Taxonomy & naming
Characidium roesseli was described by the French-based ichthyologist Jacques Géry in 1965. It belongs to the family Crenuchidae — the South American darters, which also includes the cave/leaf-spawning sailfin characins Crenuchus and Poecilocharax — within the order Characiformes. The genus Characidium is the largest in the family, comprising dozens of small, elongate, bottom-perching species distributed across South America, and the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the reference point for keeping the roster of valid names straight, since the genus has a long history of species being described, synonymised and re-split.
Characidium roesseli shares its home ecoregion, the Ucayali-Urubamba piedmont of Peru, with at least seven other congeners — C. etheostoma, C. geryi, C. pteroides, C. purpuratum, C. steindachneri, C. sterbai and C. zebra — a level of local species-richness typical of the genus, where several visually similar darters routinely co-occur in the same stream reach. This account keeps the genus as Characidium; there is no indication in the sources reviewed that roesseli has been moved to another genus.
Morphology
Characidium roesseli has the classic darter-characin build: a slim, elongate, faintly compressed body, a flattened underside and a reduced or absent swim bladder that keeps the fish tethered to the substrate rather than floating in the water column. FishBase records a maximum size of roughly 3 in, which is typical for the genus — these are small, unshowy fish built for grip and burst speed rather than open-water cruising.
The pectoral and pelvic fins are enlarged and used almost like limbs, propping the fish up off the bottom between darts; this benthic, fin-braced posture combined with the short explosive movement between resting spots is the single most recognisable behavioural and morphological signature of Characidium. Detailed colour-pattern and fin-count data specific to roesseli were not available in the sources reviewed for this article, so no species-specific description of pattern or fin formula is given here beyond the genus-typical build.
Habitat
The species is endemic to the Ucayali-Urubamba piedmont of Peru, part of the upper Rio Ucayali basin that ultimately feeds the Amazon. It occupies fast-flowing, highly-oxygenated lowland and piedmont streams, where it perches directly on the substrate in current rather than schooling in open water — a niche shared with its many Characidium congeners in the same drainage.
Specific water-chemistry data (pH, hardness, temperature) for the wild population have not been located; hobby accounts for Characidium as a genus describe soft to moderately hard, mildly acidic to neutral water in the roughly pH 5.5–7.5 range with strong oxygenation and current, which is consistent with a fast piedmont-stream specialist, but this should be read as genus-level guidance rather than a confirmed reading for roesseli specifically.
Feeding
Characidium roesseli is a micro-predator, not a grazer or generalist scavenger. In the wild it almost certainly takes small aquatic invertebrates — insect larvae, micro-crustaceans and similar benthic prey — picked off the streambed and from the current between its characteristic short dashes, consistent with genus-wide accounts of Characidium diet.
In the aquarium, darter characins of this genus are consistently reported to refuse dry flake and often ignore standard pellets, doing far better on small live or frozen foods such as bloodworm, mosquito larvae, daphnia and cyclops. A keeper should plan on sourcing appropriately sized live or frozen micro-foods rather than expecting the fish to adapt to a flake-based diet.
Mating
No species-specific courtship description for Characidium roesseli was found in the sources reviewed. Within the family Crenuchidae, breeding behaviour differs sharply between lineages: Crenuchus spilurus and Poecilocharax species are cave or leaf spawners with a brief period of male care of the clutch, while Characidium species — including roesseli's genus-mates that have been bred in captivity — are reported as egg-scatterers, with males showing brief territorial behaviour around a chosen spawning site rather than sustained courtship display or pairing ritual.
Given the absence of documented sexual dimorphism for this species, hobbyists conditioning a group for breeding typically rely on females appearing fuller-bodied when gravid rather than any reliable external marker.
Breeding
Characidium as a genus has been bred in captivity — unlike many darters — which sets it apart from its cave-spawning Crenuchus and Poecilocharax relatives in the same family. Reported spawning conditions are soft, acidic water (roughly pH 6.0–7.0) over fine-leaved plants such as Java moss or artificial spawning mops. The mode is egg-scattering: eggs are deposited among the plant mass rather than in a defended nest or cave, and there is no documented parental care of eggs or fry in Characidium — the brood-guarding behaviour seen in the family belongs to Crenuchus and Poecilocharax, not this genus.
No breeding account specific to Characidium roesseli itself was located; the description above reflects genus-level captive-breeding reports and should be read as a plausible starting point rather than a confirmed protocol for this species. Fry, once free-swimming, would be expected to need infusoria-sized first foods given the tiny size of Characidium eggs and larvae generally.
In the aquarium
Characidium roesseli is a small, current-loving nano species best suited to a dedicated setup rather than a generic community tank. Darter characins are reported to do well in nano aquariums from around 10 US gal upward, kept in a proper group of a dozen or more, since these are naturally gregarious, streambed-dwelling fish that settle better with company of their own kind. Strong filtration and good current are important — these are fast-water specialists that also do best with excellent oxygenation — and a tight-fitting lid is essential, since Characidium species are noted as expert jumpers.
Because the species refuses flake and often ignores standard pellets, keepers should be ready to offer small live or frozen foods regularly rather than relying on a set-and-forget dry diet. As a genuinely tiny predator of small invertebrates, it should only be kept with similarly small, non-aggressive tankmates that will not outcompete it at feeding time or see it as prey.
Conservation
Characidium roesseli has not been evaluated by the IUCN Red List; its status is Not Evaluated. As a narrow-range endemic restricted to the Ucayali-Urubamba piedmont of Peru, it shares a stream system with several closely related Characidium species, and — as is typical for small, obscure, single-region characins — there is essentially no published population or threat data available.
No specific conservation concerns or pressures for this species were identified in the sources reviewed. Given its restricted piedmont-stream range, general watch items for the region would include water-quality change and habitat disturbance associated with land use in the upper Ucayali basin, though this is inference from the region rather than a documented finding specific to roesseli.