Taxonomy & naming
Phenacogrammus flexus was formally described in the American Museum Novitates in 2021 by Melanie L. J. Stiassny, Solomon E. Alter, Juvénal Monsembula Iyaba, and Tobie Liyandja, in a paper treating two new Phenacogrammus species from the Ndzaa River system of the Democratic Republic of Congo. The valid name as recognised by the Catalog of Fishes is Phenacogrammus flexus Stiassny, Alter, Monsembula Iyaba & Liyandja, 2021. The type locality is a precise point on the Ndzaa River (02°58′25.08″S, 018°7′55.26″E), in Mai-Ndombe Province.
Phenacogrammus is a genus within the family Alestidae, the African tetras, which encompasses the majority of characiform fishes native to sub-Saharan Africa. The genus has long been in taxonomic flux and its limits remain a subject of ongoing revision; several species formerly placed in allied genera have been transferred to or from Phenacogrammus as molecular and morphological work has progressed. P. flexus is distinguished from congeners primarily by its reduced supraneural bone count (6 versus 7) and by the conspicuous midline pigmentation pattern described below — a combination not seen in any previously described species of the genus.
Morphology
Phenacogrammus flexus is a small, slender alestid tetra, with the maximum standard length recorded at the type locality measuring 2 in. The body form is typical for small Phenacogrammus: elongate, laterally compressed, with a terminal mouth and the paired and unpaired fins characteristic of characiforms.
The most diagnostic feature is a zigzag black pigmentation pattern that runs along the body midline from roughly mid-flank to the base of the caudal peduncle. This winding or sinuous stripe — rendered in the species name by the Latin flexus — is unlike the solid or dotted lateral bands seen in most congeners and provides a reliable field character. The skeletal character that formally separates it from allied species is the presence of six supraneural bones anterior to the first dorsal-fin pterygiophore, whereas seven is the typical count in other members of the genus. No common names are in established use for this species.
Habitat
The Ndzaa River, from which all known specimens were collected, is a clear-water stream flowing through a riparian corridor that borders grassland and savannah in Mai-Ndombe Province, western Democratic Republic of Congo. The stream system drains into the broader Congo Basin catchment. The combination of clear water and a grassland-savannah matrix is characteristic of certain mid-elevation tributaries within the Mfimi drainage.
Habitat modelling conducted by the describing authors indicated that potentially suitable habitat for P. flexus may also exist in the headwaters of the nearby Ulanzaa River, though the species has not yet been confirmed there by collecting. The known range remains restricted to the type-locality reach of the Ndzaa River, making it one of the narrower endemic tetras within the DRC — a country that harbours exceptional freshwater fish diversity overall.
Feeding
No specific dietary data are available for Phenacogrammus flexus from field observations or gut-content studies. Given its size, body form, and placement within Phenacogrammus — a genus of predominantly invertebrate-feeding omnivores — the species almost certainly preys on small aquatic and terrestrial invertebrates, including insect larvae and small crustaceans, and likely supplements this with organic detritus and plant matter where available.
In the aquarium, small alestid tetras of comparable size and form are typically straightforward feeders that accept quality micro-pellets, crushed flake, and frozen or live small invertebrates such as Artemia nauplii, micro-worms, and Daphnia. Until the species is better established in the hobby and formal husbandry observations accumulate, extrapolation from congeners is the best guide to feeding strategy.
Mating
Specific mating behaviour has not been documented for Phenacogrammus flexus in either field or aquarium settings. Like virtually all characiforms, alestid tetras are egg-scatterers that broadcast adhesive or semi-adhesive eggs into the water column or among vegetation with no parental care or nest construction.
Congeners of similar size typically spawn in fine-leaved vegetation or over the substrate, with males pursuing females and fertilising eggs as they are released. There is no evidence that P. flexus departs from this pattern, and the species would be expected to follow the open-water or plant-spawning strategy common across the family Alestidae.
Breeding
No captive breeding records for Phenacogrammus flexus have been published. The species was described only in 2021 and, as far as is known, is not yet established in the ornamental fish trade or in institutional collections in sufficient numbers for controlled breeding observations.
By analogy with small Phenacogrammus that have been bred in aquaria, eggs would be expected to be adhesive and deposited among fine-leaved plants or spawning mops, with the parents taking no further role after spawning. Incubation time, fry size, and early diet requirements remain undocumented for this species.
In the aquarium
Phenacogrammus flexus is not, as of its description, an established aquarium fish. The species was collected during ichthyological fieldwork and has not been documented in the ornamental trade. Its husbandry requirements must therefore be inferred from related species and from the habitat data recorded at collection: clear, well-oxygenated water in a wooded to savannah-edge stream in the western Congo Basin.
Should the species become available, a soft to moderately mineralised, slightly acidic to neutral water chemistry, tropical temperatures in the mid-to-upper 20s Celsius, and a setup offering planted cover and subdued lighting would represent a reasonable starting point. As a small tetra it would likely do best in groups with similar-sized, peaceful tankmates. Any hobbyist working with confirmed specimens should document observations carefully, as even basic husbandry notes would constitute a meaningful addition to the literature on this recently described species.
Conservation
Phenacogrammus flexus has not been assessed by the IUCN Red List; described in 2021, it is too recent an addition to the literature to have received a formal conservation evaluation. No conservation status designation is currently assigned.
The species is known only from a single river reach in Mai-Ndombe Province, DRC — an intrinsically precarious biogeographic situation. The Congo Basin as a whole faces escalating threats from deforestation, agricultural expansion, artisanal mining, and increasing human pressure on riparian habitats. Habitat modelling suggests limited potential range beyond the type locality. Until systematic survey work establishes whether P. flexus occurs more broadly, its conservation outlook cannot be properly evaluated, and targeted fieldwork across the Ndzaa and Ulanzaa drainages would be a valuable first step.