Taxonomy & naming
Characidium urucum was described in 2026 by Uzeda, Stabile, Reis, Sartori, Pavanelli, Margarido and da Graça, based on both morphological and molecular (mitochondrial COI) evidence from specimens collected across several tributaries of the upper Rio Grande in Minas Gerais, Brazil. It belongs to the genus Characidium, the most species-rich genus in the family Crenuchidae — the South American darters — within the order Characiformes; the genus currently comprises around 90 species distributed from Central America to Patagonia.
Molecular analysis placed C. urucum within a clade alongside C. bolivianum, C. fasciatum, C. gomesi and C. purpuratum, specifically as sister to a group combining C. cf. fasciatum from the Rio São Francisco basin with C. gomesi and C. aff. gomesi from the upper Paraná. The describing authors estimate C. urucum diverged from its closest relatives around 3 million years ago, likely tied to hydrographic rearrangements that historically connected and then separated the upper Paraná and São Francisco river systems. The specific epithet honours urucum (Bixa orellana), a plant whose seeds have long supplied a traditional red pigment across South America — a direct reference to the striking red body colour of adult fish, otherwise unknown in the genus.
Morphology
Characidium urucum is a robust, elongate darter; across the full type series standard length ranges from 1–2.6 in, with the 1.7 in male holotype toward the smaller end of that range. It is diagnosed from most congeners by a naked isthmus paired with a fully scaled chest, except for small naked patches around the pectoral-fin bases — most similar species have either a completely naked chest or scaling that stops well short of the isthmus. A further diagnostic character found by dissection is the foramen (opening) in the skull for the ophthalmic nerve, which in C. urucum is small and comma-shaped rather than the large, rounded opening seen in its closest relatives C. bolivianum, C. fasciatum and C. gomesi, through which the brain cavity is visible.
The species' most striking feature, though, is not a bony character at all: adult specimens develop an intense red hue across the body and head, a colour the describing authors chose not to use as a formal taxonomic character, since it fades in preserved material and appears restricted to adults, but that is otherwise unrecorded in Characidium. Juveniles under about 1.4 in SL instead show a yellowish body scattered with one- to three-scale dark brown spots, a pattern that superficially resembles the unrelated C. lauroi species group before fading and giving way to red as the fish matures.
Habitat
Characidium urucum is restricted to the upper Rio Grande sub-basin of the upper Paraná River basin in Minas Gerais, Brazil, with an estimated range of about 26,249 mi². At the type locality, the Rio Capivari, the describing authors classify the stream as third-order, four to eight metres wide and 7.9–59.1 in deep, with a substrate of sand interspersed with pebbles and cobbles in its riffle sections.
Notably, C. urucum was captured only in the deepest pools of these streams, specifically where the banks were tangled with submerged wood debris, and was never found in riffle sections or in stretches with an open canopy — a level of habitat specificity the describing authors flag explicitly. It was collected in sympatry with a range of other Neotropical stream fish, including a second Characidium (C. gomesi), the catfish Cetopsorhamdia iheringi, the eartheater cichlid Geophagus iporangensis, and several Hypostomus, Trichomycterus and Pareiorhina species. Sites range from roughly 945 to 1,761 ft in elevation; specific temperature, pH and hardness data were not reported.
Feeding
Gut-content analysis of six specimens found the diet of Characidium urucum to be dominated by Chironomidae (non-biting midge) larvae, with additional remnants of digested arthropod exoskeletons, confirming the species as primarily invertivorous — consistent with the substrate-picking, insect-hunting habits documented across the genus. This direct feeding data, drawn from wild-caught fish rather than inferred from relatives, is unusually well documented for a species this newly described.
Mating
No courtship behaviour has been directly observed or published for Characidium urucum, but its describing authors document a form of sexual dimorphism new to the genus: males have a noticeably more robust and rounded snout than females, whose snouts are comparatively slender, and males also carry longer paired fins — the tip of the pectoral fin extends past the origin of the pelvic fin, and the tip of the pelvic fin past the origin of the anal fin, neither of which occurs in females. Whether these traits play a direct role in courtship or male competition is not addressed in the source available for this article.
Breeding
No breeding data — wild or captive — have been published for Characidium urucum. As with the wider genus, it is presumed to be an egg-scattering species without parental care, but this has not been confirmed directly for C. urucum, and the species has no history in the aquarium hobby that might have generated such observations.
In the aquarium
Characidium urucum is not established in the aquarium trade; it was only formally described in 2026 and, given its documented preference for deep pools with submerged wood debris rather than open riffles, would likely need current-swept but structurally complex water with driftwood cover to do well if it were ever kept. As with other Characidium, it would be expected to behave as an active, substrate-perching micro-predator needing small live or frozen invertebrate foods rather than a generalist community fish, but none of this has been tested in captivity for this specific species.
Conservation
Characidium urucum has not been formally evaluated by the IUCN Red List, having been described only in 2026. Its describing authors calculated an extent of occurrence of roughly 26,249 mi², corresponding to the upper Rio Grande sub-basin, and note that although the species has not yet been recorded within any protected area, most of its known localities remain pristine or only lightly impacted, with no identifiable threats to its population at present. On that basis, the authors recommend the species be treated as Least Concern under IUCN criteria, pending a formal assessment.