Taxonomy & naming
Nannocharax uniocellatus was described by the French ichthyologist Jacques Pellegrin in 1926. It sits in the genus Nannocharax, a group of small, slender, bottom-associated African characins within the family Distichodontidae — the same family that contains the much larger, deep-bodied Distichodus species and the grazing citharinids are placed in the closely related family Citharinidae. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the governing authority for the valid name and combination; this article keeps the genus as Nannocharax per the cache and current usage.
The genus name combines Greek nanos ("dwarf") with charax, a name applied to several characin genera, reflecting the diminutive size that separates Nannocharax from its bulkier distichodontid cousins. The species epithet uniocellatus is Latin for "single-eyed [spot]", almost certainly referring to a single ocellus (eyespot) marking somewhere on the body or fins — a common naming pattern in small characins where an eyespot is a distinguishing field mark, though the cache does not describe the mark's exact position in detail.
Morphology
This is a genuinely tiny fish: FishBase records a maximum length of just 1 in standard length for males and unsexed individuals, with a total length (including the caudal fin) of about 1.5 in. That places Nannocharax uniocellatus among the smallest described members of its genus, which itself consists of small, elongate, torpedo-shaped fish quite unlike the deep-bodied silhouette most people associate with "piranha-family" characins.
Data on fine details of colouration and fin shape are sparse in the available sources beyond the species name itself, which points to a single ocellus marking as a likely identifying feature. Like other Nannocharax, it is a pelagic freshwater species rather than a strict bottom-dweller, though relatives in the genus are typically slim-bodied fish adapted to holding position in current.
Habitat
Nannocharax uniocellatus is restricted to the Congo River basin, with FishBase recording it from the Kinsuka rapids and the middle Congo River, as well as the Lualaba River, spanning the Republic of Congo and the Democratic Republic of Congo. This places it firmly within the fast-flowing, rapid-associated reaches of the Congo system rather than its quiet floodplain lakes or forest streams.
As with other Congo basin distichodontids, water here is generally warm and can run soft to moderately mineralised depending on the exact reach, though the cache does not report specific temperature or pH readings for this species. Given its documented occurrence in the Kinsuka rapids, well-oxygenated, moving water is likely an important habitat feature, consistent with the general pattern for small rheophilic (current-loving) Congo basin characins.
Feeding
Specific diet data for Nannocharax uniocellatus itself is sparse in the available sources. Related Nannocharax species are documented as carnivorous in nature, taking small live foods such as tiny invertebrates and other minute prey items suited to their small mouths and diminutive body size — a very different feeding strategy from the plant-grazing habits of their larger distichodontid relatives such as Distichodus.
Given the extreme adult size of this species (under 1 in SL), any captive diet would need to consist of correspondingly tiny live or frozen foods — infusoria-scale to micro-crustacean prey — though no aquarium feeding accounts specific to this species were found in the cache. Data sparse; keepers should not assume this species shares the herbivorous leanings of bigger Distichodontidae.
Mating
No courtship or pairing behaviour specific to Nannocharax uniocellatus was found in the available sources. The species is rare in the aquarium trade and appears to be little studied outside of its taxonomic description and basic distribution records, so nothing reliable can be said here beyond noting that data is sparse. External sexual dimorphism is likewise undocumented in the cache.
Breeding
There is no documented breeding record for Nannocharax uniocellatus in aquaria or in the wild in the available sources. As with the broader Distichodontidae and Citharinidae, African characins in this family are typically egg-scatterers with no parental care when breeding has been observed at all, and species of Distichodus in particular are notoriously difficult to spawn in a home tank. Given how rarely Nannocharax uniocellatus appears in the trade at all, it is reasonable to assume it has essentially never been bred in captivity, but this should be read as an honest absence of data rather than a confirmed breeding mode — nothing here should be taken as established fact.
In the aquarium
Nannocharax uniocellatus is rare to the point of being essentially unknown in the aquarium trade, and the cache turned up no dedicated care accounts for the species. What can be said with confidence is its size: at under 1 in standard length, this is a true nano fish, dramatically smaller than the piranhas, pacus and silver dollars that dominate this section of the atlas, and any hypothetical care approach would need to scale accordingly — a small, gently filtered, well-oxygenated setup rather than anything resembling the large tanks demanded by its more famous relatives.
Because almost nothing specific has been published on husbandry, keepers should treat this species as effectively unavailable and undocumented rather than assume it can be kept like other small characins. Given its rapids-associated wild habitat, good water movement and oxygenation would be a reasonable starting assumption if the species were ever encountered in the trade, but this is inference, not documented fact.
Conservation
The IUCN Red List assesses Nannocharax uniocellatus as Least Concern, with an assessment dated 16 February 2009. This reflects the general pattern for small, undemanding Congo basin characins that are not targeted by fisheries or heavily impacted by habitat pressures documented to date, though as a narrow-range Congo basin endemic it would be worth revisiting if development or hydropower pressure increases in the Kinsuka rapids / middle Congo / Lualaba area.
The species is not part of the aquarium trade in any meaningful volume, so collection pressure is not a relevant conservation factor here. Broader threats facing the Congo basin — regional hydropower development, sedimentation and water-quality change — are the general long-term watch items for small-range rapids specialists like this one, even though the species itself is not currently classified as threatened.