Redescription of Corydoras undulatus Regan, 1912 (Siluriformes: Callichthyidae), with comments on the identity of Corydoras latus Pearson, 1924

A redescription of Corydoras undulatus Regan, 1912 is presented. The original description of C. undulatus is very succinct, as is its diagnosis, which is based only on external morphology. Additional information in the scientific literature on this species is scarce. Specimens from the distribution area of this species were analyzed; Paraná and Paraguay river basins in Argentina, Uruguay river basin in Argentina, Brazil and Uruguay, and the Laguna dos Patos system in Brazil. Morphological analysis, principal component analysis (PCA), meristic comparison and osteological description were performed. Corydoras undulatus can be distinguished from its congeners mainly by having the following combination of characters: mesethmoid short, with anterior tip short, smaller than 50% of the entire bone length; posterior margin of the pectoral-fin spine with nearly all serrations directed towards origin of spine; pectoral-fin spine with conical serrations; and its peculiar color pattern. The analysis of the material from the different basins did not indicate relevant morphological differences, suggesting that the species presents a wide distribution in La Plata and Laguna dos Patos drainages. The shared geographic distribution between these two systems is also present in other fish species. The current work presents data about the type locality, taxonomy, osteology, distribution and ontogenetic variation of color pattern in C. undulatus. Comments on the identity of a very similar congener, Corydoras latus, will also be provided.


Introduction
Corydoras Lacépède, 1803 comprises the majority of species of the Corydoradinae, and represents the genus with the greatest species-rich among the Siluriformes. With 174 valid species, 19 described in the last ten years, besides presenting many species not yet described [1,2].
An important comprehensive study on Corydoras species from the Suriname was carried out by Nijssen [3] in which Corydoras was divided in nine groups of species based mainly on a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 data were taken following Reis [8] with modification of Tencatt et al. [9]. Morphometric measures are presented as percentages of standard length (SL) and head length (HL). Based on the morphometric data, Principal Component Analysis (PCA) was performed with PAST 1.0 statistical program. Some specimens were cleared and stained (c&s) following Taylor & Van Dyke [10]. The homology of the barbels follows Britto & Lima [11]. The osteological terminology was based on Reis [12], except for the parieto-supraoccipital instead of the supraoccipital [13], the pterotic compound instead of the supracleithrum [14] and the scapulocoracoid instead of the coracoid [15]. The nomenclature of the lateral-sensory canals and the preopercular pores is in agreement with Schaefer & Aquino [16] and Schaefer [17], respectively. The supra-preopercle sensu Huysentruyt & Adriaens [18] was treated here as a part of the hyomandibula. Vertebral counts include only free vertebrae, with the compound caudal centra (preural 1+ ural 1) counted as a single element. The stripes were counted as in Tencatt & Ohara [19].

Diagnosis
Corydoras undulatus can be distinguished from its congeners, with exception of C. bilineatus, C. elegans, C. gracilis, C. guapore, C. latus, C. mamore, C. nanus, C. napoensis Nijssen & Isbrücker, 1986, C. nijsseni, and C. paucerna, by having the following combination of features: mesethmoid short, with anterior tip short, smaller than 50% of entire bone length (vs. long, with anterior tip larger than 50% of entire bone length); posterior margin of pectoral-fin spine with nearly all serrations directed towards origin of spine (vs. all or nearly all serrations perpendicularly inserted or directed towards tip of spine); pectoral-fin spine with conical serrations (vs. laminar). It differs from C. bilineatus, C. elegans, C. gracilis, C. guapore, C. latus, C. mamore, C. nanus, C. napoensis and C. paucerna by having flanks with relatively large dark brown or black markings, forming a marbled or somewhat anastomosed pattern at least on anterior portion of flanks; blotches variably forming up to three irregular and/or intermittent longitudinal bands; first band, if present, along dorsolateral body plates, second, if present,  along midline of flank, and third, if present, along ventrolateral body plates; third band more regular and continuous in some specimens (vs. generally with clearly more regular and distinct pattern of longitudinal stripes on flanks, which alternate transversely a dark brown or black stripe with a brownish yellow stripe; marbled or somewhat anastomosed pattern, if present, restricted to anterior portion of body in C. bilineatus and C. napoensis; dorsal portion of flank with a long, continuous, regular, longitudinal dark brown or black stripe, which runs in parallel to body dorsal profile, extending at least from anterior portion of dorsal-fin base to posterior portion of caudal peduncle; stripe generally with broad anterior portion, becoming narrower towards its posterior portion; dorsally and ventrally bordered by scarcely spotted regions, which often form longitudinal brownish yellow longitudinal bands; region midline of flank and ventrolateral body plates with dark brown or black spots, which can be aligned in longitudinal rows; midline of flank variably with a slender longitudinal stripe in C. elegans; dorsal portion of flank with a long, arched, continuous black stripe, which runs parallel to body dorsal profile, extending at least from corner of mouth region to posterior portion of caudal peduncle, dorsally and ventrally bordered by scarcely spotted regions, which often form longitudinal brownish yellow longitudinal bands; ventrolateral body plates with small dark brown or black blotches, variably aligned in longitudinal rows, in C. gracilis; flank mostly covered by small dark brown or black blotches, not forming a marbled or anastomosed pattern, with a large roundish dark brown or black blotch covering caudal peduncle region in C. guapore; ventral portion of dorsolateral body plates and dorsal portion of ventrolateral body plates with a longitudinal row of brownish yellow small roundish areas, more evident on flanks anterior half; remaining areas of dorsolateral body plates with conspicuous concentration of dark brown or black chromatophores, more evident on flanks anterior half; dorsal half of ventrolateral body plates, except for small brownish yellow roundish areas, with conspicuous concentration of dark brown or black chromatophores in C. latus; with relatively small dark brown or black blotches, not forming a marbled or anastomosed pattern in C. mamore and C. paucerna; dorsal portion of flank with a long, continuous, regular, longitudinal dark brown or black stripe, which runs in parallel to body dorsal profile, extending at least from dorsal-fin base anterior portion to caudal peduncle posterior portion; dorsally and ventrally bordered by scarcely spotted regions, which often form longitudinal brownish yellow longitudinal bands; ventral portion of dorsolateral body plates and dorsal portion of ventrolateral body plates with a series of longitudinally aligned dark brown or black spots, which merge at dorsal-fin region and form a stripe, that can range from narrow to broad; ventrolateral body plates with dark brown or black blotches, generally longitudinally aligned in C. nanus); from C. nijsseni by

Description
Morphometric data shown in Table 1. Head slightly compressed in dorsal view, with convex dorsal profile in lateral view. Head roughly triangular or somewhat pentagonal in dorsal view. Snout short and rounded; strongly reduced in some specimens. Head dorsal profile convex from snout tip to anterior nostrils; ascending slightly convex or nearly straight from this point to dorsal-fin origin. Profile slightly convex along dorsal-fin base. Body profile posterior to dorsal fin slightly concave to adipose-fin spine; concave from this point to caudal-fin base. Ventral profile of body slightly convex from isthmus to pelvic-fin origin; region of gill opening slightly concave in some specimens; slightly convex or nearly straight from this point to anal-fin origin; concave until caudal-fin base. Body roughly elliptical in cross section at pectoral girdle, gradually becoming more compressed towards caudal fin.
Eye rounded, located mid-dorsally on lateral portion of head; orbit dorsally delimited by lateral ethmoid, frontal, sphenotic and antero-dorsal laminar expansion of infraorbital 1, ventrally by infraorbitals. Anterior and posterior nostrils close to each other, separated only by skin flap. Anterior nostril tubular. Posterior nostril near anterodorsal margin of orbit, separated from it by distance similar to naris diameter. Mouth small, subterminal, width approximately equal to bony orbit diameter. Maxillary barbel size ranging from moderate, not reaching anteroventral limit of gill opening, to large, slightly surpassing anteroventral limit of gill opening. Outer mental barbel generally slightly longer than maxillary barbel. Inner mental barbel fleshy, with base close to its counterpart. Small rounded papillae covering entire surface of all barbels, upper and lower lips, snout and isthmus. Mesethmoid conspicuously short, clearly smaller than frontal length; anterior tip strongly reduced, smaller than 50% of bone length (see Britto, 2003: 123, character 1, state 1; fig 1B), posterior portion wide, entirely covered by thick layer of skin in most specimens; variably with small exposed area on its posteriormost portion. Narrow nasal, laterally curved, inner margin generally with poorly-developed laminar expansion; moderately-developed in some specimens; outer margin with laminar expansion ranging from clearly reduced to poorly developed; mesial border generally contacting only frontal; variably contacting both frontal and mesethmoid. Frontal elongated, relatively wide, width larger than half of entire length; anterior projection ranging from short, size smaller than nasal length, to long, size larger than nasal length. Frontal fontanel large, slender; posterior tip extension entering anterior margin of parietosupraoccipital. Sphenotic roughly rectangular in shape, contacting parieto-supraoccipital dorsally, compound pterotic posteriorly, second infraorbital ventrally and frontal anteriorly. Compound pterotic roughly pipe-shaped, with posterodorsal portion contacting first lateralline ossicle, and ventral margin contacting infraorbital 2, opercle and cleithrum. Parietosupraoccipital wide, posterior process ranging from moderate to long in size, generally not contacting nuchal plate, contacting in single specimen (FML 7113, 31.1 mm SL); posterior portion of parieto-supraoccipital posterior process and anterior portion of nuchal plate generally covered by thick layer of skin; region between parieto-supraoccipital posterior process and nuchal plate with minute platelets in some specimens.
Two laminar infraorbitals with minute odontodes; infraorbital 1 large, ventral laminar expansion generally ranging from poorly to moderately developed; few specimens with welldeveloped expansion; anterior portion with laminar expansion ranging from poorly developed, slightly surpassing posterior margin of nasal capsule, to moderately developed, reaching to middle of nasal capsule; inner laminar expansion ranging from strongly reduced to poorly developed; small portions of external surface covered by thick layer of skin (Fig 4); infraorbital 2 small, relatively wide; posterior laminar expansion well developed; inner laminar expansion ranging from poorly to well developed; posteroventral margin contacting posterodorsal ridge of hyomandibula, dorsal tip contacting sphenotic and compound pterotic; small portions of external surface covered by thick layer of skin ( Fig 5A). Posterodorsal ridge of hyomandibula close to its articulation with opercle conspicuously slender, exposed; dorsal ridge of hyomandibula between compound pterotic and opercle covered by thick layer of skin or by posterodorsal laminar expansion of infraorbital 2; exposed areas bearing small odontodes. Interopercle almost entirely exposed, with anterior tip covered by thick layer of skin; somewhat triangular, anterior projection ranging from moderately to well developed. Preopercle elongated, relatively slender; minute odontodes on external surface. Opercle dorsoventrally elongated, relatively compact, width slightly larger than half of its length; free margin convex; posterodorsal region generally with slightly concave area; without serrations and covered by small odontodes; some portions of bony distal margin generally irregular.
Four branchiostegal rays decreasing in size posteriorly. Hypobranchial 2 somewhat triangular, tip ossified and directed towards anterior portion, posterior margin cartilaginous; ossified portion well developed, about twice size of cartilaginous portion. Five ceratobranchials with expansions increasing posteriorly; ceratobranchial 1 with small process on anterior margin of mesial portion; process strongly reduced in some specimens; ceratobranchial 3 with continuous postero-lateral margin; ceratobranchial 5 toothed on postero-dorsal surface, 35 to 38 (2) teeth aligned in one row. Four epibranchials with similar size; epibranchial 2 slightly larger, with small pointed process on laminar expansion of posterior margin; epibranchial 3 with somewhat triangular uncinate process on laminar expansion of posterior margin. Two wide pharyngobranchials (3 and 4), pharyngobranchial 3 with triangular laminar expansion on posterior margin; expansion with notches in some specimens. Upper tooth plate oval; 37 to 43 (2) teeth aligned in two rows on postero-ventral surface.
Lateral-line canal entering neurocranium through compound pterotic, generally branching twice before entering sphenotic: pterotic with single pore; preoperculomandibular branch conspicuously reduced, with a single pore opening close to postotic main canal; single specimen (MCP 46587, 29.1 mm SL) with third branch just posterior to pterotic branch. Sensory canal continuing through compound pterotic, entering sphenotic as temporal canal, which splits into two branches: one branch giving rise to infraorbital canal, other branch entering 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, 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 or three 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 roughly triangular, located just posterior to third dorsolateral body plate. Dorsalfin rays II,8 � (49), posterior margin of dorsal-fin spine with one to five poorly-developed serrations directed towards tip of spine; serrations generally restricted to distal portion of spine. Nuchal plate ranging from moderately developed to well developed; almost entirely exposed, with minute odontodes; spinelet short; spine ranging from moderately developed, adpressed distal tip slightly surpassing middle portion of dorsal-fin base, to relatively well developed, adpressed distal tip slightly surpassing posterior origin of dorsal-fin base; anterior margin with small odontodes. Pectoral fin roughly triangular, its origin just posterior to gill opening. Pectoral-fin rays I,8 � (49); posterior margin of pectoral spine with 16 to 24 conical serrations along almost its entire length; most serrations strongly developed and directed towards origin of spine; proximal and/or distal edge of spine variably with less-developed serrations; some serrations perpendicularly directed or directed towards tip of spine in some specimens; bifid serrations in some specimens (Fig 5B). Anteroventral portion of cleithrum exposed; posterolateral portion of scapulocoracoid moderately developed, exposed, with anterior portion poorly to moderately expanded, not contacting anteroventral portion of cleithrum; minute odontodes sparse on exposed areas. Pelvic fin oblong; located just below second ventrolateral body plate, and at vertical through first or second branched dorsal-fin rays. Pelvic-fin rays i,5 � (49). Adipose fin roughly triangular, separated from base of last dorsal-fin ray by generally six dorsolateral body plates. Anal fin somewhat triangular, located just posterior to 11 th or 12 th ventrolateral body plates, and at vertical through region of preadipose platelets or anterior portion of adipose-fin spine. Anal-fin rays ii,6 � (49). Caudal-fin rays i,12,i � (49), generally with four dorsal and ventral procurrent rays; bilobed, dorsal and ventral lobes generally with similar size (Fig 5C).

Color in alcohol
Overall color of body in Fig 3. Background color of body brownish yellow, with top of head dark brown. Dorsal and lateral surfaces of head densely covered by dark brown or black chromatophores, variably forming a marbled pattern of blotches; cleithrum, external surfaces of upper and lower lips, maxillary and outer mental barbels, and ventrolateral portion of snout with scattered dark brown or black chromatophores; region of isthmus just posterior to mouth and inner mental barbel variably with dark brown or black chromatophores. Flanks with relatively large dark brown or black markings, generally forming marbled or somewhat anastomosed pattern; blotches variably forming up three irregular and/or intermittent longitudinal bands; first band, if present, along dorsolateral body plates, second, if present, along midline of flank, and third, if present, along ventrolateral body plates; third band more regular and continuous in some specimens. Ventral portion of ventrolateral body plates generally with sparse dark brown or black chromatophores. Dorsal fin covered by dark brown or black chromatophores, generally more concentrated on its dorsal half; variably forming irregular diffuse dark blotches; somewhat longitudinally aligned blotches in some specimens. Pectoral fin with dark brown or black chromatophores, generally restricted to spine and rays, variably forming diffuse dark blotches. Pelvic, adipose and anal fins with dark brown or black chromatophores, forming dark blotches in some specimens. Caudal fin with dark brown or black chromatophores, generally restricted to rays; often forming dark blotches transversely aligned in irregular bars.

Color in life
Similar to preserved specimens, but with greyish yellow background color of body. Greenish yellow iridescent coloration on body (Fig 6).

Ontogenetic color pattern changes
Juvenil fish, after 7 days of birth without lateral stripe on the head behind the eye; after one month of post hatching (aprox. 8.5 mm SL) with oval blotches on dorsal plates series and posterior ventral plates; caudal fin with dark blotches transversely aligned in irregular bars. (Fig 7).

Sexual dimorphism
Males specimens exhibit a genital papillae with a tubular shape as most species of Corydoradinae and most species of lineage 6 [22]. Males are generally smaller than females with a darker color pattern tending to form clear longitudinal irregular lines on the laterals of the body vs. females presenting a more diffuse color pattern. Also, males present more elongated and pointed pelvic fins than females. In females the ventral plates between the pelvic and anal fins are more separated medially (Fig 8).

Geographic distribution
Corydoras undulatus is known from the Paraguay River basin in Paraguay and Argentina, Paraná River basin in Argentina, in the Uruguay river basin in Uruguay, Argentina and Brazil and the Laguna dos Patos in Rio Grande do Sul, Brazil (Fig 9).

Principal Component Analysis (PCA)
No differences were found among the evaluated populations of Corydoras undulatus with the analyzed variables indicating no morphometric differences among them (Fig 10).

Ecological notes
Corydoras undulatus is generally found in vegetated environments (Fig 11A) where it is more abundant, but can also be eventually found in low abundance in open waters in rivers ( Fig  11B).

Remarks
As explained by Tencatt & Evers [23], the color pattern of the Corydoradinae considerably change from their hatching till the adult stage. Corydoras undulatus is well known among fishkeeping hobbyists, and it was successfully bred under aquarium conditions by Hans-George Evers, who documented C. undulatus development during its growth, including photos of eggs deposited in an aquatic macrophyte. In the aquarium hobby, putative new species of Corydoradinae are generally inserted in a widely-known code-system with the intention of avoiding the creation of nomina nuda. (see Tencatt & Evers, 2016). Although some of the coded species strongly resembles C. undulatus, such as C88, said to be from Mato Grosso, Brazil, C132, with unknown locality, no specimen related to these codes and/or available locality could be examined herein, making it impossible to undoubtedly attribute these speciemens to C. undulatus. However, another extremely similar coded species, CW22, is said to be from the Rio Paraguay, Argentina, which matches one of the localities of our examined material. Considering the available collecting data plus photos of CW22 (Fig 7), it seems that these specimens indeed can be determined as C. undulatus.     found and are close localities to the Port of Buenos Aires where is the most likely place where those fish were shipped to Europe.
The comparative analysis of the linear measures among the different basins did not show significant differences, as well as the principal component analysis (PCA) (Fig 10). Thus, based on these results plus meristic comparisons and osteological analysis we conclude that there are no evidences to consider any of these populations as a different taxon.
After revising the pictures of C. undulatus corresponding to lots deposited in the Swedish Museum of Natural History from the Paraguay River drainage in Paraguay, we conclude that those specimens probably correspond to C. undulatus [catalog numbers: NRM 23550, NRM 31026, NRM 31483, NRM 33556, NRM 45056, NRM 45060 and NRM 65433]. Here we also confirm the presence of C. undulatus in Uruguay Republic based on voucher materials for the first time.
Alexandrou et al. [26], obtained Corydoras sensu lato in their phylogenetic analysis as a paraphyletic group composed by seven monophyletic clades. As discussed by Tencatt & Pavanelli [27], one of the most conspicuous groups within Corydoras comprises the species of the lineages 4 and 5, which is basically conformed by Nijssen & Isbrücker's (1998) "Corydoras elegans group" species, including C. undulatus, plus C. bilineatus, C. gracilis, C. napoensis, C. nijsseni and some unidentified species. Even though Nijssen & Isbrücker [4] have assigned C. latus to the C. elegans group (Nijssen & Isbrücker, 1980), this name is often erroneously attributed to female specimens of C. pantanalensis in several museums and fish collections (LFCT pers. obs.). One of the most reasonable explanations for this can be the work published by Knaack [28], in which he discussed the identity of C. aurofrenatus and C. latus. Knaack ([29]: 19) argued that the serration pattern of the pectoral-fin spine displayed by C. latus holotype is compatible to what he called the "Corydoras reticulatus group", which, according to this author, comprises C. reticulatus, C. sodalis, C. geryi and C. pantanalensis (see Knaack [29]: 51).
The close relationship between the species of Knaack's [29] "Corydoras reticulatus group" was posteriorly corroborated by Alexandrou et al. ([26]: 3, supplementary fig. 2), which recovered these four species forming a small monophyletic clade within the lineage 8. Despite of that, Knaack's [28] argumentation regarding the serration pattern of C. latus holotype and his "Corydoras reticulatus group" could be refuted herein. In a closer look, the serrations along posterior margin of the pectoral-fin spine in C. latus holotype are strongly-developed conical serrations, mostly directed towards origin of spine. Such pattern is present only in Aspidoras  (Fig 5B), whereas the typical pectoral-fin spine serration pattern of the lineage 8 species is characterized by the presence of moderately-developed laminar serrations, mostly directed towards origin of spine ( [19]: e150063, fig. 3c), and which can be used to distinguish the species within this lineage from all others congeners. In addition to the pectoral-fin spine serration pattern, the analysis of the available photos of the holotype of C. latus (Fig 12; Knaack [28]: 21) revealed the presence of a conspicuously short and rounded snout, readily distinguishing it from the congeners of the lineages 1 and 8 (vs. snout well developed and pointed, generally with a slightly concave lateral profile; nearly straight in some specimens in lineage 1 species; snout generally well developed, pointed and straight, variably with a slightly concave lateral profile; some specimens with moderately developed and/or rounded snout in lineage 8 species).
Although the correlation between C. latus and the species from the "Corydoras elegans group" can be promptly verified even through the analysis of photographs of its holotype, the recognition of its identity as species is still unclear. This species is currently known only from its holotype, which seems to be a large-sized female (about 40.0 mm SL with absence of visible genital papillae). As presented in previous publications (Nijssen & Isbrücker [32]; Knaack [28,29]), the species from this group often present a wide variation range in color pattern during their development and also between sexes. Therefore, only through the analysis of material containing juvenile, adult, male and female specimens it will be possible to clearly recognize C. latus. Interestingly, the color pattern of the holotype of C. latus (Fig 12) resembles the one displayed by females of C. bilineatus (Fig 13), both species from the rio Madeira basin, Bolivia. However, no specimen of C. latus could be examined herein, except by the photographs of its holotype, and, by this reason, no changes regarding its taxonomic status shall be suggested until further analysis.

Comparative material examined
In addition to the material listed by Espíndola et al. [33], the following specimens were analyzed. Brazil, Amazonas.