Taxonomy & naming
Characidium hasemani was described by the Austrian ichthyologist Franz Steindachner in 1915, based on material collected in the Rio Branco drainage of what is now the Brazilian state of Roraima. Characidium surumuense, also described by Steindachner the same year, is treated as a synonym of C. hasemani. The species sits in the genus Characidium within the family Crenuchidae — the South American darters and sailfin characins — a group whose members range from the shy, bottom-hugging darters like this one to the much showier, mid-water sailfin characins such as Crenuchus spilurus.
Crenuchidae taxonomy has shifted repeatedly as researchers have worked through a genus with well over 60 nominal species, many of them superficially similar small, brown-barred stream fish, and identification within Characidium remains a genuinely difficult, specialist task resting on fin-ray counts, squamation and colour-pattern detail rather than anything obvious to the aquarist. C. hasemani is known primarily from the Rio Uraricoera and Rio Mucajaí, tributaries of the Rio Branco, with an additional record from the upper Rio Xingu basin in Mato Grosso; whether that Xingu population truly belongs to this species or represents a separate, unresolved lineage is the kind of question that is common across the genus.
Morphology
Characidium hasemani is a small, elongate, cylindrical fish reaching about 1.5–2.5 in in standard length, with the slim, faintly compressed body typical of stream-bed darters rather than the deeper, laterally flattened build of open-water tetras. Like other Characidium, it carries a series of dark blotches or a broken lateral band along the flank over a pale brown-to-olive ground colour, camouflage that suits a fish that spends its life pressed against sand, gravel and pebble substrate.
The most distinctive feature of the genus is behavioural as much as structural: large, muscular pectoral and pelvic fins let the fish prop itself against the current on the bottom, holding position between short, rapid darts rather than swimming continuously in the water column the way a typical tetra does. This darting, perch-and-dash locomotion is the origin of the common name "darter" and is immediately obvious in aquarium specimens.
Habitat
The species is native to fast-flowing streams and rivers of the Rio Branco drainage in Roraima, Brazil — principally the Rio Uraricoera and Rio Mucajaí — with a further record from the upper Rio Xingu basin. Crenuchid darters as a group favour riffles and current-swept stretches with sand, gravel or pebble bottoms and a good supply of dissolved oxygen, habitat that suits a fish built to hold station against flow rather than cruise in slack water.
Water in these Amazonian headwater and tributary streams tends to run warm and soft, from mildly acidic to close to neutral. For the genus generally, published aquarium-keeping guidance cites roughly 72–82 °F and a pH around 5.5–7.5, with some sources narrowing that to a stricter 6.0–6.5 for a closer match to blackwater-influenced tributaries; species-specific field data for C. hasemani itself is sparse, so these ranges should be read as genus-level guidance rather than a precise reading for this exact population.
Feeding
Characidium hasemani is a micro-predator, not a grazer or a flake-eater. In the wild it picks aquatic insect larvae — chironomid midge larvae in particular — along with other small invertebrates from the current and the substrate surface, foraging in short bursts between perches rather than actively cruising for food.
In the aquarium this translates directly: darters often ignore dry flake and prepared foods altogether, and keepers are advised to offer small live or frozen foods that mimic their natural prey — frozen bloodworm, brine shrimp, daphnia and cyclops. A tank that is not offering this kind of food, or where faster mid-water fish out-compete a bottom-hugging darter for it, will leave the fish underfed even in a well-stocked community.
Mating
Species-specific courtship data for Characidium hasemani has not turned up in available sources, and detailed accounts of Characidium spawning behaviour generally are thin even at genus level. What is documented for the genus describes a substrate spawner: a conditioned pair (or a small group held together) becomes active over open sand, gravel or fine-leaved plants as spawning approaches, in water kept soft, warm and mildly acidic to trigger breeding condition.
External sexual dimorphism has not been clearly documented for this species; breeders working with Characidium generally report no dependable external difference between the sexes beyond a fuller body in gravid females, and pairs are more often identified by behaviour than by appearance.
Breeding
Characidium in general are egg-scatterers over the substrate: eggs are laid loose among rocks, sand, gravel or fine-leaved plants such as Java moss rather than being deposited in a nest, cave or bubble structure, and reported clutch sizes for the genus run to roughly 150 eggs, hatching in 30 to 40 hours in warm water. There is no established, well-documented parental care in the genus — this is a scatter-and-abandon strategy, distinct from the cave- and leaf-spawning, brief male-guarding behaviour reported for their crenuchid relatives Crenuchus and Poecilocharax.
Breeding accounts specific to Characidium hasemani were not found in available sources; the description above reflects genus-level hobby and aquaculture reporting for Characidium rather than confirmed observation of this species. A soft, acidic, warm breeding tank with a sand or fine-gravel bottom and dense fine-leaved plant cover for the eggs to fall into is the standard approach used for the genus, and fry are typically raised on infusoria before stepping up to brine shrimp nauplii.
In the aquarium
At under 2.5 in, Characidium hasemani is a genuine nano fish and does not need a large tank by aquarium standards, but it does need current and oxygen: a well-filtered setup with decent flow, a sand or fine-gravel bottom and some open space to dart across suits its natural behaviour far better than a still, densely planted community tank. Darters are reported to be capable jumpers, so a tight-fitting cover is worth taking seriously.
Keep the species in a group — five or more is commonly recommended for Characidium — since a lone darter in open water is a stressed, exposed fish. Feeding is the other point that catches keepers out: because this is a micro-predator that often refuses flake, plan on small live or frozen foods (bloodworm, brine shrimp, daphnia, cyclops) and make sure faster mid-water tankmates are not stripping the water column of food before it reaches the bottom. Choose tankmates carefully — calm, similarly small fish that will not out-compete or harass a shy, bottom-oriented species.
Conservation
Characidium hasemani has not been formally evaluated by the IUCN Red List and currently carries no conservation status (Not Evaluated). Its known range is restricted to a handful of tributaries of the Rio Branco in Roraima, Brazil, plus a possible additional population in the upper Rio Xingu, and no population trend or threat data specific to the species was found in available sources.
As with many small, narrow-range Amazonian stream fish that have never been the subject of a targeted assessment, the absence of a listing reflects a lack of formal evaluation rather than any evidence that the species is secure. General pressures on Amazonian headwater streams — deforestation-linked sedimentation, damming and water-quality change — are the kind of long-term watch items relevant to any fish restricted to a small number of named rivers, though none has been documented specifically against C. hasemani.