Taxonomy & naming
Characidium bahiense was described by Almeida in 1971. It belongs to the genus Characidium within the family Crenuchidae — the darters and sailfin characins — in the order Characiformes. The genus is large and taxonomically active: molecular work on Characidium populations from Bahia and neighbouring coastal drainages has flagged possible non-monophyly and cryptic diversity within what is currently recognised as a single species, meaning some populations attributed to C. bahiense may eventually be split out under taxonomic revision. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the standing authority for the valid name; nothing in the available sources suggests a change to the combination Characidium bahiense at this time.
No vernacular or trade name for this species is documented in the sources consulted, which is typical for a narrow-range darter characin that has little presence in the aquarium trade under its own name — most darter characins sold in the hobby are traded simply as "Characidium sp." or under a country-of-origin label rather than by species.
Morphology
Characidium bahiense has not been assigned a documented maximum length in the sources available for this account, but the genus Characidium is uniformly small: most species top out around 1.5–3 in, and there is no reason from the record to expect this species to fall outside that range. Like other members of the family Crenuchidae, darters of this genus show the classic rheophilic (fast-water-adapted) body plan: a flattened, elongate body and low-set paired fins that let the fish cling and perch against current on rock or gravel rather than swim continuously in open water.
Colour pattern and precise proportions specific to C. bahiense are not detailed in the available sources; data is sparse for this narrow-range species beyond its original description.
Habitat
Characidium bahiense is endemic to Brazil, recorded from coastal river basins of the Northeastern Mata Atlântica (Atlantic Forest) ecoregion, with specimens collected from the rio Capivara near Arembepe in the state of Bahia; the species' broader range has also been associated with drainages extending toward the São Francisco and Parnaíba systems. As a rheophilic darter it is a stream-bed specialist, favouring flowing water over rock and gravel substrate where its flattened body and low-set fins let it hold position against current rather than being swept downstream.
Water chemistry specific to this species is not documented in the sources consulted; genus-level accounts for Characidium describe soft, mildly acidic conditions typical of Atlantic Forest coastal streams, but a species-specific reading should not be assumed without direct data.
Feeding
Characidium bahiense has not been the subject of a dedicated diet study in the sources available, but the genus is well documented as a micro-predator: darters perch on the substrate and dart out to seize tiny invertebrates — chironomid (midge) larvae, mosquito larvae, small crustaceans such as daphnia and cyclops — rather than grazing or filter-feeding. Many Characidium refuse dry flake outright and need to be offered live or frozen foods sized to their small mouths.
In the aquarium, genus-typical feeding practice applies: small live or frozen foods such as bloodworm, mosquito larvae, daphnia and cyclops are taken readily, while flake is often ignored.
Mating
No species-specific courtship account for Characidium bahiense is documented in the sources consulted. Genus-level accounts for Characidium describe a pair spawning side-by-side over the substrate, with no elaborate courtship display beyond pairing off and settling onto a chosen spot — behaviour consistent with the group's general pattern of scattering rather than guarding eggs, in contrast to the cave/leaf-spawning, male-tending habit documented for the related sailfin characins Crenuchus and Poecilocharax.
Breeding
Characidium as a genus are documented as substrate egg-scatterers: a pair spawns side-by-side over gravel or fine substrate, producing on the order of 150 eggs per clutch in genus-level hobby accounts, which hatch in roughly 30–40 hours at warm tropical temperatures. There is no parental care described for the genus — breeders are typically removed from the spawning site once eggs are laid, since adults will consume the eggs and fry given the chance. No breeding record specific to Characidium bahiense itself is documented in the sources consulted, and the species should not be assumed to be established in the aquarium hobby; this account follows the genus pattern rather than a confirmed species-specific spawning.
In the aquarium
Characidium bahiense is not a documented aquarium staple, and species-specific husbandry data is sparse. Genus-level guidance for Characidium darters calls for a species or quiet community tank with a sand or fine-gravel substrate, moderate current, and open perching spots on rock or wood rather than a heavily planted, still-water layout — this is a stream-bed fish, not a schooling water-column one. Genus accounts cite a temperature range around 72–79 °F and soft, mildly acidic water near pH 6.5–7.0, with a strong preference for live or frozen micro-foods over flake.
Darters of this genus can be kept singly or, better, in a loose group of eight or more so that the naturally territorial perching behaviour is spread across more individuals rather than concentrated on one or two tankmates. As with any narrow-range, poorly documented species, a keeper should treat any Characidium bahiense encountered in the trade with caution about provenance and should not assume it tolerates conditions outside the modest, cool-to-moderate parameters typical of the genus.
Conservation
Characidium bahiense is assessed by the IUCN Red List as Least Concern. As a narrow-range endemic restricted to Atlantic Forest coastal drainages in and around Bahia, Brazil, it nonetheless sits in a broader ecoregion under heavy, well-documented pressure from deforestation, agricultural runoff and urban development, which is the general watch item for small-range Mata Atlântica stream fish even when a species' current assessment is not elevated. No species-specific population trend or threat detail beyond the LC assessment is documented in the sources consulted.