Taxonomy & naming
Characidium barbosai was described in 2020 by Flausino Junior, Lima, Machado & Melo as a new species of Characidium (Characiformes: Crenuchidae) distinguished in part by a unique mid-water behaviour not otherwise documented in the genus — most Characidium are strict substrate-perchers, resting on the bottom between short darting movements, but this species has been observed holding position off the substrate. The genus Characidium sits within the family Crenuchidae, the "darters" or South American darter characins, alongside the sailfin darters of Crenuchus and Poecilocharax; Crenuchidae as a whole is often informally grouped with the Neotropical characins under the broad "tetra-like" umbrella, but its members are ecologically distinct — small, benthic, and built for perching and darting rather than open-water shoaling.
The species epithet honours Gerson Natalício Barbosa, recognised for his work in Brazilian environmental law enforcement and conservation, including the "Água para o Futuro" (Water for the Future) project surveying, protecting and restoring urban springs around Cuiabá, Mato Grosso — a fitting namesake given how tightly the species' survival is tied to the health of a small number of headwater streams in that same region.
Morphology
Characidium barbosai is a small fish even by the standards of its genus: the holotype measures 1.5 in standard length, with the largest observed specimens reaching about 2 in SL. Like other Characidium, it has an elongated, subcylindrical body built for holding position against current and making short, controlled darts rather than sustained swimming — the general body plan that gives the genus its "darter" common name. Detailed colour pattern and fin-ray counts distinguishing it from congeners are set out in the original description; the species was separated from its relatives in part on the basis of its unusual mid-water resting posture, a behavioural trait rarely used to help characterise a Characidium species.
As with most recently described, narrow-range Characidium, there is no dedicated account of sexual size or shape differences for this species beyond the type series; see "Mating" below.
Habitat
The species is known only from headwater tributaries of the Serra da Borda, part of the upper Rio Guaporé basin within the larger Rio Madeira drainage, in Mato Grosso, Brazil, at elevations of roughly 984–2625 ft. This is a small, isolated plateau system, and to date the species has been recorded from only two tributaries — an extremely restricted range even by the standards of microendemic Amazonian headwater fish.
The streams it inhabits are typical of Amazonian tributary headwaters: soft, acidic blackwater conditions running over sand, gravel and leaf litter. As a Crenuchidae darter, Characidium barbosai is adapted to flowing, well-oxygenated water rather than the still floodplain habitats used by many other Neotropical characins.
Feeding
No species-specific dietary study has been published for Characidium barbosai, but the genus as a whole is made up of micro-predators: small insects, aquatic insect larvae and micro-crustaceans picked from the substrate and water column, consistent with a fish that perches and darts rather than filter-feeds or grazes. Congeners kept in the aquarium are known to largely ignore flake food and require small live or frozen prey items such as microworm, baby brine shrimp shrimp and cyclops — there is no reason from the cached sources to expect C. barbosai to differ from this genus-wide pattern, though this is an inference rather than a direct observation of the species.
Mating
Nothing specific to Characidium barbosai's courtship has been documented — the species is known only from its type series and a handful of survey records. Across the genus, Characidium are described as egg-scatterers with minimal courtship ritual documented in the literature; males of some congeners are reported to briefly tend eggs in plant clumps after spawning, but detailed pairing or courtship behaviour is not well characterised for the genus as a whole, and none of it has been confirmed for this species specifically.
Breeding
Characidium barbosai has not been bred in the aquarium, and breeding data specific to the species is sparse to non-existent — it is not part of the aquarium trade. What is known comes from its genus: Characidium generally scatter adhesive eggs, most often over fine-leaved plants or moss in flowing, soft, acidic water, with only minimal parental care — occasional brief male attendance at the egg site has been reported in some congeners, but nothing approaching the sustained cave-guarding behaviour seen in the related sailfin darters (Crenuchus, Poecilocharax). Congener accounts describe clutches of roughly 150 eggs hatching in 36–48 hours, with fry started on infusoria before graduating to baby brine shrimp. None of this has been verified for C. barbosai itself, and it should be read as a genus-level extrapolation rather than a documented account of this species.
In the aquarium
Characidium barbosai is not established, or realistically establishable, in the hobby: it is a 2020-described microendemic known from two tributaries on a single Brazilian plateau, it is Critically Endangered, and there is no aquarium-trade history for it to draw on. Any care information here is therefore extrapolated from its genus and family, not from direct experience with the species, and should be treated as a starting point rather than a settled protocol.
Crenuchidae darters generally are nano fish under 4 in — C. barbosai, at under 2 in, is small even among them — that need highly oxygenated, flowing water simulating a fast stream: a sand-gravel-pebble substrate, strong circulation, and soft, acidic water in the pH 5.5–7.5 range at 64–81 °F. They are noted as capable jumpers, so tight cover is essential. Diet must include small live or frozen foods rather than flake. Given its conservation status and total lack of trade history, this species should not be sought for the hobby; any future aquarium interest would need to be grounded in ex-situ conservation breeding rather than collection from its two known tributaries.
Conservation
Characidium barbosai is assessed by the IUCN Red List as Critically Endangered (CR), reflecting its extraordinarily restricted range — only two known tributaries on an isolated headwater plateau in the upper Rio Guaporé basin, Mato Grosso, Brazil. The primary threat is habitat degradation from deforestation and agricultural expansion in the region, pressures that can affect an entire species' range through changes to just one or two small streams.
As a fish described only in 2020, C. barbosai is emblematic of a broader pattern in Amazonian ichthyology: microendemic headwater species are still being discovered, described and immediately flagged as threatened, often before their basic biology — breeding behaviour, full diet, population trend — is understood. Its species epithet, honouring a Brazilian conservationist involved in urban spring protection near Cuiabá, underscores how closely its fate is tied to local watershed management rather than to any broad-scale threat.