Taxonomy & naming
Characidium duplicatum was described in 2021 by Armbruster, Lujan and Bloom as part of a broader revision of Guiana Shield Characidium, the same body of work that redescribed the related Characidium crandellii. It belongs to the family Crenuchidae — the South American darters and their relatives, including the sailfin characins Crenuchus and Poecilocharax — within the order Characiformes. The genus Characidium is large and taxonomically difficult, with many superficially similar species distinguished chiefly by fin-ray counts, scale counts and subtle colour-pattern details rather than obvious external features.
The species epithet duplicatum is Latin for "doubled" or "duplicated," a neuter adjective referring to a diagnostic feature of two unbranched anal-fin rays, a character used to separate it from closely related Characidium species in the same Guiana Shield river system. As with much of the genus, Characidium duplicatum is effectively unknown in the aquarium trade and is not marketed under any common name; the handful of vernacular names used for the genus generally apply to a small number of more widely traded species such as C. fasciatum, not to this species specifically.
Morphology
Characidium duplicatum is a small, slender, cylindrical fish typical of the genus, built for perching and darting along the substrate rather than for sustained open-water swimming. The type series is modest in size: the holotype measures 1.5 in standard length, with a paratype series ranging roughly 1 in SL, placing this species firmly among the smaller Crenuchidae darters. Like other Characidium, it has enlarged, muscular paired fins (pectoral and pelvic) that it uses to prop itself against the bottom between short darting movements, a distinctive locomotor style that gives the genus its "darter" common name.
Diagnostic identification rests on fine anatomical detail rather than obvious colour or shape differences from its relatives — in this case the paired, unbranched anal-fin rays referenced in the species name. Data are sparse on live colouration and pattern beyond the original description; as with many recently described stream darters, a full account of colour in life has not been widely published.
Habitat
Characidium duplicatum is known from the Kuribrong River at Grass Shoals, in the Potaro-Siparuni Region of Guyana, within the Essequibo River drainage — part of the Guiana Shield, an ancient upland region with numerous short, clear, fast-flowing rivers draining crystalline bedrock. Within that drainage the species appears to be distributed more broadly through the Essequibo basin, though it is described as rarely encountered, consistent with the generally low detection rates of small, cryptically coloured, bottom-dwelling darters in fast Guiana Shield streams.
As a bottom-dweller in Guiana Shield rivers, Characidium duplicatum most likely favours the clear, well-oxygenated, moderate-flow habitats typical of the region — rocky or sandy stream beds with leaf litter and current — though specific published water-chemistry data for this species are not available. Data sparse beyond the type locality description.
Feeding
No diet study has been published specifically for Characidium duplicatum, but the genus as a whole is well documented as a micro-predator of small invertebrates. Stomach-content studies of related Characidium species consistently show a diet dominated by aquatic insect larvae, particularly chironomid midge larvae, along with other small insects and micro-crustaceans picked from the stream bed and drifting current — not detritus, algae or plant matter.
In captivity, genus members readily take small frozen and live foods such as bloodworm, brine shrimp, Daphnia and Cyclops, and they generally ignore dry flake food or take it only reluctantly. A keeper attempting to house this species should plan on small, meaty foods offered near the substrate rather than surface or column feeding.
Mating
No courtship or spawning behaviour has been documented specifically for Characidium duplicatum. Within the family Crenuchidae, the sailfin characins (Crenuchus, Poecilocharax) are known cave- or leaf-spawners with brief male care of the clutch, but true darters in the genus Characidium are understood more generally as substrate egg-scatterers, without the male-guarded cave-spawning behaviour of their sailfin relatives. No sex-specific colour or fin differences have been reported for this species; data sparse.
Breeding
There is no published breeding account, in the wild or in captivity, for Characidium duplicatum specifically. Aquarium literature on the wider genus Characidium describes egg-scattering over fine-leaved plants or spawning mops in soft, acidic water, with no parental care of eggs or fry once scattering is complete — distinct from the male-tended cave spawns documented in the sailfin genera Crenuchus and Poecilocharax within the same family. Given the complete absence of species-specific data, any breeding attempt with C. duplicatum would need to proceed cautiously from the general genus pattern rather than a confirmed protocol for this species.
In the aquarium
Characidium duplicatum has essentially no aquarium history and is not known to be available in the trade; any care guidance here is extrapolated cautiously from the wider genus rather than from direct experience with this species. Characidium generally are modest-sized, social bottom-dwellers best kept in a group, in a tank with a soft substrate, moderate current and cover rather than as a solitary novelty. As micro-predators, they often ignore dry flake and need small live or frozen foods — bloodworm, brine shrimp, Daphnia, Cyclops — offered close to the bottom.
Related genus members such as C. fasciatum are kept in the range of roughly 64–75 °F with nitrate held below 50 mg/L and regular partial water changes, values that may offer a reasonable starting point given this species' Guiana Shield origin, but they are not a substitute for specific data on C. duplicatum. Given how recently the species was described and how little is documented about it in captivity, this is not a beginner's fish and not one to source without confidence in its provenance.
Conservation
Characidium duplicatum has not been formally assessed by the IUCN Red List and carries no conservation status; as a species described only in 2021 from a single Guiana Shield drainage, it falls into the common pattern for recently described stream fishes of remaining data-deficient by default rather than actively evaluated. The Essequibo basin's Guiana Shield headwaters are, on balance, less heavily impacted by agriculture and urbanisation than many other South American river systems, though mining activity (including gold mining) is a known regional pressure on water quality in parts of Guyana's interior.
Given the rarity noted in the original description and the narrow known range, the species would benefit from basic distributional and population survey work; in the absence of an assessment, no specific conservation claim beyond "not evaluated" can responsibly be made.