Taxonomic review of the typical long-snouted species of Corydoras (Siluriformes: Callichthyidae) from the río de La Plata basin

The typical long-snouted species of Corydoras from the río de La Plata basin were reviewed herein, and the previously proposed synonymy of Corydoras ellisae was corroborated. Corydoras areio and C. aurofrenatus are diagnosed from their congeners, excluding those in lineage 1, by the following features: temporal sensory canal in sphenotic with two pores; upper tooth plate of branchial arch with three or four series of teeth; fleshy flap at mouth corner. Corydoras areio differs from all lineage 1 congeners by having infraorbital 2 with relatively wider posterior laminar expansion; absence of large patches of black pigmentation on the body and absence of conspicuous concentration of dark brown or black chromatophores on anterior portion of the dorsal fin; and presence of blotches on flanks not aligned in longitudinal series. Corydoras aurofrenatus differs from all lineage 1 congeners by having ventral surface of head and trunk densely covered by small, not coalescent platelets; middle portion of flank with two or three dark brown or black patches (below the dorsal-fin, below the adipose-fin base, and on the caudal peduncle base, diffuse and variably present), patches decreasing in size posteriorly; poorly developed fleshy flap at the corner of mouth; anteroventral portion of cleithrum exposed. Palavras-chave: rio Paraguai, Corydoradinae, Corydoras diphyes , Corydoras ellisae , Osteologia.

Eigenmann, Kennedy (1903) provided a revised catalogue of a fish collection from Paraguay sent to the Indiana University by Juan Anisits. This collection comprises a total of 750 specimens from many localities of Paraguay, including material collected by Carl Ternetz at Asuncion, and also from Descalvados, State of Mato Grosso, Brazil. In addition, the authors described several new species. One of them is Corydoras aurofrenatus Eigenmann & Kennedy, 1903, a typical long-snouted species from lineage 1 sensu Alexandrou et al. (2011) described based on a single specimen captured in Aguada, near arroyo Trementina, río Paraguay basin, Paraguay. Eigenmann, Kennedy (1903) mentioned that this species displays a very peculiar color pattern among Corydoras species, with "no color on sides, belly or breast" (p. 508), dorsal and caudal fins spotted and "a broad yellow band across the snout" (p. 508).
Corydoras species lacking any kind of conspicuous pigmentation on the body are very uncommon. Considering the río de La Plata basin, the only species sharing the 4/36 ni.bio.br | scielo.br/ni Paraguay, the synonymy proposed by Axenrot, Kullander (2003) was corroborated. Because the original descriptions of C. areio and C. aurofrenatus lack standard diagnoses and several morphological information (mainly concerning osteology), in addition to the difficulty in clearly distinguishing them, the aim of this study is to provide redescriptions for both species, allowing their clear recognition.

MATERIAL AND METHODS
Measurements were obtained using a precision digital calipers in tenth of millimeter. Morphometric and meristic data were taken following Reis (1997) with modifications of Tencatt et al. (2013). Morphometrics are reported as percentages of standard length (SL) and head length (HL). Homology of barbels follows Britto, Lima (2003). The specimens used for osteological analysis were cleared and stained (cs) following the protocol of Taylor, Van Dyke (1985). Osteological terminology was based on Reis (1998), except for the use of the parieto-supraoccipital instead of supraoccipital (Arratia, Gayet, 1995), compound pterotic instead of pterotic-supracleithrum (Aquino, Schaefer, 2002) and scapulocoracoid instead of coracoid (Lundberg, 1970). Nomenclature of the latero-sensory canals and preopercular pores are according to Schaefer, Aquino (2000) and Schaefer (1988), respectively. The supra-preopercle sensu Huysentruyt, Adriaens (2005) will be treated here as a part of the hyomandibula according to Vera-Alcaraz (2013). Vertebral counts include only free centra, with the compound caudal centra (preural 1+ ural 1) counted as a single element. The last two dorsal-fin rays were counted as distinct elements. Pharyngeal teeth were counted in both sides of the branchial arches. In the descriptions, numbers between brackets represent the total number of specimens with those counts. Literature in which it was not possible to corroborate the species identification (through voucher specimens, drawings or photographs) were not included in the synonymic lists. The majority of the specimens examined herein were obtained in museums/ichthyological collections, and therefore no specific licenses were needed.
Institutional abbreviations. ANSP, The Academy of Natural Sciences, Philadelphia; BMNH, Natural History Museum, London; CPUFMT, Coleção Britto, 2003:123, character 1, state 0; fig. 1A), and 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 similar to 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, laterally curved, inner margin laminar, and mesial border contacting frontal and mesethmoid. 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, second infraorbital ventrally and frontal anteriorly ( Fig. 2A). 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, and 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. 2A). Parieto-supraoccipital wide, posterior process long and contacting nuchal plate and region of contact between posterior process and nuchal plate covered by thick layer of skin.
Two laminar infraorbitals with minute odontodes. Infraorbital 1 large, ventral laminar expansion ranging from moderately-to well developed; anterior portion with well-developed laminar expansion, reaching to or slightly surpassing anterior margin of nasal capsule; inner laminar expansion strongly reduced ( Fig. 2A). Infraorbital 2 small, widened, with posterior laminar expansion well developed, and posteroventral margin contacting posterodorsal ridge of hyomandibula, posterodorsal edge contacting sphenotic and compound pterotic; inner laminar expansion poorly developed ( Fig. 2A). Posterodorsal ridge of hyomandibula close to its articulation with opercle conspicuously 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 to 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, and ossified portion conspicuously well developed, its size three times or more than cartilaginous portion. Five ceratobranchials with expansions increasing posteriorly; ceratobranchial 1 generally with strongly reduced process on anterior margin of mesial portion; ceratobranchial 3 with continuous laminar expansion on postero-lateral margin; ceratobranchial 5 toothed on posterodorsal surface, with 22 to 26 (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 8/36 ni.bio.br | scielo.br/ni triangular laminar expansion variably notched on posterior margin. Upper tooth plate oval, 42 to 51(2) 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 two or three openings, first on posterior edge, second, when present, on posterolateral portion and generally fused with first pore, and third on anterior edge. Infraorbital canal running through entire infraorbital 2, extending to infraorbital 1 and opening into two pores. Preoperculomandibular branch giving rise to preoperculomandibular 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 (39), II,9 (1), posterior margin of dorsal-fin spine with seven to 10 poorly-developed serrations directed towards tip of spine, serrations arranged on distal half of posterior margin; small odontodes on anterior and lateral surfaces of spine. Nuchal plate well developed, almost entirely 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 (26), posterior margin of pectoral spine with 16 to 17 with moderately-to welldeveloped conical serrations along its entire length, most serrations directed towards pectoral-fin origin, and some serrations perpendicularly directed or directed towards tip of spine; small odontodes on anterior, dorsal and ventral surfaces of spine (Fig. 3A). Anteroventral portion of cleithrum exposed; posterolateral portion of scapulocoracoid exposed; small odontodes 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 (3), i,7 (1), ii,6 (16). 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.
Two to four laterosensory canals on trunk. First ossicle tubular, second ossicle laminar and the remaining encased in third, fourth and fifth dorsolateral body plate, respectively. Body plates with minute odontodes scattered over exposed area, conspicuous line of odontodes confined on posterior margins. Dorsolateral body plates 23 (30), 24 (9). Ventrolateral body plates 20 (8), 21 (31). Dorsolateral body plates along dorsal-fin base 6. Dorsolateral body plates between adipose-and caudal-fin 7 (13), 8 (7). Preadipose platelets 1 (1), 2 (6), 3 (26), 4 (5), 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 the junction of frontal and lateral ethmoid, ventral margin 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.   Fig. 1. Ground color of body yellow. Top of head dark brown. Region just above posterodorsal margin of orbit with nearly straight, horizontally elongated dark brown or black blotch, forming eyebrowlike marking, blotch slightly arched, following outline of orbit in some specimens, variably diffuse or absent. Dorsal surface of snout with conspicuous concentrations of dark brown or black chromatophores, variably forming rounded or irregular blotches generally diffuse. Ventral region of infraorbital 1 with conspicuous concentration of dark brown or black chromatophores, with pigmentation extending ventrally in anterior-and posterior-most infraorbital 1 edges in some specimens. Opercle with border and middle portion yellow, remaining area with conspicuous concentration of dark brown or black chromatophores. Cleithrum with conspicuous concentrations of dark brown or black chromatophores on its dorsolateral surface, variably forming irregular small blotches; blotches absent or diffuse in some specimens. 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 the last one on posterior portion of caudal peduncle. First blotch conspicuously circular, remaining blotches irregular or rounded. Dorsolateral body plates with relatively small rounded, irregular or elongated dark brown blotches. Ventrolateral body plates generally unspotted, dark brown rounded, irregular or elongated blotches close to midline of flank in some specimens. Last dorsoand ventrolateral body plates with conspicuous concentration of dark brown or black chromatophores, forming generally diffuse, irregular, transversally elongated blotch; some specimens with conspicuous blotch. Spots on flanks faded in few specimens. Dorsal-fin with conspicuous concentration of dark brown or black chromatophores, generally more evident on rays, forming small spots; some specimens with diffuse spots. Pectoral and pelvic fins with dark brown or black chromatophores, generally more concentrated on rays and not forming spots. Adipose fin with dark brown or black chromatophores, generally more concentrated on spine, especially on its distal twothirds and ventral portion of its origin. Anal fin with conspicuous concentration of dark brown or black chromatophores, generally more evident on rays of its middle portion, forming small spots; spots roughly aligned transversally and generally diffuse. Middle portion of caudal-fin base 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 four to ten slender bars; diffuse bars in some specimens.

Coloration in life.
Similar to color pattern of preserved specimens but with lighter ground color of body, and with greenish yellow iridescent coloration (Fig. 4). Eyebrowlike blotch generally more evident. Spots faded in some specimens.
Sexual dimorphism. The presence of lanceolate genital papilla is a common feature in males of all Corydoradinae species (see Nijssen, Isbrücker, 1980b;Britto, 2003). Additionally, the males present a fibrous tissue on pectoral spine, which is generally covered by hypertrophied odontodes, and also a segmented filament on the tip of the spine, as illustrated for C. solox Nijssen & Isbrücker, 1983(see Nijssen, Isbrücker, 1983 fig. 10g).

Geographical distribution.
Corydoras areio is known from the rio Piquiri (rio Itiquira) and the ribeirão Parnaíba, tributaries of the rio São Lourenço basin, State of Mato Grosso, and also from the rio Negro (rio Taboco) and rio Taquari basins, State of Mato Grosso do Sul, Brazil (Fig. 5).
Ecological notes. In the rio Taboco and rio Taquari basins, Corydoras areio was generally found inhabiting small streams (Fig. 6), mainly associated with deep, lentic habitats during the day, ranging from about 50 cm to 1 m depth, with substrate predominantly composed of sand. During day, the species displays a more active behavior, swimming away at the sign of any movement, even from outside of the water. On the other hand, the species can be easily observed and captured in shallow beaches of streams (about 10 cm depth) during the night, where they stay nearly motionless. At the beaches of the ribeirão dos Veados (Fig. 6A), the species was observed in syntopy with C. polystictus and Corydoras aff. polystictus. The species was rarely captured in the main channel of the rio Taboco (Fig. 6B), where it is also associated with the sandy substrate of the river banks. In the rio Taboco basin, C. areio occurred in syntopy with Corydoras aff. aeneus (Gill, 1858). In most collecting sites, C. areio specimens were observed burying themselves in the sandy substrate, especially during capture attempts. Another interesting feature of C. areio observed in natural habitat is the presence of translucid trunk in smaller specimens (up to 30.0 mm SL), which possibly enhances their camouflage on sand. Additionally, it was possible to observe signs of severe deforestation in the region of both river basins, basically for agricultural and cattle raising purposes, which reflects the high levels of siltation in most of the local water bodies (LFCT pers. obs.).
Remarks. An interesting point concerning Corydoras areio is its exact type-locality, since Knaack (2000) did not provide an exact locality, pointing that the species was captured in streams of the "Córr." (surely an abbreviation for córrego) Areio, in the vicinity of Vila Nova, east of Cuiabá, Mato Grosso State, Brazil. According to Knaack (2000: 47-48), C. areio seems to be found in two streams that cross the road MT-373, one of them in the kilometer 8.5 and the other one in the kilometer 13.5, near the city of Poxoréo. Searches for "Vila Nova" in Mato Grosso only resulted in a small town at the margins of the rio das Garças, upper rio Araguaia basin, which seems unlikely to be related to the C. areio type-locality. Even though it was not possible to track these exact points, the córrego Areia basin, tributary from the rio Poxoréo, itself a tributary from rio Vermelho, where the rio São Lourenço flows seems the most plausible potential area for the type-locality, especially considering that the córrego Areia itself and some of its tributaries cross the road MT-373 close to the city of Poxoréo (in a range of about 10 kilometers). In a recent collecting trip led by LFCT, the córrego Areia was sampled where it crosses the road MT-130, less than 1 km from the urban area of Poxoréo, where no specimens of C. areio were captured. Despite this, considering the data provided 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.  Knaack (2000), it seems reasonable to consider the córrego Areia drainage around Poxoréo (15°50'41"S 54°26'44"W) as the type-locality of C. areio.
Another curious subject regarding the C. areio original description refers to the institution where the holotype and nine paratypes were deposited, which was not mentioned in detail, having the catalogue number as the only available information, BZM 33113 and BZM 33136, respectively. A search in the databases available in Sabaj (2019) and Fricke, Eschmeyer (2020) revealed no match for the acronym "BZM". However, Knaack himself (1966: 364) provided further information on "BZM" as follows: "Das Material, welches der Neubeschreibung von Eigenmann und Ward [1907] zu Grunde lag, wurde von Anisits in Paraguay eigesammelt und davon Paratypen auch dem Berliner Zoologischen Museum überlassen (Asuncion, Pilcomayo, BZM Nr. 17249; Matto Grosso, Corumba, BZM Nr. 17258)" (= The material on which the new description by Eigenmann, Ward (in Eigenmann et al., 1907) was based was collected by Anisits in Paraguay, and the paratypes were also given to the Berlin Zoological Museum (Asunción, Pilcomayo, BZM 17249; Mato Grosso, Corumbá, BZM 17258)). Therefore, it seems reasonable to conclude that Knaack created "BZM" for "Berliner Zoologischen Museum", not knowing that the "Zoologischen Museum" in Berlin (a former name of the Museum für Naturkunde) was recognized by the acronym "ZMB" (see Sabaj, 2019).
Although Knaack (2000:47) planned to send the type series of C. areio to a fish collection in São Paulo (  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 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 19/36 ni.bio.br | scielo.br/ni 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. 20/36 ni.bio.br | scielo.br/ni 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 parietosupraoccipital 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 21/36 ni.bio.br | scielo.br/ni 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. Caudalfin rays i,12,i, generally four dorsal and ventral procurrent rays.

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)(1993)(1994)(1995)(1996)(1997)(1998)(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 25/36 ni.bio.br | scielo.br/ni 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,  patches decreasing in size along the middle portion of the flank, first one below the dorsal fin, second one below the adipose fin, and third one, if present, diffuse on the posterior portion of the caudal peduncle. Even the specimens of C. aurofrenatus with diffuse coloration present black chromatophores in the three aforementioned regions, thus, the only difference between the two morphotypes is the intensity of the dark brown or black coloration. In all aforementioned congeners, no distinct dark brown or black patch below the adipose fin was observed.
One of the most similar congeners to C. aurofrenatus is C. negro, which is reinforced by its frequent misidentification as C. aurofrenatus in different fish collections around the world (SAS pers. obs.). The confusion in distinguishing both species is probably due to the incipient information regarding the identity of C. negro, which is basically restricted to its original description. Additionally, material of this species in museums and fish collections is scarce, as Knaack (2004:81) deposited only 10 of the 167 available specimens in regular collections, keeping 157 specimens as paratypes in his private collection, which seems to have been lost (see Tencatt, Pavanelli, 2015:293). The relatively small distance between type localities of both species may have also contributed to this problem, since Paraguay and Bolivia are neighbor countries. Beyond the difference in color pattern presented in the diagnosis, C. negro also presents an apparently uncommon feature in Corydoradinae, a well-developed opercular membrane, covering the anteroventral portion of the cleithrum, contrary to C. aurofrenatus, which presents a poorly-developed opercular membrane, leaving the anteroventral portion of the cleithrum exposed (Fig. 11). Corydoras areio is a very peculiar species known only from the upper rio Paraguay basin, Brazil. Its most similar congeners are C. cervinus and C. sarareensis, both from the rio Guaporé basin in Brazil, from which it can be promptly distinguished by its color pattern. Corydoras areio has rounded, irregular or elongated, dark brown or black blotches on the flanks, generally restricted to the region of the dorsolateral body plates, contrary to C. cervinus and females of C. sarareensis, which have numerous, small, rounded, irregular or elongated, dark brown or black spots aligned in both longitudinal rows along the flanks and also in vertical rows on the lateral body plates. Additionally, C. areio presents a transversally elongated brown blotch at the end of the caudal peduncle, which is absent in C. cervinus and females of C. sarareensis. Male specimens of C. sarareensis are readily distinguished from C. areio by having dark brown or black rounded, irregular and elongated blotches on the snout, forming a marbled or striated pattern (vs. conspicuous concentrations of dark brown or black chromatophores on the snout, variably forming rounded or irregular blotches, not forming a marbled or striated pattern, and blotches generally diffuse).