some stretches present submerged and marginal vegetation, including the partially exposed submerged roots of marginal trees. The igarapé Miuçuzinho is a relatively large tributary of the igarapé Muiuçu, itself a tributary of the rio Tapajós, characterized by having a relatively turbid water, with width around 20.0 to 30.0 m, and depth ranging from about 40 cm to more than 1.50 m, and substrate mostly composed by sand, with stretches accumulating leaf litter and submerged logs/branches, as well as partially exposed submerged roots of marginal trees, in the sampled site. In both localities, H. noxium was never observed during day time, being only observed, and captured during the night. The new species was observed by LFCT and WMO close to the margins, in sites with leaf litter and partially exposed submerged roots of marginal trees, in small numbers (up to four individuals) and relatively distant from each other. At the type-locality, H. noxium cooccurs with several other Corydoradinae species, namely: Brochis sp. C 151 and CW176 (which we consider to be the same species), C. caramater , Corydoras sp. CW066 and CW174, and Hoplisoma sp. C 152. In the igarapé Miuçuzinho, the new species cooccurs with Corydoras sp. CW174, Hoplisoma sp. C 152 and CW193, while in the igarapé do Pinto it cooccurs only with Hoplisoma sp. CW193. Another interesting information on the new species was provided by the local fishermen, which reported that specimens of H. noxium must be separated from any other fish species just after capture, otherwise they can rapidly kill them, making the transport water milky and foamy on the surface, which was also observed by LFCT and WMO. Additionally, the same fisherman reported that getting “stung” by them (via dorsal and pectoral spines) is clearly more painful than any other Corydoradinae in the region, eventually causing minor allergic/inflammatory processes.

Etymology. The specific epithet “ noxium ” derives from the Latin “ noxa ”, a noun meaning “injury, damage”, plus “ ium ”, a Latin suffix used to form adjectives from nouns.

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 typelocality, the igarapé do Buriti (= igarapé Sonrizal), plus two additional records, the igarapé Miuçuzinho and the igarapé do Pinto, all tributaries of the rio Tapajós basin in

Pará State, Brazil. Despite the relatively restricted geographical distribution, part of the drainage (headwaters) where the three streams flow is protected by some Conservation

Units, especially the Floresta Nacional do Amaná, Floresta Nacional do Urupadi, and also by indigenous territories of the Munduruku ethnic group. 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 noxium would be classified as Least Concern (LC).

Remarks. Subtle differences in color pattern were observed in individuals of the three different populations examined herein, especially when comparing specimens from the type-locality with those from the igarapé do Pinto. Specimens from the igarapé do Buriti typically present more evident mask-like blotch and dark pigmentation associated with dorsal fin, i.e., on its anterior portion and on region around anterior portion of its base ( Figs. 11A–D), while in the specimens from the igarapé do Pinto, such dark markings are typically less evident, or even absent in the case of the dark patches on dorsal fin ( Figs. 11E–G). The specimens from the igarapé Miuçuzinho are more similar to those from the type-locality, but apparently tend to have more vertically elongated dark blotches on flanks.

Curiously, in both igarapé do Buriti and igarapé Miuçuzinho, the new species cooccurs with congeners with conspicuous mask-like blotch and dark blotches on anterior portion of dorsal fin and the region surrounding anterior portion of dorsal-fin base, especially Brochis sp. C 151/CW176, Corydoras sp. CW174 and Hoplisoma sp. C 152 (the first not captured in the igarapé Miuçuzinho). In the igarapé do Pinto, only Hoplisoma sp. CW193 was found to cooccur with the new species, which lacks a mask-like blotch and dark blotches on anterior portion of dorsal fin and the region surrounding anterior portion of dorsal-fin base. In this context, it is possible that the color pattern of the syntopic (regarding water body, not mesohabitat) species can, at least in part, explain the differences in color between different populations of H. noxium . Nevertheless, it is important to highlight that our observations on specimens from the igarapé Miuçuzinho are based on a small sample, composed only by four juvenile specimens (CPUFMT 8619, 27.9–38.1 mm SL).

Regarding snout morphology, H. noxium present a relatively wide range of variation, which can be moderately to relatively well developed, with dorsal profile slightly convex to somewhat straight or even slightly concave on region of anterior portion of mesethmoid, and anterior tip smoothly rounded or pointed ( Figs. 1, 11, 13, 14, 17). Considering that specimens with different snout shapes, as well as slightly different color patterns, were found in the same population, and that, regardless of this, all of them share key features used to delimit species and/or genera within Corydoradinae , such as the morphology of the infraorbitals 1 and 2, dorsal- and pectoral-fin spines serrations, and mesethmoid (see Dias et al. (2024)); Description and Discussion sections), we considered them as congeneric and conspecific. This treatment diverges from current interpretation used in the aquarium trade, which is based essentially on the snout morphology, an approach that proved unreliable in the light of our analysis. As illustrated by previous studies ( e.g., Tencatt et al., 2019; 2021; 2024a,b), the snout shape can conspicuously change along individual’s development, tending to become more pronounced with growth, which seems to be the case of H. noxium ( Fig. 18). The specimen with more pronounced snout is the largest among paratypes ( Fig. 17). Nevertheless, some species of Hoplisoma may present more pronounced snout in intermediary stages of development, with early and final stages displaying blunter snouts, as in congener H. thanatos (see Tencatt et al., 2022a:11, fig. 3).

Hoplisoma noxium has been bred under aquarium conditions by Roland van Ouwerkerk, who documented its ontogenetic development from 8.0 to 32.0 mm TL, showing general changes in external morphology and color pattern ( Fig. 18). Considering the average quality of the several available photos, especially of the earliest stage (8.0 mm TL), the following description is focused mainly on general external morphology and color pattern. Even though only the photos that better illustrate the general aspect of each development stage were selected to compose Fig. 18, all of them were used for the following descriptions. Specimen with 8.0 mm TL apparently in early post-flexion stage ( Fig. 18); head slightly depressed, becoming gradually deeper along individual’s growth; snout relatively short and rounded; eye moderate in size; median fin fold present, slightly absorbed, extending from posterior margin of dorsal fin to genital opening; dorsal, pectoral and caudal fins with distinct rays, including dorsal- and pectoral-fin spines; pelvic fin distinct, with presence of distinct rays uncertain; caudal-fin asymmetrical, its dorsal portion slightly longer than ventral. Body covered by dark brown or black chromatophores, more evident on head and anterior portion of dorsal-fin base; 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 slightly more diffuse dark markings, forming longitudinal series of moderate-sized blotches on flanks; 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, which tends to be gradually less evident along individual’s growth, completely fading in some specimens; dorsal fin with diffuse roughly longitudinal dark stripes, and diffuse roughly transversal stripes on caudal fin.

Specimen with 10.0 mm TL apparently in early juvenile stage ( Fig. 18), median fold apparently almost completely absorbed, with all fins more developed, especially adipose, pelvic, and anal fins; rays on pelvic and anal fins distinct, caudal fin smoothly bilobed; body plates apparently present. Body slightly more pigmented, with wider mask-like blotch; longitudinal series of moderate-sized dark blotches on flanks more evident and well defined; pectoral, pelvic and anal fins with small, diffuse dark blotches, which tend to fade along individual’s growth, especially on pectoral and pelvic fins. Specimen with 12.0 mm TL in early juvenile stage ( Fig. 18), with fin fold completely absorbed, all fins clearly more developed; snout slightly more pronounced, body plates poorly developed; dark markings on flanks clearly smaller, except for some slightly larger blotches along flank midline and along dorsum, more evident around anterior portion of adipose fin; ill-defined whitish to brownish yellow or pale yellow/beige blotches apparently present on head, especially on its dorsal surface and opercle. Specimen with 32.0 mm TL in juvenile stage ( Fig. 18), with more pronounced and pointed snout; caudal-fin ventral lobe more evident; body plates conspicuously more developed, apparently fully covering flanks; overall color pattern as described for fully-grown adults.