Taxonomy & naming
Brachypetersius pseudonummifer was described by Max Poll in 1967 in his revision of the dwarf African characins (Révision des Characidae nains africains). The type locality is Avakubi, a town on the Aruwimi River in what is now the Democratic Republic of the Congo, and the holotype is deposited at the Musée Royal de l'Afrique Centrale (MRAC 2919) in Tervuren, Belgium.
The species has a convoluted synonymic history that mirrors broader instability across the small African alestid group. Earlier material was placed under Micralestes altus (Nichols & Griscom, 1917) and later treated as Petersius nummifer (Poll, 1945). Jacques Géry reclassified the fish as Phenacogrammus pseudonummifer in 1996. The genus Brachypetersius itself is small and its limits relative to the larger, more species-rich Alestidae genera have not been fully resolved by molecular work. Eschmeyer's Catalog of Fishes treats Brachypetersius pseudonummifer Poll, 1967 as the valid name, and that usage is followed here.
Morphology
Brachypetersius pseudonummifer is a compact, laterally compressed characin reaching a maximum recorded total length of 3 in, though most individuals encountered in collections appear closer to the type measurement of 3 in TL. Like other small alestids it is slender and streamlined, built for life in open water rather than among dense vegetation.
Detailed published descriptions of external morphology are sparse in the accessible literature. The genus Brachypetersius shares the family's characteristic adipose fin and the deep, forked tail typical of active mid-water feeders. Coloration in preserved material tends to be silvery, as is common across many Alestidae, and live-color accounts are not established in the available sources. Sexual dimorphism has not been documented for this species.
Habitat
The species is recorded from the middle Congo River drainage in the Democratic Republic of the Congo, specifically from the Aruwimi, Lindi-Tshopo, and Lualaba rivers, as well as the turbulent waters near Wagenia Falls. FishBase classifies it as a pelagic freshwater fish, indicating it occupies the open-water column rather than the substrate or dense marginal vegetation.
Water conditions associated with its range centre on soft, slightly acidic to neutral water: pH 6.0–7.0, hardness 0–10 dH, and temperatures between 20 and 81 °F. These parameters reflect the broad character of middle Congo tributaries, which drain ancient basement rock and carry water that is typically soft and poorly buffered. The Wagenia Falls locality is noteworthy — that section of the Congo is one of the most biodiverse freshwater habitats on earth, with strong currents and complex hydraulics supporting a remarkable endemic fish fauna.
Feeding
FishBase assigns Brachypetersius pseudonummifer a mean trophic level of 3.1 ± 0.4, placing it firmly in the insectivore-to-omnivore range. This figure is consistent with what is known for small, open-water Alestidae generally: the fish likely feeds on small aquatic and terrestrial invertebrates taken from the surface film or mid-water, including insect larvae, small crustaceans, and aerial insects.
No stomach-content analyses specific to this species appear in the accessible literature. In the aquarium, small alestids in this size class and trophic guild generally accept a range of foods including live and frozen invertebrates, small floating pellets, and quality flake, with live and frozen fare promoting better condition and more natural foraging behaviour.
Mating
No specific breeding observations for Brachypetersius pseudonummifer have been published in the accessible literature. By analogy with other small open-water Alestidae, spawning is expected to follow the egg-scattering pattern typical of the family: adults scatter non-adhesive or weakly adhesive eggs among fine-leaved aquatic vegetation or into the current, with no parental care and no defense of eggs or fry.
In species of this trophic and ecological guild, male courtship is generally brief and opportunistic, with males pursuing ripe females and spawning events triggered by seasonal water changes — rising temperature, falling pH, or increasing current — associated with the onset of the rainy season in the Congo basin.
Breeding
There are no published captive-breeding accounts for this species in the available sources. Aquarists wishing to attempt reproduction would likely follow protocols developed for comparably sized West African alestids: conditioning adults on a varied live and frozen diet, then providing a spawning tank with fine-leaved plants or synthetic mops, soft and slightly acidic water in the low-to-mid 20s Celsius, and a modest current or rainfall simulation.
Eggs in related species are small and numerous, with adults consuming them readily if left together post-spawning; separation of adults after egg deposition and rearing fry in a clean, well-filtered tank with infusoria and then newly hatched brine shrimp as first foods is the standard approach. Until captive records exist for this species specifically, any breeding details must remain provisional.
In the aquarium
Brachypetersius pseudonummifer is not established in the ornamental trade and does not appear to be kept or bred in captivity with any regularity. Its natural range in the Congo basin's middle reaches, including the biologically rich but geographically remote Wagenia Falls area, means it is seldom collected for export.
For the rare aquarist who encounters specimens, the ecological profile suggests it should be maintained as a schooling species in a tank long enough to accommodate active open-water swimming, with subdued lighting, soft and slightly acidic water (pH 6.0–7.0, dH below 10, temperature 72–79 °F), and tankmates of similar size and temperament. Small alestids can be skittish and startle-prone; a well-planted, dark-substrate setup reduces stress. Given the data scarcity on this species, direct reports from experienced keepers would be a valuable contribution to understanding its captive requirements.
Conservation
Brachypetersius pseudonummifer is assessed as Least Concern (LC) on the IUCN Red List, a status recorded in FishBase as dating from an assessment made on 16 February 2009. The Least Concern designation reflects the species' distribution across multiple river systems in the middle Congo basin — a large and hydrologically complex drainage — rather than any concentrated endemism.
The middle Congo and its major tributaries, including the Aruwimi and Lualaba, remain among the least disturbed major river systems in Africa, and no specific threats to this species have been documented. That said, the Congo basin faces growing pressure from artisanal mining, deforestation in the catchment, and infrastructure development, and the 2009 assessment predates the most recent decade of change in the region. The species' occurrence near Wagenia Falls, a site of exceptional ichthyological interest, means that any impacts on that locality could affect a disproportionate number of endemic or restricted-range fishes simultaneously.