Corydoras aurofrenatus Eigenmann & Kennedy, 1903

Tencatt, Luiz Fernando Caserta, Santos, Sérgio Alexandre dos & Britto, Marcelo Ribeiro de, 2020, Taxonomic review of the typical long-snouted species of Corydoras (Siluriformes: Callichthyidae) from the río de La Plata basin, Neotropical Ichthyology 18 (4), pp. 1-36 : 16-26

Corydoras aurofrenatus Eigenmann & Kennedy, 1903

( Figs. 2B , 3B , 7 , 8 , 9 , 11A )

Corydoras aurofrenatus Eigenmann, Kennedy, 1903:506–508 (original description; type-locality: Aguada near arroyo Trementina, Paraguay). — Nijssen, Isbrücker, 1980a:215, 217, table X (listed; member of the “ Corydoras acutus group”; morphometric data of the holotype). — Axenrot, Kullander, 2003:262, (occurrence in the río Paraguay basin; discussion on color pattern variations; proposed as senior synonym of Corydoras ellisae ). — Reis, 2003:295 (listed). — Fuller, Evers, 2005:74 (photo in life of an alleged topotype; presence in the aquarium hobby). — Ferraris, 2007:113 (listed). — Tencatt, Pavanelli, 2015:294 (listed as comparative material). — Tencatt, Britto, 2016: e150062 (listed as comparative material). — Tencatt, Evers, 2016:e150019 (listed as comparative material). — Tencatt, Ohara, 2016a:438 (listed as comparative material). — Tencatt, Ohara, 2016b:e150063 (listed as comparative material). — Tencatt et al., 2016:e150089 (relation with Corydoras paleatus taxonomic history; diagnosis from Corydoras froehlichi ; listed as comparative material). — Espíndola et al., 2018:(listed as comparative material). — Tencatt et al., 2019:468 (listed as comparative material).

Corydoras flaveolus (non Ihering, 1911). — Ellis, 1913:398, 407, Pl. XXVIII fig. 1 (identification key; catalogue).

Corydoras ellisae Gosline, 1940:13 , 17, 18 (original description; type-locality: arroyo Poná, Sapucay, Paraguay; identification key). — Axenrot, Kullander, 2003:262, (occurrence in the río Paraguay basin; proposed as junior synonym of Corydoras aurofrenatus ). — Reis, 2003:297 (listed). Fuller, Evers, 2005:119 (photo in life; presence in the aquarium hobby). — Britski et al., 2007:154–155 (identification key; brief description). — Ferraris, 2007:116 (listed). — Tencatt et al., 2013:263 (listed as comparative material). — Tencatt et al., 2014a:96 (listed as comparative material). — Tencatt et al., 2014b:78 (listed as comparative material). — Tencatt et al., 2016:e150089 (diagnosis from Corydoras froehlichi ).

Diagnosis. Corydoras aurofrenatus can be distinguished from its congeners, except for the species within lineage 1, by the presence of the following features: temporal sensory canal in sphenotic with two pores anterior to the branch that gives rise to infraorbital canal (vs. with a single pore), upper tooth plate of branchial arch with three to four series of teeth (vs. two series of teeth), area at the corner of the mouth, ventral to maxillary barbel, with a fleshy flap (vs. fleshy flap absent). Corydoras aurofrenatus can be distinguished from the species of the lineage 1, except for C. acutus , C. areio , C. cervinus , C. coriatae Burgess, 1997 , C. desana Lima & Sazima, 2017, C. filamentosus Nijssen & Isbrücker, 1983 , C. fowleri Böhlke, 1950 , C. geoffroy Lacépède, 1803 , C. maculifer , C. negro Knaack, 2004 , C. ourastigma Nijssen, 1972 , C. oxyrhynchus , C. sarareensis Dinkelmeyer, 1995 , C. semiaquilus Weitzman, 1964 , C. septentrionalis Gosline, 1940 , C. simulatus Weitzman & Nijssen, 1970 , C. solox , C. stenocephalus Eigenmann & Allen, 1942 , C. treitlii Steindachner, 1906 and C. zawadzkii by the absence of a dark brown or black stripe transversally crossing the eye (vs. presence of such stripe, forming the typical mask-like blotch). Corydoras aurofrenatus is diagnosed from C. coriatae , C. fowleri and C. semiaquilus by having ventral surface of head and trunk densely covered by small, not coalescent platelets (vs. ventral surface of head and trunk covered by relatively large, coalescent platelets). Corydoras aurofrenatus can be promptly distinguished from C. acutus , C. areio , C. cervinus , C. desana , C. filamentosus , C. geoffroy , C. maculifer , C. negro , C. ourastigma , C. oxyrhynchus , C. sarareensis , C. septentrionalis , C. simulatus , C. solox , C. stenocephalus , C. treitlii and C. zawadzkii by the color pattern of the middle portion of its flank, composed by two or three dark brown or black patches, with first one below dorsal-fin, second one below adipose-fin base and third one, if present, diffuse, and on caudal peduncle base, patches decreasing in size posteriorly (vs. covered by small, rounded black spots, with a longitudinal dark brown or black stripe along midline of flank; stripe variably fragmented and generally more evident on posterior half of flanks in C. acutus , C. filamentosus and C. vittatus ; covered by numerous, small, rounded, irregular or elongated, dark brown or black spots aligned in both longitudinal rows along flanks and in vertical rows on lateral body plates in C. cervinus and females of C. sarareensis ; covered by small, rounded, irregular or elongated, dark brown or black spots roughly aligned in longitudinal rows; spots variably fused, forming slender longitudinal stripes in C. maculifer ; with longitudinal series of small, rounded or irregular black spots; series of spots just below midline of flank variably fused, forming a slender longitudinal black stripe; region just above midline of flank with wider longitudinal black stripe in C. zawadzkii ; with conspicuous concentration of dark brown or black chromatophores on dorsolateral body plates; variably, dorsolateral body plates entirely or almost entirely covered by intensely dark brown or black coloration; region of ventrolateral body plates close to flank midline with irregular dark brown or black spots or conspicuous concentration of dark brown or black chromatophores in C. geoffroy , C. negro , C. stenocephalus , C. solox and C. treitlii ; with two dark brown or black blotches, first one larger and vertically elongated, below dorsal fin, and second one smaller, roughly rectangular or rounded, on base of caudal peduncle; region between the two blotches with smaller and variably diffuse dark brown or black markings in C. desana ; generally with two dark brown or black patches, first one larger, extending from the region just anterior to dorsal fin to region close to adipose-fin anterior origin; first patch more intensely pigmented below dorsal fin, becoming diffuse posteriorly; and second one smaller, on base of caudal peduncle; patches fused in some specimens, forming a single large patch on flanks in C. septentrionalis ; generally with two dark brown or black patches, first one below dorsal fin and second one on posterior portion of caudal peduncle; first patch generally larger than second one, variably smaller, diffuse or even absent; second patch generally smaller than first one, vertically elongated; region between the two patches with scattered dark brown or black chromatophores in C. simulatus ; with conspicuous concentrations of dark brown or black chromatophores on anterior half of flank, forming rounded, irregular or vertically elongated blotches; presence of fused blotches, forming a large patch below dorsal fin in some specimens; posterior half of flank with rounded or irregular blotches roughly aligned in longitudinal rows; blotches variably more intensely pigmented close to flank midline in males of C. sarareensis ; with dark brown or black, small, irregular or rounded blotches on dorsolateral body plates and on ventrolateral body plates on region close to flank midline; blotches on dorsolateral body plates on the posterior half of the flanks larger in C. areio ; anterior portion of flanks with small, rounded or irregular, dark brown or black spots, and a large, oblong, dark brown or black patch on caudal peduncle in C. ourastigma ; diffuse dark brown or black chromatophores scattered all over the body; chromatophores conspicuously concentrated along with posterior margin of lateral body plates in C. oxyrhynchus ). Additionally, Corydoras aurofrenatus can be distinguished from C. septentrionalis by the presence of a poorly developed fleshy flap of skin at the corner of the mouth (vs. moderately to well developed, forming a barbel-like structure). Corydoras aurofrenatus is further distinguished from C. negro by the presence of short opercular membrane, leaving anteroventral portion of cleithrum exposed (vs. long, covering anteroventral portion of cleithrum).

Description. Morphometric data presented in Tab. 1 . Head compressed with convex dorsal profile, roughly triangular in dorsal view. Snout conical, conspicuously pointed. Head profile slightly concave from tip of snout to anterior nares, ascending slightly convex from this point to tip of posterior process of parieto-supraoccipital. Profile slightly convex along dorsal-fin base. Postdorsal-fin body profile concave to adipose-fin spine; concave from this point to caudal-fin base. Ventral profile of body nearly straight from isthmus to pectoral girdle, and slightly convex from this point until pelvic girdle. Profile nearly straight from pelvic girdle to base of first anal-fin ray, concave from this point until caudal-fin base. Body roughly elliptical in cross section at pectoral girdle, gradually becoming more compressed toward caudal fin.

Eye rounded, located dorsolaterally on head. Orbit delimited anteriorly by lateral ethmoid, anterodorsally by frontal, posterodorsally by sphenotic, posteriorly by infraorbital 2, and ventrally by infraorbital 1. 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 similar to naris diameter. Mouth small, subterminal, width nearly equal to bony orbit diameter. Maxillary barbel long in size, reaching anteroventral limit of gill opening. Outer mental barbel slightly longer than maxillary barbel. Area at corner of mouth, ventral to maxillary barbel, with reduced fleshy flap. Inner mental barbel fleshy, base of each counterpart slightly separated from each other. Small rounded papillae covering entire surface of all barbels, upper and lower lips, snout and isthmus.

Mesethmoid long, anterior tip well developed, larger than 50% of the bone length (see Britto, 2003: 123, character 1, state 0; fig. 1A), posterior portion relatively narrow, entirely covered by thin layer of skin. Middle portion of mesethmoid with well-developed lateroventral process; region of process with width slightly larger than width of posterior portion of mesethmoid. Nasal capsule delimited anterodorsally by mesethmoid, anteriorly and ventrally by lateral ethmoid, and posteriorly and dorsally by frontal. Nasal slender, curved laterally, inner margin laminar, with mesial border contacting frontal and mesethmoid, variably contacting only frontal. Lateral ethmoid conspicuously expanded anteriorly, with anterodorsal expansion contacting only mesethmoid, and anteroventral expansion connected to lateroventral process of mesethmoid. Frontal elongated, strongly narrow, width clearly smaller than half of its entire length; anterior projection short, size smaller than nasal length. Frontal fontanel large, conspicuously slender, posterior tip extension markedly entering anterior margin of parieto-supraoccipital. Sphenotic somewhat trapezoid, contacting parieto-supraoccipital dorsally, compound pterotic posteriorly, infraorbital 2 ventrally and frontal anteriorly ( Fig. 2B ). Compound pterotic 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. 2B ). Parieto-supraoccipital wide, posterior process long and contacting nuchal plate, and region of contact between posterior process and nuchal plate exposed.

Two laminar infraorbitals with minute odontodes. Infraorbital 1 large, ventral laminar expansion ranging from moderately- to well developed; anterior portion with laminar expansion ranging from moderately developed, almost reaching to anterior margin of nasal capsule, to well developed, slightly surpassing anterior margin of nasal capsule; inner laminar expansion strongly reduced ( Fig. 2B ). Infraorbital 2 small, widened, with posterior laminar expansion well developed, posteroventral margin contacting posterodorsal ridge of hyomandibula, posterodorsal edge contacting sphenotic and compound pterotic; inner laminar expansion moderately developed ( Fig. 2B ). Posterodorsal ridge of hyomandibula close to its articulation with opercle slender, exposed, reduced and bearing small odontodes. Dorsal ridge of hyomandibula between compound pterotic and opercle covered by thick layer of skin. Interopercle covered by thin layer of skin, subtriangular, anterior projection well-developed. Preopercle relatively slender, elongated, minute odontodes sparse on external surface. Opercle dorsoventrally elongated, width equal or smaller than half of entire length, free margin slightly convex, without serrations and covered by small odontodes.

Four branchiostegal rays decreasing in size posteriorly. Hypobranchial 2 somewhat triangular, tip ossified and directed towards anterior portion, posterior margin cartilaginous, ossified portion moderately to well developed, ranging from slightly larger to more than twice size of cartilaginous portion. Five ceratobranchials with expansions increasing posteriorly; ceratobranchial 1 with small process on anterior margin of mesial portion; ceratobranchial 3 notched on posterolateral margin, variably with continuous laminar expansion; ceratobranchial 5 toothed on posterodorsal surface, with 22 to 28 (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 triangular uncinate process on laminar expansion of posterior margin, process variably trapezoid. Two wide pharyngobranchials (3 and 4). Pharyngobranchial 3 with triangular laminar expansion, variably notched, on posterior margin. Upper tooth plate oval, 34 to 53 (3) teeth roughly aligned in three or four rows on posteroventral surface. Lateral-line canal reaching cephalic laterosensory system through compound pterotic, branching twice before reaching sphenotic: pterotic branch, with single pore. Preoperculomandibular branch conspicuously reduced, with single pore opening close to postotic main canal. Postotic main canal becoming widened just posterior to pterotic branch. Sensory canal continuing through compound pterotic, 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, with one and two pores, respectively. Supraorbital canal branched, running through nasal bone. Epiphyseal branch relatively long, pore opening close to frontal fontanel. Nasal canal with three openings, first on posterior edge, second, on posterolateral portion generally fused with first pore, and third on anterior edge. Infraorbital canal running through entire second infraorbital, extending to infraorbital 1 and opening into two pores. Preoperculomandibular branch giving rise to preoperculo- mandibular canal, which runs through entire preopercle with three openings, leading to pores 3, 4, and 5, respectively.

Dorsal fin subtriangular, located just posterior to third dorsolateral body plate. Dorsal-fin rays II,7 (1), II,8 (26), II,9 (3), posterior margin of dorsal-fin spine with four to seven poorly-developed serrations directed towards tip of spine, serrations arranged on distal half of its posterior margin; small odontodes on anterior and lateral surfaces of spine. Nuchal plate well developed, exposed, with minute odontodes. Spinelet short, spine moderately developed, adpressed distal tip slightly surpassing posterior origin of dorsal-fin base, and anterior margin with small odontodes. Pectoral fin roughly triangular, its origin just posterior to gill opening. Pectoral-fin rays I,9 (15), I,10 (14), posterior margin of pectoral spine with 13 to 21 moderately- to well-developed conical serrations along its entire length, most serrations directed towards pectoral-spine origin, and some serrations perpendicularly directed; small odontodes on anterior, dorsal and ventral surfaces of spine ( Fig. 3B ). Anteroventral portion of cleithrum exposed; posterolateral portion of scapulocoracoid exposed; minute odontodes sparse on exposed areas. Pelvic fin oblong, located just below third ventrolateral body plate, and at vertical through first branched dorsal-fin ray. Pelvic-fin rays i,5. Adipose fin roughly triangular, separated from base of last dorsal-fin ray by generally six dorsolateral body plates. Anal fin subtriangular, located just posterior to 12 th ventrolateral body plates, and at vertical through anterior margin of adipose-fin spine. Anal-fin rays ii,5 (1), ii,6 (24). Caudal fin bilobed, markedly furcated, with dorsal lobe slightly larger than ventral lobe. Caudal-fin rays i,12,i, generally four dorsal and ventral procurrent rays.

Four to five laterosensory canals on trunk. First ossicle tubular, second ossicle laminar, and remaining lateral-line canals, when present, encased in third, fourth and fifth dorsolateral body plates, respectively. Body plates with minute odontodes scattered over exposed area, a conspicuous line of odontodes confined on posterior margins. Dorsolateral body plates 22 (1), 23 (19), 24 (10), 25 (4). Ventrolateral body plates 20 (14), 21 (11), 22 (9). Dorsolateral body plates along dorsal-fin base 5 (1), 6 (24), 7 (10). Dorsolateral body plates between adipose- and caudal-fin 7 (10), 8 (12), 9 (3). Preadipose platelets 2 (2), 3 (20), 4 (8), 5 (3). 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 junction of frontal and lateral ethmoid, anterior and ventral margins of nasal capsule, and dorsal surface of snout with small, irregular platelets bearing odontodes. Ventral surface of head and trunk densely covered by small irregular platelets bearing odontodes.

Vertebral count 22 (2). Ribs 5 (2), first pair conspicuously large, its middle portion closely connected to first ventrolateral body plate. Complex vertebra moderately developed.

Color in alcohol. Overall color of body in Fig. 8 . Ground color of body yellow or brownish yellow. Top of head and snout dark brown. Dorsal surface of snout with conspicuous concentration of dark brown or black chromatophores, not forming blotches. Ventral region of infraorbital 1 with conspicuous concentration of dark brown or black chromatophores, with pigmentation extending ventrally in anteriorand posterior-most infraorbital 1 edges in some specimens. Opercle with border and middle portion yellow or brownish yellow, remaining area conspicuous concentration of dark brown or black chromatophores. Cleithrum with conspicuous concentration of dark brown or black chromatophores on its dorsolateral surface, generally more evident on middle portion. Dorsal series of diffuse dark brown or black blotches, first on anterior portion of dorsal-fin base, second on posterior portion of dorsal-fin base, third on adipose-fin base and last one on posterior portion of caudal peduncle. Middle portion of flanks with two or three dark brown or black patches, first below dorsal-fin base, second below adipose-fin base and third, if present, diffuse, on caudal peduncle base. Patches decreasing in size posteriorly, presence of darker patches with scarce and diffuse black pigmentation between them, generally restricted to dorsolateral-body plates, in some specimens; patches diffuse in some specimens. Dorsal-fin rays with conspicuous concentration of dark brown or black chromatophores, generally more evident on rays, forming small spots; diffuse spots in some specimens. Pectoral, pelvic and anal fins with dark brown or black chromatophores, generally more concentrated on rays and not forming spots; anal fin variably with small diffuse spots roughly aligned transversally on its middle portion. Adipose fin with dark brown or black chromatophores, generally more concentrated on spine, especially on its distal two-thirds and ventral portion of its origin, and on posterodorsal portion of membrane. Middle portion of caudal-fin base variably with small and diffuse dark brown or black dot. Caudal fin with conspicuous concentrations of dark brown or black chromatophores, generally more evident on rays, forming blotches roughly aligned transversally in five to eight slender transversal bars.

Color in life. Similar to the color pattern of preserved specimens, but ground color of body light and with greenish yellow iridescent coloration. Additionally, region just above posterodorsal margin of orbit with nearly straight, horizontally elongated dark brown or black blotch, forming eyebrow-like marking; blotch slightly arched, following outline of orbit in some specimens ( Fig. 9 ).

Sexual dimorphism. Same as described for Corydoras areio .

Geographical distribution. Corydoras aurofrenatus is known from several tributaries of the rio Paraguay basin in Brazil and Paraguay ( Fig. 5 ).

Ecological notes. During collecting trips for the Proyecto Vertebrados del Paraguay (1992–1999) conducted by the NRM and the Museo Nacional de Historia Natural del Paraguay ( MNHNP), C. aurofrenatus was captured from small streams to the main channel of rivers within Paraguayan territory. The arroyo Laguna Penayo ( Fig. 10A ) is a stream with about 20 m width and 0.5 m deep, very slight to stagnant current, turbid water, and substrate composed mainly by clay. An unnamed stream tributary from the río Apa ( Fig. 10B ) with about 25m width and 1 m deep, stagnant current, dark brown, very turbid water, and substrate composed by sand, rocks and trunks. The río Araguay-Guazú ( Fig. 10C ) is relatively small river with about 35 m width and 1 m deep, moderate current, turbid water, and substrate composed mainly by sand. The río Jejuí-Guazú ( Fig. 10D ) is a small river with brown semitransparent water, and substrate composed mainly by sand, gravel and leaf heaps. A small, unnamed stream tributary from the río Paraguay ( Fig. 10E ) with light brown water, and substrate composed mainly by sand. Most sites presented some degree of anthropogenic impact. Habitat information summarized herein is available at the NRM online database (http:// artedi.nrm.se/nrmfish/).

Remarks. In a revisionary study for Callichthyidae, Ellis (1913) mentioned the presence of C. flaveolus Ihering, 1911 (described from the rio Tietê basin) in the río Paraguay basin, at Sapucay, Paraguay. In the illustration of one of the examined specimens ( Ellis, 1913:Pl. XXVIII, fig. 1), it is possible to observe that the specimen presents a long and conical snout, contrary to C. flaveolus , which displays a short and rounded snout ( Tencatt et al., 2014a:93, fig. 4). Ellis’s (1913) confusion is probably due to the comparison between C. aurofrenatus and C. flaveolus made by Ihering (1911:386), which stated that his new species is morphologically similar to C. aurofrenatus , from which it differs by its color pattern. Gosline (1940) also conducted a review on Callichthyidae , in which he described Corydoras ellisae based on that material identified as C. flaveolus by Ellis (1913:407, pl. 28, fig. 1). Gosline (1940) mentioned that C. ellisae and the other new species described therein ( C. septentrionalis ) are remarkably similar but regarded them as different species by some morphological features and geographic distribution ( Gosline, 1940:18). As previously discussed by Axenrot, Kullander (2003), there is no unequivocal way to distinguish C. aurofrenatus from C. ellisae , which led the authors to consider them conspecific. A synonymy corroborated herein in a broader analysis.

Some fish catalogs (e.g. Menni, 2004; Liotta, 2005; Arias et al., 2013; Mirande, Koerber, 2015; Fricke et al., 2020) recorded C. aurofrenatus from Argentina and Bolivia. However, it was not possible to confirm these records in any way (e.g. analysis of voucher specimens, drawings or photos). Although they are possibly correct, we consider only checked information to assign species distribution.

Material examined. All from the rio Paraguay basin: Brazil. Mato Grosso: CPUFMT 243, 4, 18.4–27.9 mm SL, Alto Paraguai , córrego Estorda , 14°28’19”S 56°34’57”W, S. Silva, 6 Sep 2008 . CPUFMT 1635, 2, 31.3–43.6 mm SL, Poxoréo, córrego Lajeadinho , 16°15’38”S 54°21’52”W, V. Oliveira, 5 Sep 2009 . CPUFMT 1697, 1, 43.3 mm SL, Poxoréo, córrego Macaco , 16°14’37”S 54°22’29”W, V. Oliveira, 22 Aug 2010 . CPUFMT 1702, 4, 15.0– 51.6 mm SL, Poxoréo, córrego Macaco , 16°14’37”S 54°22’29”W, V. Oliveira, 31 May 2011 . CPUFMT 1709, 1, 41.7 mm SL, Poxoréo, córrego Lajeadinho , 16°15’38”S 54°21’52”W, V. Oliveira, 25 Aug 2010 . CPUFMT 1997, 1, 41.6 mm SL, Porto Estrela, unnamed stream, córrego Salobro basin, 15°31’55”S 57°17’17”W, T. Arnhold, 15 Jan 2012 . CPUFMT 2303, 9, 31.0– 48.3 mm SL, Cuiabá, córrego Aricazinho , 15°32’21”S 55°55’05”W, L. Pereira, 25 Oct 2013 . CPUFMT 2323, 2, 35.1–42.7 mm SL, Cuiabá, córrego Aricazinho, 15°32’21”S 55°55’05”W, L. Pereira. CPUFMT 2324, 1, 20.5 mm SL, Cuiabá , córrego Aricazinho , 15°32’21”S 55°55’05”W, L. Pereira. CPUFMT 2344, 9, 25.6–34.8 mm SL, córrego Aricazinho , 15°32’21”S 55°55’05”W, L. Pereira, 16 Feb 2014 . CPUFMT 2362, 12, 28.9 mm SL, 43.1 mm SL, Cuiabá, córrego Aricazinho , 15°32’21”S 55°55’05”W, L. Pereira, 12 Apr 2014 . CPUFMT 2381, 1, 37.9 mm SL, Cuiabá, córrego Aricazinho , 15°32’21”S 55°55’05”W, L. Pereira, 29 Jun 2014 . CPUFMT 3575, 2, 29.9–32.8 mm SL, Nossa Senhora do Livramento, Baia dos Coqueiros (córrego Landi ), 16°22’12”S 56°17’24”W, F. Machado, 28 Nov 1990 . MNRJ 20541 , 4 , 28.8–38.5 mm SL, Cuiabá, small tributary of the rio Aricá-Açu, BR364 , East from Cuiabá , 15º40’21”S 55º56”37”W, F. A. G. Melo, P. A. Buckup & M. R. S. Melo, 18 Feb 2000 . MNRJ 31382 , 1 , 44.6 mm SL, Chapada dos Guimarães, a stream in the Santa Cruz neighborhood, rio Coxipó basin, 15º36’20”S 56º03’07”W, M. N. Mehana, L. Centofante & D. Krinski, 21 Feb 2007 . MZUSP 44283 , 1 , 30.3 mm SL, Rosário Oeste, ribeirão Chiqueirão , 15°07’15”S 56°38’45”W, Expedition MZUSP / USNM / MCP /UFMT staff, 10 Aug 1991 . MZUSP 44336 , 2 , 36.0–43.0 mm SL, Porto Estrela , unnamed stream, 15°19’35”S 57°11’34”W, Expedition MZUSP / USNM / MCP /UFMT staff, 11 Aug 1991 . MZUSP 44351 , 3 , 37.6–47.2 mm SL, Porto Estrela, córrego Cachoeirinha , 15°44’28”S 57°19’56”W, Expedition MZUSP / USNM / MCP /UFMT staff, 11 Aug 1991 . MZUSP 91053 , 5 , 16.2–30.5 mm SL, Salto do Céu, córrego dos Veados, 15º08’10”S 57º57’28”W, H. A. Britski, O. Froehlich, A. Catella & F. Marques. NUP 3239, 7, 25.3–45.3 mm SL, Rosário Oeste, córrego Forquilha , 14°44’58”S 56°07’39”W, Nupélia staff, 15 Jun 2001 . NUP 10227, 1, 17.6 mm SL, Nobres, córrego Cancela , 14°42’31”S 56°15’52”W, Nupélia staff, 12 Dec 2002 . NUP 16191, 33, 20.1–53.8 mm SL, 2 cs, 38.2-41.6 mm SL, Rosário Oeste, córrego Cancela, Nupélia staff, 15 Jan 2001. Mato Grosso do Sul: ZUFMS 5353 , 4 , 14.2–23.4 mm SL, Porto Murtinho, córrego Rapadura, rio Tarumã basin, 21°29’47”S 57°32’42”W, F. Severo-Neto, 28 Mar 2017 . ZUFMS 5704 , 2 , 22.6-27.5 mm SL, Porto Murtinho, córrego Tereré, rio Tererê basin, 21°21’23”S 57°36’45”W, F. Severo-Neto, D. A. Lopes & T. T. M. Taveira, 20 May 2018 . Paraguay. Amambay: NRM 30085 , 1 , 49.7 mm SL, Bella Vista, unnamed stream tributary from the río Apa , A. M. C. Silfvergrip et al., 14 Jun 1994 . Caaguazú: NRM 45022 , 2 , 11.9–14.3 mm SL, río Tebicuary , where it crosses the road Cnel. Oviedo – Villarica, Åhlander et al., 12 Nov 1999 . Canindeyú: NRM 31506 , 6 , 21.2–46.4 mm SL, unnamed stream, S. O. Kullander et al., 12 Oct 1994 . NRM 32591 , 10 of 19, 32.0– 47.6 mm SL, río Jejuí-Guazú , 76 km on the road leading to Curuguaty, S. O. Kullander et al., 11 Oct 1994 . Concepción: NRM 23529 , 10 of 33, 31.4–45.7 mm SL, Paso Barreto , arroyo Laguna Penayo where it crosses the road Concepción-Paso Barreto, S. O. Kullander et al., 20 Aug 1993 . Cordillera: NRM 42315 , 1 , 36.9 mm SL, arroyo Yhaguy , where it crosses Ruta 2 at about Km 83, E. Åhlander et al., 21 Mar 1998 . Guairá: NRM 42361 , 1 , 43.6 mm SL, arroyo Guazú at road bridge in Carlos Fannel, Åhlander et al., 27 Mar 1998 . NRM 42703 , 2 , 28.4–30.5 mm SL, río Tebicuary , where it crosses the road Mauricio J. Troche – Independencia, Åhlander et al., 27 Mar 1998 . ZMB 16911 , 1 , 26.7 mm SL, Villa Rica (= Villarrica ), Anisits. ZMB 17046 , 1 , 45.0 mm SL, Villa Rica (= Villarrica), Anisits . Paraguarí: MTD F 26329 , 1 , 42.6 mm SL, Sapucay (= Sapucai), arroyo Poná , 25°43’55’’S 57°12’28’’W, A. Zarske, 1 Oct 1997 . MTD F 26330 , 1 , 39.4 mm SL, Sapucay (= Sapucai), arroyo Poná , 25°43’55’’S 57°12’28’’W, A. Zarske, 1 Oct 1997 . ZMB 33323 , 1 , 42.5 mm SL, Sapucay (= Sapucai), arroyo Poná, J. Knaack, 21 April 2001 . ZMB 33324 , 1 , 40.0 mm SL, Sapucay (= Sapucai), arroyo Poná, J. Knaack, 21 Apr 2001 .

Figures

FIGURE 2 | Lateral view of the head of cleared-and-stained specimens of (A) Corydoras areio, ZUFMS-PIS 1314, 38.7 mm SL, and (B) Corydoras aurofrenatus, NUP 16191, 41.6 mm SL. The black lines represent the limits of the bones. Yellow arrows indicate additional pore on the sphenotic bone. Abbreviations: cpt: compound pterotic. Scale bars = 1.0 mm, io1: infraorbital 1, io2: infraorbital 2, ioc: infraorbital canal, iop: interopercle; op: opercle, pop: preopercle, sph: sphenotic.

FIGURE 3 | Pectoral-fin spine of cleared-and-stained specimens of Corydoras areio, ZUFMS-PIS 1314, 38.7 mm SL, and Corydoras aurofrenatus, NUP 16191, 38.2 mm SL, showing the well-developed conical serrations directed towards pectoral-spine origin, disposed along the posterior margin of the (A) right spine of C. areio (8.8 mm long) and of the (B) left spine of C. aurofrenatus (8.6 mm long). Yellow arrows indicate serrations that are directed towards tip of spine or perpendicularly directed.

FIGURE 5 | Map of geographic distribution of Corydoras areio and C. aurofrenatus from the rio Paraguay basin, in Brazil and Paraguay, respectively: white star and white circles denote the possible type locality and non-type material of C. areio, respectively; black star and red circles denote the type locality and non-type material of C. aurofrenatus, respectively, blue star indicates the type locality of Corydoras ellisae (a junior synonym of Corydoras aurofrenatus). Each symbol may represent more than one locality.

FIGURE 7 | Corydoras aurofrenatus, holotype, CAS 20661, 40.9 mm SL, Paraguay, Aguada, near arroyo Trementina. Dorsal (top), lateral (middle) and ventral (bottom) views. Copyright © California Academy of Sciences, with all rights reserved.

FIGURE 8 | Color pattern variation in preserved specimens of Corydoras aurofrenatus, showing the intensely pigmented pattern, NUP 16191, 45.8 mm SL, Brazil, Mato Grosso, Rosário do Oeste, córrego Cancela, in dorsal (A) and lateral (C) views, and the diffuse pattern, NRM 23529, 44.6 mm SL, Paraguay, Concepción, arroyo Laguna Penayo, in dorsal (B) and lateral (D) views.

FIGURE 9 | Uncatalogued live specimens of Corydoras aurofrenatus, showing (A) a specimen with conspicuous blotches on flanks, said to be from “Paraguay”, and (B) a specimen with diffuse pigmentation on flanks, from the rio Amonguijá, Mato Grosso do Sul, Brazil. Photo by Hans-Georg Evers and Heriberto Gimênes, respectively.

FIGURE 10 | Habitats in which Corydoras aurofrenatus was found during the Proyecto Vertebrados del Paraguay, in Paraguay, showing (A) the arroyo Laguna Penayo, (B) an unnamed stream tributary from the río Apa, (C) the río Araguay-Guazú, (D) the río Jejuí-Guazú, and (E) a small, unnamed stream tributary from the río Paraguay.

FIGURE 11 | Lateral view of the head of (A) Corydoras aurofrenatus (non-type specimen, MTD F 19127, 46.9 mm standard length) and (B) Corydoras negro (holotype, MTD F 28472, 41.8 mm standard length), showing the short opercular membrane of C. aurofrenatus, with anteroventral portion of cleithrum exposed (cep), and the long opercular membrane of C. negro, with anteroventral portion of cleithrum covered (ccv) by it.