A new species of Corydoras Lacépède, 1803 (Siluriformes: Callichthyidae) from the río Madre de Dios basin, Peru

A new species of Corydoras is described from the río Madre de Dios basin, Peru. The new species can be distinguished from its congeners by presenting the following features: a longitudinal black stripe along midline of flank; mesethmoid short, with anterior portion poorly developed; serrations on posterior margin of pectoral spine directed towards the tip of the spine; dorsal fin with the region of the first branched ray, including membrane, with concentration of black pigmentation, the remaining areas with irregular black blotches; absence of a vertically elongated black blotch across the eyes; conspicuously rounded moderately-developed black spots on the snout; and ventral expansion of infraorbital 1 moderately developed.


Introduction
The Callichthyidae are small-to medium-sized catfishes that are easily recognizable by the presence of two longitudinal series of dermal plates on the flanks.Currently comprised of more than 200 species, the family is split into eight genera (Reis, 2003;Eschmeyer, 2015).Corydoras Lacépède, 1803 is the largest of these genera, harboring more than 170 species, making it the most species-rich genus of Siluriformes (Reis, 2003;Eschmeyer, 2015).The first comprehensive taxonomic studies involving Corydoras began over 70 years ago with the revisionary work of Gosline (1940), which performed a taxonomic review of Callichthyidae.Thirty years later, the next extensive study was published by Nijssen (1970), with the review of the Corydoras species from Suriname.Additionally, Nijssen (1970) also proposed nine groups of species based mainly in color pattern and external morphology.Nijssen & Isbrücker (1980) carried out a review of Corydoras, making some reformulations in Nijssen's (1970) species groups, reducing them from nine down to five.
The first phylogenetic study encompassing a large number of Corydoras species was conducted by Britto (2003).His work on Corydoradinae revealed a large polytomy among Corydoras with most species grouped into nine clades.Britto's (2003) results also pointed Scleromystax Günther, 1864 as valid and sister-group of Aspidoras Ihering, 1907.Additionally, Corydoras was found paraphyletic, therefore, Brochis Cope, 1871 was placed in its synonymy in order to maintain the monophyly of Corydoras.Alexandrou et al. (2011) studied the correlation of competition and phylogenetic relationships in the community structure of Müllerian co-mimics species of Corydoradinae.This work presented an extensive phylogenetic hypothesis which corroborates the paraphyly of Corydoras, revealing nine clearly different lineages of species.Contrary to Britto's (2003) work, Alexandrou et al. (2011) study does not present a taxonomic approach and by this reason they do not proposed any taxonomic change in order to maintain the monophyly of Corydoras.

Material and Methods
Measurements were obtained by the use of digital calipers to the nearest tenth of a 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).For the osteological analysis, some specimens were cleared and stained (c&s) according to the protocol of Taylor & Van Dyke (1985).Osteological terminology was based on Reis (1998), except for the use of parietosupraoccipital instead of supraoccipital (Arratia & Gayet, 1995), compound pterotic instead of pterotic-supracleithrum (Aquino & Schaefer, 2002), and scapulocoracoid instead of coracoid (Lundberg, 1970).Nomenclature of latero-sensory canals and preopercular pores are according to Schaefer & Aquino (2000) and Schaefer (1988), respectively.The suprapreopercle sensu Huysentruyt & Adriaens (2005) will be treated here as a part of the hyomandibula according to Vera-Alcaraz (2013).Vertebral counts followed Britto et al. (2009).
In the description, numbers between brackets represent the total number of specimens with those counts.Numbers with an asterisk refer to the counts of the holotype.
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; ceratobranchial 3 with continuous postero-lateral margin; ceratobranchial 5 toothed on postero-dorsal surface, 36 to 45 (3) 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 curved mesially uncinate process on laminar expansion of posterior margin.Two wide pharyngobranchials (3 and 4), pharyngobranchial 3 with irregular laminar exapansion on posterior margin.Upper tooth plate oval; 38 to 43 (3) teeth aligned in two rows on postero-ventral surface.Lateral-line canal entering neurocranium through compound pterotic, splitting into two branches before entering sphenotic: pterotic branch with a single pore; preoperculomandibular branch conspicuously reduced, with a single pore opening close to postotic main canal.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 a single pore.Supraorbital canal not branched, running through nasal bone.Epiphyseal pore opening at supraorbital main canal.Nasal canal with three openings, first on posterior edge, second on posterolateral portion 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 triangular, located just posterior to second dorsolateral body plate.Dorsal-fin rays II,7 (1), II,8* ( 19), posterior margin of dorsal-fin spine with 10 to 14 serrations directed towards tip of spine; serrations absent proximally.Nuchal plate moderately developed; exposed, with minute odontodes; spinelet short; spine moderately developed, adpressed distal tip reaching to or slightly surpassing origin of last dorsal-fin branched ray; anterior margin with small odontodes.Pectoral fin triangular, its origin just posterior to gill opening.Pectoral-fin rays I,7* (15), I,8 (5); posterior margin of pectoral spine with 21 to 27 small serrations along its entire length; serrations directed towards pectoral-spine tip (Fig. 3).Pelvic fin oblong, located just below first ventrolateral body plate, and at vertical through first branched dorsalfin ray.Pelvic-fin rays i,5.Adipose fin roughly triangular, separated from base of last dorsal-fin ray by generally seven dorsolateral body plates.Anal fin triangular, 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), ii,5,i ( 15), ii,6 (1), ii,6,i (1).Caudal-fin rays i,11,i (1), i,12,i* ( 19), generally four dorsal and ventral procurrent rays; bilobed; dorsal lobe slightly larger than ventral lobe.Two or three laterosensory canals on trunk; first ossicle tubular, second ossicle laminar, and third lateralline canal, if present, encased in third dorsolateral body plate.Body plates with minute odontodes scattered over exposed area, a conspicuous line of odontodes confined on posterior margins; dorsolateral body plates 22 (2), 23* (16), 24 (2); ventrolateral body plates 20 (12), 21 (8); dorsolateral body plates along dorsal-fin base 6; dorsolateral body plates between adipose-and caudal-fin 6 (1), 7* (15), 8 (4); preadipose platelets 2* (11), 3 (9); 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 with platelets above lateral ethmoid.Ventral surface of trunk generally without platelets; few specimens with scarce small platelets.
Color in alcohol.Overall color pattern in Figure 1.Ground color of body grayish yellow, with top of head and snout dark brown.Ventral margin of orbit, above infraorbital 1, blackened.Maxillary barbel covered by black chromatophores.Dorsal and lateral portion of head, dorsal two thirds of dorsolateral body plates anterior to adipose fin, cleithrum and dorsal portion of ventrolateral body plates anterior to dorsal fin with conspicuously rounded black spots.Dorsal portion of ventrolateral body plates between dorsal-fin origin and adipose-fin origin with vertically elongated black spots.Dorsal portion of caudal peduncle close to caudal-fin origin blackened.Midline of flank posterior to dorsal-fin origin with longitudinal black stripe; posterior margin of dorso-and ventrolateral body plates conspicuously blackened, forming longitudinal zigzag.Longitudinal black stripe along midline of flank fragmented in juvenile specimens.Dorsolateral body plates close to longitudinal stripe on midline of flank unspotted.Dorsal fin covered by irregular black spots; membrane between dorsal spine and first branched dorsal-fin ray with concentration of black chromatophores, generally more concentrated in its dorsal half.Pectoral and pelvic fins unspotted, with concentration of brownish chromatophores on their rays.Anal fin with brown spots; spots aligned forming one to two oblique brown bars in some specimens; first and second anal fin rays markedly blackened in holotype.Adipose fin hyaline with distal margin blackened.Caudal fin with generally three transversal black bars.
Color in life.Very similar to preserved specimens but with ground color of body rosy orange; body covered by greenish yellow iridescent coloration (Fig. 4).
Sexual dimorphism.Additionally to the lanceolate genital papilla in males, which is present in all Corydoradinae (see Nijssen & Isbrücker, 1980;Britto, 2003), the males of Corydoras knaacki have an oblong pelvic fin, while in females the pelvic fin is rounded.
Conservation status.Despite the new species seems to be relatively well distributed in the río Madre de Dios basin, part of its currently known region of occurrence, the río Inambari basin, was recently affected by gold mining and road building.Lujan et al. (2013) demonstrated that the aforementioned threats have already affected the stream community structure of some tributaries to the río Inambari as a whole.However, since there is no available data of the possible direct effects of the gold mining and/or road building in the populations of C. knaacki, and also due to the quite plausible possibility that the new species may occurs in other non-affected water bodies of the region, the most reasonable category for Corydoras knaacki for the time being, according to the International Union for Conservation of Nature (IUCN) categories and criteria (IUCN Standards and Petitions Subcommittee, 2014), is Least Concern (LC).
Remarks.Corydoradinae catfish are well known in the aquarium hobby and have been collected for the ornamental fish trade for many decades.Many newly encountered species are clearly recognized for being scientifically undescribed.To avoid the creation of nomina nuda by using trade names, Evers (1993) suggested to implement a code-system for all undetermined species, giving each species a "C-number" ("C" for "Corydoradinae") in the German aquarist magazine DATZ (Die Aquarien-und Terrarienzeitschrift).Posteriorly, this system was carried on by the "Corydoras World" website (www.corydorasworld.com),and the codes were changed to CW-numbers ("CW" for "Corydoras World").Corydoras knaacki, new species, is well known in the aquarium hobby, and has been previously known under the code number CW 032.Corydoradinae species display specific coloration and pigmentation pattern in juveniles, changing pattern during development after hatching onto reaching their adult coloration.Corydoras knaacki has been reproduced under aquarium conditions by the second author, and the color pattern of juvenile specimens (with less than 20.0 mm SL) are provided herein (Fig. 7), documenting the unique development of this species during growth.

Discussion
The species with the most similar color pattern to Corydoras knaacki are C. bondi, from the Yuruari, Corantijn and Rupununi rivers basins (Fig. 8a), C. sipaliwini, from coastal rivers basins of Guyana and Suriname (Fig. 8b), and C. coppenamensis, from the rio Coppename basin (Fig. 8c).Despite the resemblance, the new species can be clearly distinguished from C. bondi and C. sipaliwini by the presence of conspicuously rounded black spots on head while in both congeners the spots are irregular, being diffuse in some specimens of C. bondi.The relative size of the spots on the snout is also useful to diagnose C. knaacki from C. bondi and C. sipaliwini.In the new species, the spots are moderate in size, contrary to the very small spots of C. bondi, and the larger spots of C. sipaliwini.
Corydoras knaacki can be clearly distinguished from its most similar congener, C. coppenamensis, by the presence of ventral laminar expansion of infraorbital 1 moderately developed (vs.well developed), and also by the anterior expansion of infraorbital 1, which is moderately developed in the new species (Fig. 2a, b An interesting feature observed in the new species is the presence of three unbranched anal-fin rays in some specimens.Generally, Corydoras species possess only one or two unbranched rays in anal fin.Another variable feature observed in C. knaacki is the contact between infraorbital 2 and compound pterotic in some specimens.Despite of the presence of the contact between infraorbital 2 and compound pterotic, it does not occur in the same way as it does with the other congeners, by means of a triangular or rectangular expansion (Britto, 2003: 129).In the new species, the infraorbital 2 posterior laminar expansion does not possess a secondary expansion in its posterodorsal portion (Fig. 2c,  d).The suture between the sphenotic and the compound pterotic seems to determine the presence of contact between infraorbital 2 and compound pterotic (Fig. 2c, d), and not the shape of the infraorbital 2 posterior laminar expansion itself.
Recently, Vera-Alcaraz (2013) conducted the more comprehensive phylogenetic hypothesis for Callichthyidae so far, suggesting the resurrection of some genera to accommodate the species currently attributed to Corydoras in order to maintain its monophyly.Among the resurrected genera is Hoplisoma Swainson, 1838, which comprises the typical shortsnouted species, from the lineages 6, 7 and 9 sensu Alexandrou et al. (2011), and also the straight-snouted species from the lineage 8 sensu Alexandrou et al. (2011).In Vera-Alcaraz's (2013) phylogenetic hypothesis, the large clade containing the short-and straight-snouted species is well delimited and clearly does not belong to the Corydoras clade, which comprises the typical long-snouted species of the lineage 1 sensu Alexandrou et al. (2011).Corydoras knaacki is a typical short-snouted species that clearly fits within the Hoplisoma clade sensu Vera-Alcaraz (2013).However, since the phylogenetic hypothesis conducted by Vera-Alcaraz ( 2013) is still unpublished, the new species will be regarded as Corydoras until formal publication of any generic revision derived from Vera-Alcaraz's work.
. trilineatus, by the presence of a longitudinal black stripe along midline of flank (vs.midline of flank unspotted; with spots; not forming a conspicuous stripe); from C.bifasciatus, C. gomezi, C. haraldschultzi, C.  incolicana, C. isbrueckeri, C. leopardus, C. noelkempffi,  C. ornatus, C. orphnopterus, C. pinheiroi, C. pulcher,  C. robineae, and C. robustus  by the presence of a short mesethmoid, with anterior portion smaller than 50% of the bone length (vs.long, equal or larger than 50% of the bone length); and serrations on posterior margin of the pectoral spine directed towards the tip of the spine (vs.directed towards the origin of the spine); from C. acrensis, C. baderi, C. habrosus, C. julii, C. nattereri, C. sterbai and C. trilineatus by the presence of black pigmentation on the first branched ray, including membrane, the remaining areas with irregular black blotches (vs.anterodorsal portion of dorsal fin with a large black blotch, the remaining areas with irregular black spots in C. acrensis, C. julii and C. trilineatus; dorsal fin entirely hyaline in C. baderi; dorsal fin covered by black spots in C. habrosus and C. sterbai; dorsal fin brownish, without blotches; membranes covered by black chromatophores, more concentrated on its anteriormost portion in C. nattereri).Additionally, C. knaacki can be distinguished from C. boesemani by the absence of a vertically elongated black blotch across the eyes (vs.presence); from C. bondi and C. sipaliwini by the presence of conspicuously rounded moderately-developed black spots on the snout (vs.irregular small black spots; or scattered black chromatophores, in C. bondi; larger irregular black spots in C. sipaliwini); from C. coppenamensis by the presence of ventral expansion of infraorbital 1 moderately developed (vs.well developed).

Fig. 2 .
Fig. 2. Lateral view of the head of c&s specimens of Corydoras knaacki, showing the moderately-developed ventral expansion of the infraorbital 1 in (a) NUP 17308, 36.9 mm SL, and (b) NUP 17308, 35.9 mm SL.Additionally, the two variable condition in which (c) the infraorbital 2 do not contact compound pterotic and (d) in which the contact of infraorbital 2 contacts compound pterotic are also displayed.The dotted lines in (c) and (d) represent the suture between sphenotic and compound pterotic.Abbreviations: io1: infraorbital 1, io2: infraorbital 2, sph: sphenotic, cpt: compound pterotic.Scale bar = 1.0 mm.

Fig. 5 .
Fig. 5. Geographical distribution of Corydoras knaacki.Red star represents type-locality, swamps tributaries to the río Inambari, and the black circle, which includes more than one lot, represents the tributaries to the río Manu, río Madre de Dios basin.

Fig. 6 .
Fig. 6.Type-locality of Corydoras knaacki, showing the swampy area in the vicinity of the town of Santa Rita, draining into the río Inambari, río Madre de Dios basin, Santa Rita, Madre de Dios, Peru.Juveniles of Corydoras knaacki, between 8 up to 15 mm SL, are abundant in the shallow swampy area, forming bigger shoals.Specimens of Corydoras aff.aeneus with similar size to the specimens of C. knaacki were observed mixing in the same shoal with the new species, which is more abundant.Adult specimens of C. knaacki, up to 38 mm SL, can be found in small groups only in the deeper parts of the creeks, generally shaded by trees and palms.The adults do not form mixed shoals with Corydoras aff.aeneus.

Fig. 7 .
Fig. 7. Ontogenetic development of the color pattern in Corydoras knaacki, showing uncatalogued specimens with (a) 10.0 mm SL, (b) 14.0 mm SL, (c) 16.0 mm SL, and (d) 19.0 mm SL.Juveniles in the first six weeks show these typical color patterns until the coloration turns into the adult pattern with a size of 20 mm onwards.

Table 1 .
Morphometric data of Corydoras knaacki.N = number of specimens and SD = standard deviation.Mesethmoid short; anterior tip poorly developed, smaller than 50% of bone length (seeBritto, 2003: 123, character 1, state 1; fig.1B); posterior portion relatively narrow, partially exposed and bearing minute odontodes.Nasal slender, curved laterally, with inner margin laminar; mesial border contacting frontal and mesethmoid.Frontal elongated, narrow, with width slightly smaller than half of entire length; anterior projection short, size smaller than nasal length.Frontal fontanel large, oval; posterior tip extension slightly entering anterior margin of parietosupraoccipital.Parieto-supraoccipital wide, posterior process long and contacting nuchal plate; region of contact between posterior process and nuchal plate covered by thick layer skin.
), while C. coppenamensis displays a very large anterior expansion, conspicuously expanded toward nasal capsule, very similar to what is observed in C. lymnades (see Tencatt et al., 2013: 260, fig.2a).The color pattern of the caudal fin also differs C. knaacki from C. bondi and C. coppenamensis (presence of conspicuous thickened black bars on caudal fin vs. diffuse black spots, not forming well-defined transversal black bars in C. bondi; scattered spots forming irregular slender black bars; or diffuse black spots, in C. coppenamensis).