Taxonomy & naming
Characidium bolivianum was described by the American ichthyologist Andrew Chase Pearson in 1924, part of his broader early-twentieth-century work cataloguing the fish fauna collected from Bolivian rivers. It sits in the genus Characidium within the family Crenuchidae, a family that Catalog of Fishes (Eschmeyer, California Academy of Sciences) places in its own lineage alongside the sailfin characins (Crenuchus, Poecilocharax) — collectively known as the South American darters and sailfin tetras. Despite the family name's superficial resemblance to "Characidae" (the true tetras), Crenuchidae is a distinct, much smaller family defined by a suite of anatomical characters, including reduced or absent lateral-line scales in many genera and, in Characidium specifically, a body plan built for perching rather than open-water shoaling.
The species epithet bolivianum reflects the country of its type locality, Bolivia, where it remains essentially endemic to the upper Madeira drainage. As with many small, regionally restricted Characidium, taxonomic revisions within the genus are ongoing, and species boundaries in this group have historically been difficult to resolve from external characters alone; this account follows the currently accepted combination, Characidium bolivianum Pearson, 1924.
Morphology
Characidium bolivianum is a slender, torpedo-shaped little fish reaching a maximum recorded size of about 2 in standard length, in line with the genus as a whole, whose species typically top out between roughly 3.5 and 3 in SL. The body is elongate and only lightly compressed, with a pointed snout, a subterminal mouth suited to picking food from the substrate, and a caudal fin that is often marked with dark bars or blotches — a common Characidium field mark, though exact patterning varies by species and population.
Unlike the deep-bodied, laterally flattened silhouette of piranhas and pacus elsewhere in this atlas, darters like C. bolivianum have paired fins — particularly the pectorals — positioned and proportioned to let the fish prop itself against the substrate and hold station in flowing water, a posture that gives the genus its common name. Colouration is generally subdued: silvery to olive-brown flanks, a dark lateral stripe or series of blotches, and no bright ornamentation, consistent with camouflage on stony or sandy streambeds.
Habitat
This species is known from the upper Madeira River basin in Bolivia, specifically the Mamoré and Beni river systems, where it inhabits flowing water over gravel, sand and rocky substrate rather than the still, vegetated margins favoured by many characins. Darters in this genus are characteristically found in streams with a perceptible current, where they perch on the bottom facing into the flow and make short, quick dashes — hence "darter" — to intercept food or reposition, rather than swimming continuously in open water.
Detailed water-chemistry data specific to C. bolivianum's native streams is sparse in the available record; the genus as a whole is associated with clear to lightly stained, well-oxygenated, moderate-flow habitats typical of Amazonian and sub-Andean tributary systems, and this account does not assume extreme blackwater conditions without direct evidence.
Feeding
Characidium bolivianum is a micro-predator, not a grazer or a flake-eater by inclination. In the wild it takes small aquatic invertebrates — chironomid (midge) larvae and other insect larvae are a documented staple for the genus — picked from the current or the substrate in short, targeted strikes rather than continuous foraging. This sit-and-wait, perch-and-dart feeding style mirrors its resting posture: the fish holds a position on a rock or the gravel bed and moves only when a food item drifts into range.
In the aquarium, Characidium species are reported to strongly prefer small live and frozen foods — bloodworm, brine shrimp, Daphnia and Cyclops — over dry prepared foods, and keepers should expect to offer these regularly rather than relying on flake, which many individuals largely ignore. A varied diet of appropriately small live/frozen fare is the safest way to keep this species in good condition long-term.
Mating
Specific courtship behaviour for Characidium bolivianum has not been documented in the available literature, and this account does not invent detail beyond what is known for the genus. Within Crenuchidae more broadly, the darters (Characidium) are understood to be egg-scatterers rather than nest-builders or mouthbrooders, in contrast to their sailfin-characin relatives Crenuchus and Poecilocharax, which are cave- or leaf-spawners with brief male care of the clutch.
No reliable external sexual dimorphism is documented for this species in the sources reviewed; as with many small Characidium, well-conditioned, gravid females may appear fuller-bodied than males, but this is not confirmed specifically for C. bolivianum and should be treated as provisional.
Breeding
Characidium bolivianum has not been documented as bred in the home aquarium in the sources available for this account, and the genus's reproductive biology is comparatively poorly recorded compared with showier aquarium characins. Based on the broader pattern for Characidium as substrate-associated darters, spawning is presumed to follow the genus norm of scattering eggs over the substrate or fine debris rather than guarding a nest, with no parental care — but this has not been specifically confirmed for C. bolivianum, and prospective breeders should treat it as an open question rather than a settled fact.
Given the sparse data, this species is best approached as one that has not been established as a captive-bred aquarium fish; any wild-caught individuals in the trade almost certainly derive from collection within its native Bolivian range rather than commercial or hobby breeding.
In the aquarium
Characidium bolivianum is a genuinely nano-scale fish — under 2.5 in — well suited to a modest aquarium provided the setup reflects its stream-dwelling habits rather than a generic community-tank template. A tank with a sand or fine-gravel bottom, smooth stones, and enough water movement to create a perceptible current gives this darter the perching and holding-station behaviour it shows in the wild; strong oxygenation and clean, well-filtered water matter more to it than décor. Because it is a bottom-perching sit-and-wait feeder rather than a mid-water shoaler, it should not be judged against typical tetra behaviour — a healthy individual spends much of its time stationary on the substrate, darting only to feed or reposition.
Diet requires attention: offer small live or frozen foods (bloodworm, brine shrimp, Daphnia, Cyclops) regularly, since this species and its genus-mates are widely reported to largely ignore flake. Tankmates should be other small, peaceable fish that will not out-compete a bottom-hugging micro-predator at feeding time. As an uncommon species with a narrow native range and no established captive-breeding record, availability in the trade is likely to be sporadic and wild-collected.
Conservation
Characidium bolivianum is assessed by the IUCN Red List as Least Concern, an assessment reflecting FishBase's citation of a 2014 evaluation. This status is consistent with a species that, while narrowly endemic to the upper Madeira basin (the Mamoré and Beni river systems in Bolivia), does not appear to face documented, elevated extinction pressure within that range.
As with many small, regionally restricted Characidium, detailed population trend data and threat assessments are limited, and the Least Concern rating should be read as reflecting a lack of documented decline rather than an exhaustive study of the species. Broader pressures on Amazonian and sub-Andean headwater systems — deforestation, sedimentation and hydrological change from damming — are general long-term watch items for fish endemic to this region, even though none is specifically documented as currently affecting this species.