Taxonomy & naming
Characidium onca was described in 2021 by Melo, Brito Ribeiro & Lima in Neotropical Ichthyology, based on a holotype (MZUSP 125807, 1.5 in SL) collected from córrego Taquara, a tributary of the ribeirão Gama within the Reserva Ecológica do IBGE near Brasília, Distrito Federal, Brazil. It belongs to the family Crenuchidae — the South American darters — within the order Characiformes, and to the genus Characidium, a large group of small, elongate, bottom-associated stream characins that behave more like gobies or darters than like open-water tetras.
The species epithet "onca" is the Portuguese word for jaguar, chosen for the dark, rounded spots scattered across the fish's gold-yellow flanks, echoing the big cat's rosettes. As with many recently described Characidium, its closest relatives and exact placement within the genus are still being worked out by specialists; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the reference point for keeping the valid combination current.
Morphology
Characidium onca is a small, slender, cylindrical characin typical of its genus, reaching a maximum recorded size of about 1.5 in standard length in mature females, roughly 2 in total length — a genuinely nano fish. The holotype measured 1.5 in SL. The body carries the dark, rounded, jaguar-like spotting on a gold-to-yellow ground colour that gives the species its name, set against the fish's typically slim, torpedo-shaped darter body plan with a flattened underside suited to resting on the substrate.
Sexual dimorphism is present: mature females run deeper-bodied than males, a difference linked to egg development, with recorded female sizes spanning roughly 1–1.5 in SL. Fine details of fin shape and colour differences between the sexes have not been documented beyond this body-depth distinction in the sources available.
Habitat
The species is known only from small, clear, flowing (lotic) headwater streams in Cerrado savanna habitat, with riparian vegetation along the banks, within the Reserva Ecológica do IBGE on the outskirts of Brasília. The type locality sits in the ribeirão Gama drainage at approximately 15°54'55"S, 47°54'24"W. As currently understood, Characidium onca is a narrow-range endemic restricted to an area of roughly 9 mi², making it one of the most geographically confined characins on record.
No detailed water chemistry readings (pH, hardness) for the type stream are given in the available sources; what is documented is that the habitat is flowing, clear headwater stream water in a Cerrado biome, a setting generally cooler and better-oxygenated than the classic Amazonian blackwater imagery associated with characins. Related Characidium such as C. fasciatum and C. rachovii are described as preferring subtropical stream temperatures around 64–75 °F and mildly acidic-to-neutral pH near 5.6–7.5, which gives a reasonable proxy range for the genus pending direct data on C. onca.
Feeding
Characidium as a genus are sit-and-wait micro-predators, not grazers or flake-eaters by nature: they perch on the substrate and dart out to snatch chironomid midge larvae, other small aquatic insects, and tiny invertebrates drifting past. There is no species-specific dietary study for C. onca in the available sources, but its close relatives' documented diet — small insects and invertebrates picked from the stream bottom — is the best available guide for this species too.
In captivity, genus-typical practice is to offer small live or frozen foods such as bloodworm, brine shrimp, Daphnia and Cyclops rather than relying on flake, which darter characins often ignore. No aquarium feeding data specific to C. onca exists yet, since the species has no documented history in the hobby.
Mating
No courtship behaviour has been documented for Characidium onca specifically; the species' description is based on wild-collected specimens and habitat data, not aquarium or field breeding observations. Sexual differentiation in the species is limited to the body-depth difference noted above — gravid females are visibly deeper-bodied than males, which is the only external cue currently available for distinguishing ripe fish.
For the genus generally, Characidium are substrate spawners: paired fish spawn over the stream bottom rather than building a nest or bubble nest, with no evidence of mouthbrooding or the cave/leaf-guarding behaviour seen in the darters' Crenuchidae relatives Crenuchus and Poecilocharax.
Breeding
Characidium onca has no documented breeding record in the aquarium or in captivity — it is a species known from a single 2021 taxonomic description and field survey, not from hobby culture. Its family, Crenuchidae, does include genuinely breedable nano fish (the sailfin characins Crenuchus and Poecilocharax, which are cave or leaf spawners with brief male care of the clutch), but Characidium itself is understood to be a substrate egg-scatterer: pairs release eggs over open bottom rather than tending a nest site, and related Characidium in the hobby literature are reported to abandon the eggs after spawning, with no ongoing parental care.
Given that, parental care in this species should be assumed minimal to absent, consistent with the broader genus pattern, but this is an inference from relatives rather than a direct observation of C. onca — no aquarium spawning of this specific, newly described and Critically Endangered species has been reported.
In the aquarium
Characidium onca is not established in the aquarium trade, and given its Critically Endangered status and its entire known range sitting inside a Brazilian ecological reserve, commercial availability is effectively nonexistent and should stay that way — this is a conservation-priority species, not a shopping-list one. The genus more broadly (Characidium spp.) does turn up occasionally in specialist nano and biotope circles, kept in small groups in a heavily oxygenated, current-swept stream-biotope tank with a fine sand or gravel bottom, smooth stones and driftwood the fish can perch against; tankmates need to be equally small and peaceful, since Characidium are shy micro-predators, not aggressive or territorial fish in the piranha sense.
Because C. onca is a true nano species (adults barely reaching 2 in), and because its diet leans toward small live and frozen invertebrate foods rather than flake, any responsible keeper of Characidium in general should expect a quiet, current-loving, sit-and-dart fish rather than an active shoaler — and should recognise that this particular species, specifically, has no ethical route into the hobby at present.
Conservation
Characidium onca is assessed as Critically Endangered, one of the most acute conservation statuses of any characin on the site. The species is known from a single locality with an extent of occurrence smaller than 62 mi², concentrated within roughly 9 mi² of Cerrado headwater stream near Brasília. Threats identified at description include habitat fragmentation driven by the city's urban expansion and the introduction of exotic predatory species downstream of its native stream reach.
The fact that its entire known population sits inside the Reserva Ecológica do IBGE offers some protection, but a single-locality endemic with naturally low abundance has essentially no buffer against a local habitat event. There is no evidence of the species entering the aquarium trade, wild-collected or otherwise; its conservation story is a field-biology and habitat-protection matter, not a fisheries or hobby-collection one.