Taxonomy & naming
Knodus transandeanus was described in 2026 by Jhon G. Albornoz-Garzón, Paula Pulido-Santacruz, Javier E. García-Melo, Mónica Torres and Carlos DoNascimiento in Neotropical Ichthyology, based on a multilocus molecular phylogeny of the family Stevardiidae combined with a morphological and osteological study. The holotype, IAvH-P 21076, is an adult male of 1.87 in standard length collected from quebrada La Concordia, a tributary of the río Cascajales in the middle río Magdalena basin, El Carmen de Chucurí, Santander, Colombia. Eschmeyer's Catalog of Fishes lists Knodus transandeanus Albornoz-Garzón, Pulido-Santacruz, García-Melo, Torres & DoNascimiento, 2026 as the valid name, placed by the Catalog in Stevardiidae, subfamily Diapominae; this atlas follows the traditional and still widely used placement of the genus in Characidae.
The genus Knodus has long been diagnosed almost entirely by plesiomorphic (ancestral, non-diagnostic) characters, and its validity relative to the closely related genus Bryconamericus has been repeatedly questioned since the 1940s. The describing authors' molecular analysis corroborates Knodus as a distinct clade and, based on the taxonomic distribution of proposed synapomorphies, also transfers the Venezuelan species Bryconamericus motatanensis into Knodus as a new combination. With the new species, the authors estimate Knodus at 41 valid species, a number the paper itself calls provisional given several morphologically defined nominal species that do not form single clades in molecular analysis.
Morphology
Knodus transandeanus is a moderately elongate characid reaching at least 1.95 in standard length among the type series (holotype 1.87 in SL), broadly comparable in size to other trans-Andean members of the Bryconamericus–Knodus complex. It is distinguished from the great majority of its congeners by a combination of meristic and morphometric characters: 19–23 branched anal-fin rays, 6–7 scale rows between the dorsal-fin origin and the lateral line, a complete lateral line of 35–38 pored scales, a caudal peduncle proportionally shorter and deeper than in several close relatives, and premaxillary teeth with five cusps. The caudal fin is scaled at the base of each lobe — the classic, if homoplasy-prone, diagnostic feature of Knodus — with scales extending from half to three-quarters of each lobe.
Adult males (1.19–1.95 in SL) carry small bony hooks along the posterior margin of the pelvic-fin rays (first to sixth branched rays) and the anal-fin rays (third unbranched to ninth or tenth branched ray), a common breeding-condition feature in characids of this group that is absent in females. In alcohol the body is light brown with a faint, vertically elongated humeral blotch and a silvery midlateral stripe running from the first lateral-line scale to the base of the caudal fin; freshly collected specimens showed reddish tones concentrated on the caudal peduncle and at the bases of the dorsal, anal and caudal fins. In life the dorsum is brown, the venter silvery, the upper iris yellow, and a bright, bluish-toned lateral stripe runs from the humeral region to the caudal peduncle; the dorsal and anal-fin bases are red in some individuals, and the adipose fin is reddish.
Habitat
Knodus transandeanus is known only from two small tributaries — quebrada La Concordia and quebrada La Leona — of the río Cascajales, itself a tributary of the río La Colorada in the middle río Magdalena basin. The type locality lies on the western slope of the Cordillera Oriental near Parque Nacional Natural Serranía de los Yariguíes, Santander, Colombia, at elevations between roughly 670 and 2461 ft. The describing authors record the type-locality stream as averaging about 23 ft in width, with a substrate of sand, pebbles and rocks and abundant riparian vegetation — a clear, moderately fast Andean-foothill stream rather than a lowland blackwater system.
The species was collected syntopically with a distinctive assemblage of Magdalena-basin fishes, including Hoplias malabaricus, Lebiasina chucuriensis, Astyanax yariguies, Creagrutus guanes, C. magdalenae, Hemibrycon plutarcoi, Pimelodella floridablancaensis, Magdalenichthys yariguies, two Trichomycterus species, Ancistrus caucanus, Dolichancistrus carnegiei, Farlowella yarigui, Astroblepus verai, Geophagus steindachneri and Poecilia caucana — several of them, like K. transandeanus itself, narrow endemics of this single middle-Magdalena tributary system. No water chemistry data (temperature, pH, hardness) were reported in the description.
Feeding
Stomach-content analysis of two cleared-and-stained specimens found remains of aquatic and terrestrial insects, indicating that Knodus transandeanus is an insectivorous micropredator that forages both within the stream and at or near the surface for terrestrial prey falling into the water — a common feeding strategy among small characids in shaded Andean-foothill streams with overhanging riparian vegetation. No further dietary or foraging-behaviour data were reported in the description.
Mating
No spawning behaviour was documented for Knodus transandeanus in the description, which is a taxonomic and osteological study rather than an ecological one. Adult males of at least 1.19 in SL bear small bony hooks on the pelvic- and anal-fin rays, a feature used in many characids to grip the female during spawning and generally associated with an egg-scattering reproductive mode. By analogy with other well-studied Knodus and Bryconamericus species — a genus-level generality, not a species-specific finding — mating is presumed to involve brief pursuit and clasping among vegetation or leaf litter, with no courtship display or parental investment documented for the genus.
Breeding
No breeding or captive-propagation data exist for Knodus transandeanus, which has not been kept in aquaria. Based on the reproductive biology documented for other small Characidae of similar body plan — an egg-scattering mode with adhesive, non-guarded eggs dispersed over vegetation or substrate and no parental care — a similar pattern is plausible for this species, but this is a genus-level inference rather than a confirmed fact. Nothing is known of its fecundity, egg size, incubation period or growth rate.
In the aquarium
Knodus transandeanus is not known to be present in the aquarium trade. It was described from a series collected during faunal surveys in the Serranía de los Yariguíes and, as a narrow-range species restricted to a single small tributary system, is unlikely to be commercially collected or exported. Should the species ever become available, its natural habitat — a clear, moderately flowing Andean-foothill stream with a rocky and sandy bottom — suggests it would do best in a well-oxygenated, unheated to moderately warm aquarium with reasonable current, kept as a shoaling group as is typical for characids of this size and body plan; this is a general inference from habitat and family-level biology rather than any documented aquarium experience with the species.
Conservation
Knodus transandeanus has not been formally assessed by the IUCN Red List; its status is Not Evaluated (NE). The describing authors note that the species is known only from two small tributaries of the río Cascajales, being absent from the upper and lower río Magdalena and from the basin's larger tributaries such as the Cauca, Cesar and San Jorge rivers, and interpret this narrow range as a biogeographic relict pattern shared by several other trans-Andean endemics of the middle Magdalena (such as the catfish Farlowella yarigui and the characin group Astyanax yariguies). No specific threats were reported, but a range this restricted — a single small drainage on the western slope of the Cordillera Oriental — would leave the species inherently vulnerable to habitat degradation, and a formal conservation assessment has not yet been carried out.