Taxonomy & naming
Characidium longum was described in 2006 by Donald C. Taphorn, Carlos G. Montaña and Paulo Buckup, part of a wave of Characidium descriptions from South American authors in the mid-2000s that substantially expanded the known diversity of the genus. It belongs to the family Crenuchidae — the South American darters and sailfin characins — within the order Characiformes; Crenuchidae also contains the more famous sailfin characins Crenuchus and Poecilocharax, but Characidium itself is the darter lineage: small, elongate, bottom-associated stream fish rather than the showy, finnage-display nano fish their family-mates are known for.
The genus Characidium is taxonomically dense, with dozens of species distinguished mainly by subtle differences in scale counts, fin-ray counts, colour pattern and body proportions, and species-level identification in the field or in the trade is genuinely difficult without a specialist reference. No secondary literature or aggregator record (FishBase, GBIF) was located confirming a common name for C. longum, so none is listed here; it is best referred to by its scientific name.
Morphology
Characidium longum has the typical darter-tetra body plan shared across the genus: an elongate, only mildly compressed body, a fairly flat ventral profile, and low-set paired fins that let the fish prop itself against the substrate rather than hold station in open water like a typical tetra. Coloration across Characidium generally runs to a base of silvery-tan or olive with a dark lateral stripe or series of blotches and fine speckling on the flanks, though the exact pattern for C. longum specifically was not confirmed in the sources reviewed.
No confirmed maximum length is available for this species; across the genus, adult sizes typically run from roughly 1.5–4.5 in depending on the species, with most aquarium-relevant Characidium at the smaller end of that range. Given the lack of a specific figure, prospective keepers should plan for a small, slender fish in the low single digits of centimetres rather than assume genus-average size.
Habitat
Characidium as a genus are rheophilic stream fish — adapted to flowing water over rocky or sandy substrates, typically in clear to lightly tannin-stained South American streams with good dissolved oxygen. They are specialised for a benthic, current-exposed lifestyle rather than the still floodplain and blackwater habitats favoured by many other small characins, and they are frequently found alongside other darters and small catfish on riffle and run habitat.
Specific locality data for Characidium longum was not confirmed in the available sources; the describing authors' geographic focus points to a South American stream system, most likely within Venezuela or Colombia, but the exact river drainage and type locality are not confirmed here and should not be assumed. Water conditions for the genus generally run soft and mildly acidic to neutral, in the range of roughly pH 6.5–7.0, at moderate tropical-to-subtropical temperatures.
Feeding
Characidium are micro-predators, not grazers or flake-eaters by preference. In the wild they pick small aquatic invertebrates — chironomid (midge) larvae, other insect larvae, small crustaceans and similar micro-fauna — from the stream bed and drift, darting forward from a resting position to snatch prey rather than cruising to forage.
In the aquarium this translates directly: Characidium do best on small live or frozen foods such as bloodworm, mosquito larvae, daphnia and cyclops, and many individuals will ignore dry flake food entirely or accept it only reluctantly once well established. Feeders should offer a steady supply of small live or frozen fare rather than relying on flake or pellet as a primary diet.
Mating
Detailed courtship behaviour specific to Characidium longum has not been documented in the sources reviewed. Across the genus, spawning is described as substrate-associated, with pairs or loosely aggregated individuals scattering eggs over sand, gravel or fine debris on the stream bed rather than forming a defended nest site of the kind seen in true piranhas.
No reliable external sexual dimorphism is documented for this species; data is sparse enough that keepers should not expect an obvious visual difference between the sexes without direct observation of gravid females or spawning behaviour.
Breeding
Characidium as a genus are generally egg-scatterers over the substrate, without the sustained parental care seen in their sailfin-characin relatives Crenuchus and Poecilocharax, which are cave or leaf spawners with brief male tending of the clutch. Some Characidium accounts note occasional brief male attendance near a scatter site, but this is not well documented at species level and should not be assumed as a reliable behaviour for C. longum specifically.
There is no record in the sources consulted of Characidium longum being bred in the aquarium hobby; data is sparse, and this species should be considered essentially unbred in captivity at this time rather than assumed to follow a specific, confirmed protocol.
In the aquarium
Characidium longum is best suited to a quiet, well-filtered nano or species tank rather than a boisterous community setup — the genus is sensitive to poor water quality and does best with gentle-to-moderate current and high dissolved oxygen, mimicking a stream riffle rather than still water. A tank with a sand or fine-gravel bottom, some smooth rock or driftwood for cover, and subdued rather than bright lighting suits the fish's natural bottom-perching, dart-and-rest behaviour.
Because it is a micro-predator that often refuses flake, keepers should be prepared to offer small live or frozen foods regularly rather than rely on prepared dry food alone. Tankmates should be limited to other small, peaceful species that will not outcompete it at feeding time or bully it off the substrate; given the lack of confirmed size data, err on the side of treating this as a small, delicate nano fish rather than a robust generalist.
Conservation
Characidium longum has not been formally assessed by the IUCN Red List; its status is Not Evaluated. This is common among micro-endemic Crenuchidae darters, many of which are known only from their original description and a handful of subsequent records, with no dedicated conservation assessment carried out.
Without confirmed distribution or population data, no specific conservation pressures can be attached to this species here. In general, South American stream darters restricted to particular drainages can be vulnerable to habitat degradation, damming and water-quality change even where no formal assessment exists, but this should be read as a general genus-level caution rather than a documented finding for C. longum specifically.