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.

Key words: Corydoradinae; CW32; rio Amazonas basin; río Inambari basin; rio Madeira basin; Taxonomy

Diagnosis. Corydoras knaacki can be distinguish from its congeners, with exception of C. acrensis , C. baderi, C. bifasciatus Nijssen, 1972, C. boesemani , C. bondi , C. coppenamensis , C. gomezi , C. habrosus , C. haraldschultzi , C. incolicana Burgess, 1993, C. isbrueckeri , C. julii , C. leopardus , C. nattereri , C. noelkempffi , C. ornatus , C. orphnopterus , C. pinheiroi Dinkelmeyer, 1995, C. pulcher , C. robineae Burgess, 1983, C. robustus , C. sipaliwini , C. sterbai and C. 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).

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. 

Ecological notes. Corydoras knaacki was captured in a swampy area with brownish "tea-colored" stagnant and very clear water. Some congeners, like Corydoras aff. aeneus , Corydoras cf. elegans and Corydoras cf. stenocephalus , were observed in the type-locality of C. knaacki (Fig. 7). Additionally, some Characiformes were also observed in syntopy, like Aphyocharax sp., Hoplias aff. malabaricus (Bloch, 1794), Hyphessobrycon sp. and Pyrrhulina vittata , Regan, 1912.

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 .

 Etymology. Corydoras knaacki is named after Dr. Joachim Knaack (2 January 1933 - 5 December 2012), German physician and biologist. He was an amateur ichthyologist and aquarist who devoted more than 60 years of his life for the study of South American catfishes, especially Corydoras . A genitive noun.

Luiz Fernando Caserta Tencatt and Hans-Georg Evers. 2016. A New Species of Corydoras Lacépède, 1803 (Siluriformes: Callichthyidae) from the río Madre de Dios Basin, Peru.
 Neotrop. ichthyol. 14(1); DOI:  10.1590/1982-0224-20150019