Taxonomy & naming
Characidium heinianum was described by Zarske & Géry in 2001. It belongs to the genus Characidium within the family Crenuchidae — the South American darters — a family that also includes the more aquarium-familiar sailfin characins Crenuchus and Poecilocharax. Characidium is by far the largest genus in the family, with dozens of small, superficially similar, bottom-associated species spread across South American river systems, many of them, like this one, described from a single river basin and rarely seen outside the ichthyological literature.
The genus name reflects the group's darting, perch-and-dash behaviour rather than any showy feature, and species-level identification within Characidium is difficult and relies on subtle differences in scale counts, fin-ray counts and colour pattern rather than anything visible to a casual observer. As with many recently described, narrow-range Characidium, C. heinianum has not been the subject of a broader taxonomic revision since its description, and data on it remain sparse.
Morphology
Characidium heinianum is a small, slender, cylindrical fish reaching a maximum recorded length of about 2 in standard length (SL) — a true nano species. Like other Characidium, its body plan is built for resting on the bottom: reduced swim-bladder function and a body shape that lets it sit propped on its paired fins rather than hovering in the water column, punctuated by short, quick darts when it moves.
Detailed published descriptions of colour pattern and fin morphology for this species are limited, but the genus as a whole typically shows a series of dark bars or blotches along the flank, a pale to translucent body, and a torpedo-like head-to-tail taper typical of stream-bed darters. Data on this species specifically is sparse beyond the original description and FishBase's summary record.
Habitat
The species is known from the upper Mamoré River basin in Bolivia, part of the greater Amazon drainage in the eastern Andean foothills and lowlands. Like other stream-dwelling Characidium, it is associated with flowing water over sand, gravel or fine sediment rather than still lake-like habitats, where its resting-and-darting behaviour lets it hold position against current and dash for cover among submerged debris or vegetation when disturbed.
Published water-chemistry data specific to C. heinianum is not available; the genus in general favours clear-to-tea-coloured, moderately warm South American stream water. Given the sparse record for this exact species, keepers and researchers should treat any specific pH or hardness figure for this species as an extrapolation from the wider genus rather than a documented fact.
Feeding
Characidium heinianum is a micro-predator, consistent with the rest of its genus: darter characins feed mainly on small aquatic invertebrates such as insect larvae (notably chironomid midge larvae), along with other tiny live prey items taken from the substrate and drifting in the current. They are not generalist flake-eaters and often ignore dry foods entirely.
In captivity, the documented approach for Characidium is to offer small live or frozen foods — bloodworm, Daphnia, Cyclops and similar micro-fare — matched to the fish's small mouth size. No feeding data specific to C. heinianum has been published, but this pattern is well established across the genus and the wider darter-characin group.
Mating
No courtship behaviour specific to Characidium heinianum has been documented in the available literature. Within Crenuchidae more broadly, breeding biology differs sharply between the sailfin characins (Crenuchus, Poecilocharax), which use cave- or leaf-spawning with brief male care, and the darters of Characidium, which are understood to be substrate egg-scatterers with no nest or cave association. Data on courtship and sexing for this particular species is sparse; it has essentially no aquarium-breeding record to draw on.
Breeding
Characidium as a genus are egg-scatterers over the substrate, without the cave-spawning and male parental care seen in their sailfin cousins Crenuchus and Poecilocharax. Practical hobby accounts of the wider darter-characin group describe captive breeding occurring in soft, acidic water furnished with fine-leaved plants, where eggs are scattered and receive no parental care once laid.
No breeding account specific to Characidium heinianum has been published, and the species has essentially no documented history in the aquarium hobby. Given how rarely it is even kept, any breeding report for this exact species should be treated as anecdotal rather than established fact; the pattern above reflects the genus, not confirmed observation of C. heinianum itself.
In the aquarium
Characidium heinianum is a genuine nano species — at a maximum of about 2 in it needs only a small footprint, but darter characins are stream-bottom fish and do best with some current, a sand or fine-gravel substrate, and cover from wood, leaf litter or fine-leaved plants rather than a bare, still tank. It is essentially unknown in the aquarium trade; general Characidium care guidance (moderate temperature around 72–79 °F, slightly acidic to neutral pH around 6.0–7.5) is the best available proxy, but no dedicated care record exists for this species.
As a micro-predator, it needs small live or frozen foods rather than flake, and tankmates should be similarly small and peaceful — the fish itself is not aggressive, but its tiny size makes it vulnerable to anything that can fit it in its mouth. Given how rarely this species is documented in captivity, prospective keepers should treat any specific husbandry claim beyond size, diet type and general water warmth as unverified.
Conservation
The IUCN Red List assesses Characidium heinianum as Data Deficient (assessment dated 2014), reflecting how little is known about its population size, trend, range extent or the pressures it faces within the upper Mamoré basin. This is a common status for narrow-range, recently described Characidium species that have not been the subject of dedicated field surveys since their description.
No specific threats to the species have been documented in the available sources. As with many small, undocumented Neotropical stream fish, the general pressures on Amazonian headwater habitats — deforestation, sedimentation and water-quality change — are plausible long-term concerns, but there is no cited evidence tying them specifically to this species' status.