Taxonomy & naming
Characidium occidentale was described by Pedro Buckup and Ricardo Reis in 1997, part of a wave of Characidium species-level revisions the two authors carried out through the 1990s that split what had been a handful of broadly defined species into many geographically restricted ones. It belongs to the family Crenuchidae — the South American darters, which also includes the much more brightly coloured sailfin characins Crenuchus and Poecilocharax — within the order Characiformes.
The genus Characidium itself sits in the subfamily Characidiinae and is characterised by an elongated, subcylindrical body, a reduced or absent swim bladder posterior chamber, and a habit of resting propped against the substrate rather than swimming in open water, all features shared with true darters (family Percidae) despite no close relationship to them — a case of convergent evolution toward a bottom-perching lifestyle in flowing water. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the reference for the currently valid combination Characidium occidentale; this article follows that placement and does not attempt to reassign the genus.
Morphology
Characidium occidentale is a small fish even by the standards of its genus: the type material includes a paratype (MCN 6801) of just 2 in standard length, and available data point to a typical adult size around 2 in. Like its relatives it has an elongated, slightly compressed body, a flattened ventral profile, and large pectoral and pelvic fins that it uses to prop itself on wood, gravel or leaf litter rather than hovering in the water column.
Colour pattern in Characidium generally runs to a series of dark blotches or a broken lateral stripe along a pale-to-olive body, camouflage suited to a life spent perched on stream-bed debris rather than schooling in open water; the reduced posterior swim-bladder chamber typical of the genus gives these fish their characteristic short, darting bursts of movement followed by long stationary pauses, quite unlike the continuous cruising of most tetras.
Habitat
Characidium occidentale is a South American stream fish. Broader biogeographic work on the genus documents a historical dispersal of Characidium out of the Amazon paleobasin into southeastern South America, including the Paraná and Iguaçu drainages — a subtropical part of the continent's freshwater system that runs noticeably cooler than the equatorial Amazon proper. The species-level type locality data available in the research cache used for this article did not resolve a precise named river system, so the entry here is intentionally conservative rather than guessed.
What is well documented for the genus as a whole, and holds for this species, is a preference for flowing, well-oxygenated water over sand, gravel or fine substrate — the streambed habitat that suits a fish adapted to perch and dart rather than swim continuously in the water column.
Feeding
Characidium occidentale is a micro-predator, not a grazer or a flake-eating generalist. Stomach-content work on the better-studied congeners C. fasciatum and C. cf. fasciatum shows a diet dominated by chironomid (non-biting midge) larvae along with other small aquatic and terrestrial insects, consistent with a fish that ambushes prey moving across or drifting near the substrate rather than filter-feeding or grazing algae.
In the aquarium this translates directly: darters of this genus are frequently reported to refuse dry flake outright and need to be offered small live or frozen foods — bloodworm, white or black mosquito larvae, cyclops and daphnia — to feed reliably. Keepers intending to house Characidium occidentale should plan on live/frozen micro-foods as the staple rather than the exception.
Mating
Species-specific courtship behaviour for Characidium occidentale has not been documented in the sources available for this article. What is known for the genus and its close crenuchid relatives points to substrate-associated spawning triggered by soft, acidic water, with fine-leaved plants or similar cover used as a spawning site rather than open water or a excavated pit.
External sexual dimorphism has not been confirmed for this species specifically; keepers of related Characidium generally rely on body shape (gravid females fuller-bodied) and behaviour rather than any reliable colour difference to distinguish the sexes.
Breeding
Characidium species are egg-scatterers over the substrate rather than cave or leaf spawners with parental care — a different mode from their crenuchid relatives Crenuchus and Poecilocharax, which guard small clutches with brief male care. Congeneric darters kept in the hobby have been bred by conditioning pairs in soft, acidic water furnished with fine-leaved plants; reported clutches run to roughly 150 eggs, hatching in 36–48 hours, with no parental care of the eggs or fry documented.
Fry require infusoria in the first days after hatching before graduating to newly hatched brine shrimp nauplii. There is no evidence in the available sources that Characidium occidentale specifically has been bred in captivity, and this account should be read as extrapolated from its genus rather than confirmed for the species — data on this point is genuinely sparse.
In the aquarium
Characidium occidentale is a small, unfussy-looking but demanding-to-feed nano fish, well suited to a biotope-style stream tank rather than a generic community aquarium. It should be given a well-oxygenated setup with some current, a sand or fine-gravel bottom, and pieces of wood or smooth stone it can perch on — this is a fish that spends most of its time resting on the substrate and darting between vantage points, not cruising mid-water, so open swimming space matters less than a varied, current-swept floor.
The single biggest keeping mistake is assuming it will take flake like a typical tetra: Characidium is a genuine micro-predator and often refuses dry foods, so keepers should plan on a steady supply of small live or frozen foods (bloodworm, mosquito larvae, cyclops, daphnia) as the diet's backbone. Water should be kept soft and acidic to neutral, consistent with the streams the genus inhabits; tankmates should be similarly small and peaceful, since this is not a fish built to compete with fast, boisterous schoolers at the feeding surface.
Conservation
Characidium occidentale has no dedicated IUCN Red List assessment and is listed here as Not Evaluated. This is common for narrowly distributed, taxonomically recently split Characidium species — the genus underwent extensive revision in the 1990s, and many of the resulting species have not yet been assessed individually for conservation status.
No species-specific threat data is available in the sources reviewed for this article. In general terms, small-range Neotropical stream fish of this kind are potentially vulnerable to the pressures affecting South American freshwater systems broadly — deforestation-linked sedimentation, water-quality change and damming — but without a formal assessment or documented population data for this species, that remains a general watch item rather than a specific finding.