Taxonomy & naming
Neolebias gracilis was described by the German ichthyologist Hermann Matthes in 1964. It sits in the family Distichodontidae — the African characin family that also contains the much larger, plant-grazing Distichodus — within the small genus Neolebias, a group of miniature African characins distributed across Congo Basin and West African river systems. Neolebias itself sits at the opposite end of the family's size range from its well-known relatives: where Distichodus species can reach half a metre and shred aquarium plants, Neolebias are diminutive, delicate nano fish.
The species is documented almost entirely through its FishBase record and passing mentions in genus-level accounts of its better-known relatives, Neolebias ansorgii and N. trilineatus. There is no evidence in the available sources of taxonomic dispute or a historical name change for N. gracilis; it is treated here under the name assigned by Matthes.
Morphology
Neolebias gracilis is small even by the standards of its own genus, reaching a documented maximum of just 1.5 in standard length. The species epithet "gracilis" — Latin for slender or graceful — points to a narrow, elongated body shape consistent with its close relatives in the genus, though detailed published descriptions of its colouration and fin proportions were not found in the available sources. Data on this species' external appearance is sparse, and this article does not speculate beyond what is documented.
Habitat
Neolebias gracilis is known only from the Ruki River, a tributary of the Congo River in the Democratic Republic of the Congo — a single-drainage endemic with no other confirmed range. Freshwater sources describe the species as pelagic within its river habitat. Family-level data for Neolebias places typical conditions at roughly 72–82 °F and pH 6.0–8.0, consistent with the generally warm water of the Congo Basin, though species-specific measurements for N. gracilis were not found in the cache and should not be assumed identical to the genus average.
As a Congo Basin endemic confined to a single tributary, N. gracilis illustrates how narrow the range of some African characins can be, in sharp contrast to wide-ranging, multi-country relatives elsewhere in the family.
Feeding
No diet study specific to Neolebias gracilis was found in the available sources. Closely related genus members — N. ansorgii, N. trilineatus and N. unifasciatus — are documented as carnivorous to omnivorous micropredators, taking small invertebrates such as mosquito larvae and daphnia. Given the shared genus and comparable small body size, N. gracilis most plausibly follows the same pattern, but this is an inference from relatives rather than a confirmed observation for this species, and it should be read as such.
Mating
No courtship or spawning behaviour has been documented for Neolebias gracilis specifically. Related Neolebias species are described as very shy fish that require a quiet, densely planted species setup, and it is reasonable to expect similar sensitivity in N. gracilis given its comparable size and habitat, but no direct account of its mating behaviour was found in the available sources.
Breeding
Neolebias gracilis has no documented breeding history in the aquarium hobby, and the available sources note that the species is presumed unavailable in the aquarium trade entirely. Closely related Neolebias species (including N. trilineatus) are egg-scatterers whose breeding is rarely achieved in captivity even when the fish are available and kept in a dedicated species setup. Given the total absence of trade or husbandry records for N. gracilis itself, this article reports no confirmed breeding mode for the species in aquaria and does not extrapolate a specific spawning account beyond noting the genus is generally egg-scattering and difficult to breed at home.
In the aquarium
There is no evidence that Neolebias gracilis has ever been available in the aquarium trade. It is a genuinely obscure, single-river Congo Basin endemic, and the available sources describe it as presumed absent from hobby circulation rather than simply uncommon. Prospective keepers should not expect to find this species offered for sale; any care guidance would have to be extrapolated from its shy, nano-sized relatives (Neolebias ansorgii, N. trilineatus) and should be treated as speculative rather than established practice for this species specifically.
Conservation
Neolebias gracilis is assessed by the IUCN Red List as Data Deficient, reflecting insufficient information for a definitive conservation assessment rather than any confirmed stability or decline. Confined to a single tributary of the Congo River, the species has not been the subject of the kind of survey work that would establish population trends or specific threats. As with many narrow-range Congo Basin endemics, the honest conservation picture here is one of genuine data scarcity rather than documented safety.