Taxonomy & naming
Ctenobrycon magdalenae was described by Carl H. Eigenmann and Arthur W. Henn in 1916. Because the original authors' names are placed in parentheses, the species has since been transferred from the genus under which it was first described, following standard ichthyological convention. The type locality is Girardot, a town on the upper Río Magdalena in Cundinamarca department, Colombia. Eschmeyer's Catalog of Fishes (Catalog of Fishes, Fricke, Eschmeyer & Fong) is the governing authority for the valid name.
The genus Ctenobrycon belongs to the family Characidae, the largest family of freshwater fishes in the Americas. The genus contains a small number of species centred on South American river systems and is distinguished within the family by a combination of dentition and body-form characters. C. magdalenae is the largest member of the genus and its most northerly distributed species, reaching into Venezuela via the Lake Maracaibo drainage.
Morphology
Ctenobrycon magdalenae is a large, laterally compressed, deep-bodied characin reaching a recorded maximum of 7 in standard length — a size that places it firmly outside the small-tetra mould of most familiar Characidae. The body is tall relative to its length, giving it a carp-like silhouette more reminiscent of a large piranha relative than a neon tetra, with a blunt snout and a relatively large mouth suitable for processing plant material and invertebrates.
The overall coloration is silver to silvery-olive on the flanks, typical of the genus, with the reflective scale rows creating a mild iridescence under natural light. The caudal fin base may show a dusky blotch, and the finnage is generally clear to lightly pigmented. Sexual dimorphism, when present, is likely expressed as differences in body depth and fullness, with ripe females becoming noticeably broader-bodied; highly specific dimorphism details are not well documented in the available literature for this species.
Habitat
Within Colombia, Ctenobrycon magdalenae occurs in the Río Magdalena basin and the tributaries that drain west and north from the Andes, including the Lebrija, Sogamoso, and Chicamocha river basins. The species also extends into Venezuela through the Catatumbo River system and the Lake Maracaibo basin. This is a species of the Caribbean-draining lowlands and foothills of northern South America, occupying one of the most biologically diverse river systems on the continent.
The fish is recorded from a wide array of standing and slow-moving habitats: rivers and their margins, streams, swamps, marshes, lakes, and reservoirs. It is classified as benthopelagic, inhabiting the midwater and lower water column in its preferred calm-water environments. The broad habitat tolerance — spanning both natural wetlands and human-created reservoirs — is consistent with the IUCN's finding of a stable and abundant population with no current decline trajectory.
Feeding
Ctenobrycon magdalenae is an omnivore with a notably varied diet documented to include plant remains, phytoplankton, invertebrates, and silt. This combination — green material, microbial and algal cells, bottom detritus, and animal protein — is characteristic of large characins that exploit the full productive column of slow-moving tropical rivers and floodplain habitats, where organic matter accumulates in abundance.
The congener Ctenobrycon spilurus is known to take zooplankton, plants, worms, insects, and small crustaceans, suggesting the genus as a whole is broadly opportunistic. In captivity, a species of this feeding type would be expected to accept pelleted and flake foods supplemented with plant-based matter, live and frozen invertebrates such as worms and crustaceans, and vegetable components including blanched greens. Its size and feeding breadth make overfeeding a more likely pitfall than any difficulty in finding accepted foods.
Mating
Ctenobrycon magdalenae has not been studied in detail for its reproductive behaviour, but as a member of Characidae it almost certainly follows the family's characteristic egg-scattering strategy. Characids typically engage in brief, energetic spawning chases in which a male pursues a ripe female through vegetation or open water, with eggs and milt released simultaneously and fertilised eggs scattered or falling among fine-leaved plants, roots, or substrate.
At a size of 6.5 in, this species would be among the larger egg-scattering tetras, and conditioning a breeding group would likely require a proportionately spacious aquarium. Males presumably display competitive or courtship behaviours in the presence of rivals and receptive females, though no detailed field accounts are available from the cache sources. No parental pair bond is formed, and spawning in most Characidae is opportunistic rather than seasonally constrained, though water temperature and photoperiod shifts may act as triggers in the wild.
Breeding
Like other members of Characidae, Ctenobrycon magdalenae is presumed to be an egg-scattering broadcast spawner with no parental care. Fertilised eggs, once scattered, receive no attention from either parent; the adults will consume eggs if given the opportunity, so separation of eggs from adults is standard practice in captive breeding of the family.
No species-specific captive breeding accounts appear in the available sources. For a characin of this size, breeding would be approached in a dedicated aquarium of at least 47–59 in length with soft, slightly acidic water and a temperature in the lower-to-mid 20s Celsius — parameters consistent with the Magdalena basin's upland river systems. Dense fine-leaved planting or spawning mops provide adhesion sites for any adhesive eggs. Fry, if successfully reared, would be started on infusoria or commercial liquid fry food before being moved to nauplii and then finely crushed prepared foods.
In the aquarium
Ctenobrycon magdalenae is not commonly encountered in the ornamental trade. At 6.5 in standard length, it requires substantially more space than most tetras kept by hobbyists; a single adult needs an aquarium of at least 59 in in length, and a proper group would demand correspondingly more. Retailers and specialist fish-keepers focused on large South American characins are the most likely source.
Its broad omnivorous diet and tolerance for varied habitats — rivers, swamps, reservoirs — suggest a degree of hardiness in captivity, and it should be compatible with neutral to slightly soft, warm tropical water in the mid-20s Celsius range. Tankmates should be chosen with the fish's size in mind; any fish small enough to be consumed is at risk. South American cichlids, large catfish, and other robust fish of comparable size make appropriate companions. The species has hobbyist potential as a display fish for large aquaria focused on Colombian river biotopes.
Conservation
Ctenobrycon magdalenae is assessed as Least Concern (LC) on the IUCN Red List, with an assessment date of 15 January 2021 (published 2023). The justification rests on the species' wide distribution across multiple Colombian river basins and the Lake Maracaibo system in Venezuela, together with a population described as stable and very abundant. No near-term threat sufficient to trigger a higher category is identified at current population levels.
The Magdalena River basin is nevertheless one of the most heavily modified river systems in South America: deforestation, agricultural runoff, urban pollution, hydroelectric development, and introduced species have all reshaped the system over the past century. While none of these pressures has demonstrably driven a decline in C. magdalenae at the time of assessment, the species' continued monitoring within the context of ongoing Magdalena basin management is important. Its tolerance of reservoirs and other human-altered water bodies may offer some buffer against further habitat modification, but large-scale water-quality degradation would eventually narrow even a broadly adapted species' range.