Taxonomy & naming
George Albert Boulenger described this fish in 1899 as Tilapia polyacanthus, in the fifth part of his series on the fishes of the Congo published in the Annales du Musée du Congo. His material came from Pweto, at the southern end of Lake Mweru on the border of what is now the Democratic Republic of the Congo and Zambia; the unique holotype (MRAC 225, a specimen of about 4.5 in total length) remains in the Royal Museum for Central Africa at Tervuren. The placement in Tilapia was a default of its era, when most African cichlids that did not obviously belong elsewhere were swept into that catch-all genus. The species was subsequently moved repeatedly as workers tried to accommodate the small rheophilic cichlids that did not fit the lacustrine haplochromine mould: Regan transferred it to Haplochromis in 1922, Thys van den Audenaerde placed it in his subgenus and later genus Rheohaplochromis in the 1960s, and in 1994 Peter Humphry Greenwood and Sven O. Kullander, in a taxonomic review and redescription of Tilapia polyacanthus and Tilapia stormsi, assigned it to Schwetzochromis. De Vos & Seegers (1998) finally combined it into Orthochromis, where Eschmeyer's Catalog of Fishes and FishBase now treat it as valid, a placement upheld by the molecular and morphological work of Schedel and colleagues (2014, 2018). Within the family it sits in the subfamily Pseudocrenilabrinae among the haplochromine-allied riverine cichlids, and it gives an informal sense of the small genus of perhaps a dozen-plus rapids-adapted Orthochromis species of the Upper Congo and the Lake Tanganyika tributaries. Because so many populations once attributed to it have proven to be other species, the name has historically been applied far more widely than its true range warrants.
Morphology
Orthochromis polyacanthus is a modest fish: FishBase gives a maximum of about 4.5 in total length, and the holotype measured roughly 4.5 in, so this is a cichlid of palm-length rather than show-tank proportions. The body is elongate and only moderately compressed, the low, tapering profile typical of fishes that live pressed to the bottom of moving water where a deep, slab-sided body would only catch the current. The fin counts recorded for the species are diagnostic of its rapids lineage: a long dorsal fin carrying about 18 to 19 spines before nine soft rays, three anal spines with seven soft rays, and 31 to 32 vertebrae. That high spine count is the source of the epithet polyacanthus, 'many-spined'. Coloration in life is comparatively subdued by the standards of the rift-lake cichlids, running to brownish or greyish ground tones, often with darker vertical barring or blotching along the flanks and the iridescent cheek and fin markings common to haplochromine-allied fishes; museum-based redescriptions by Greenwood & Kullander (1994) document the pattern, dentition, scalation and osteology in detail. Sexual dimorphism follows the maternal-mouthbrooder pattern of the group rather than being strongly marked: breeding males tend to be the more intensely coloured sex and develop the spawning markings on the anal fin, while females are plainer, but the two sexes are notoriously hard to tell apart when small.
Habitat
The reliable range of Orthochromis polyacanthus is narrow. Despite a long list of historical records scattered across the Congo basin, the careful redescription by Greenwood & Kullander (1994) concluded that the species is apparently endemic to Lake Mweru and its immediate environs, and that records from other rivers and sites in the Democratic Republic of the Congo and Zambia are either unconfirmed, questionable, or refer to other (often newly described) Orthochromis species. FishBase echoes this, treating the wider Congo and Zambian reports as doubtful. The type locality at Pweto sits where the Luvua River drains Lake Mweru, a setting of flowing, broken water rather than open lake. Like the rest of the genus the fish is rheophilic, a current-dweller of swift, well-oxygenated rivers and rapids where it lives among sand, water-smoothed pebbles and boulders on the bed. Boulenger's own later collections came partly from the Lake Mweru and Lake Tanganyika expedition material of the early twentieth century, underscoring that this is a fish of the Upper Congo's flowing waters. FishBase records a narrow measured temperature band of about 77–79 °F for the species; the broader genus tolerates warm, near-neutral to mildly alkaline tropical water. No precise in-situ depth has been published, but as a benthic rapids fish it occupies the shallow, fast bed of the river, on the order of the uppermost metre or two of the water column rather than any great depth.
Feeding
The natural diet of Orthochromis polyacanthus has not been the subject of a dedicated study, and FishBase carries no quantified trophic data for it. Its ecology can be read instead from its anatomy and its relatives. The rheophilic Orthochromis are small benthic foragers of the rapids, working the films of algae (aufwuchs) and the associated community of aquatic insect larvae and other small invertebrates that coat the stones and sand of fast water. Fishes of this guild typically rasp and pick at the biofilm on rock surfaces while taking the larvae, nymphs and micro-crustaceans sheltering among the substrate, making them broadly omnivorous bottom-grazers rather than specialist predators. In aquaria the species is unfussy and readily accepts flake and frozen foods including bloodworm and brine shrimp, which is consistent with a flexible, opportunistic feeder rather than a narrow dietary specialist. Within its rapids community it functions as one of the small grazer-invertivores that crop the productive but physically demanding band of broken water.
Mating
Detailed field observations of courtship in Orthochromis polyacanthus specifically have not been published, but its reproductive style is well constrained by its membership in the maternal-mouthbrooding haplochromine-allied lineage and by accounts of its close congeners. These are territorial fishes, particularly toward their own kind: males establish and defend a patch of the river bed against rival males and display to females from it. Breeding is polygynous in the haplochromine manner, a single male courting and spawning with several females in succession rather than forming a lasting pair bond. Courtship centres on the male's heightened spawning coloration and on the markings of the anal fin, which in this group play a role in drawing the female to take up the eggs. Aggression in the genus is directed mainly intraspecifically, which shapes both wild spacing and the way the fish must be managed in captivity, where lone males will harass each other and overcrowd subordinate fish.
Breeding
Orthochromis polyacanthus is a maternal mouthbrooder: FishBase records reproduction in the species as 'mouthbrooding by females', the female alone incubating the clutch. As in other haplochromine-allied mouthbrooders, the female lays a small batch of relatively large, yolk-rich eggs, takes them into her mouth, and the male fertilises them there or as she collects them; she then carries the developing eggs and larvae in her buccal cavity through to the free-swimming stage, foregoing feeding for much of that period. No clutch count has been published for this particular species; genus-typical broods in the small rheophilic Orthochromis and their close relatives are modest, on the order of a few dozen eggs, in keeping with the energy cost of oral incubation and the small adult size. Parental care is entirely maternal and ends not long after the fry are released, with no biparental guarding of a nest of the kind seen in substrate-spawning cichlids. The reliance on mouthbrooding rather than an exposed substrate clutch is itself an adaptation to flowing water, where eggs left on the open bed would be swept away or quickly consumed.
In the aquarium
Orthochromis polyacanthus is an uncommon import and something of a specialist's fish; Practical Fishkeeping noted it appearing only sporadically among wild Congo shipments and being easily confused with the other poorly known members of its genus. It is best thought of as a rheophilic cichlid first and a cichlid second: the single most important thing a keeper can provide is the swift, clean, oxygen-rich water it evolved in. A spacious tank of roughly 35.5 in or more suits a small group, furnished to mimic a rapid, with a sand base, smooth water-worn pebbles and rounded boulders to break the flow and define territories, and some driftwood. Strong filtration and brisk water movement are not optional extras but the core of the setup, and excellent water quality is essential; these fish do poorly in still, organically loaded tanks. Reported aquarium parameters run to roughly 73–82 °F and a pH around 6.0 to 7.0, in line with the warm, near-neutral water of the Upper Congo, and the fish are tolerant of moderately hard, mildly alkaline conditions as well. Feeding is straightforward, with flake and frozen foods such as bloodworm and brine shrimp readily taken. Because they are maternal mouthbrooders and difficult to sex when young, the standard approach is to grow on a group and keep one male with several females, rehoming surplus males that clash over territory. They are territorial toward their own kind but generally leave dissimilar tankmates alone, so suitable companions are robust Congo-region characins or catfishes rather than other small bottom-oriented cichlids that would compete for the same ground. This is not a beginner's centrepiece fish; its appeal is to keepers interested in riverine biotopes and in a genus that remains genuinely under-documented.
Conservation
The IUCN Red List assesses Orthochromis polyacanthus as Least Concern, in an assessment carried out on 16 February 2009 by T. Moelants and published in 2010. The listing reflects the species' occurrence in the Lake Mweru system without evidence of a population-level decline; it carries no CITES listing and no CMS listing. That said, the assessment predates much of the taxonomic clarification of the genus, and the species' true conservation picture is complicated by its narrow, genuinely endemic range around Lake Mweru once the spurious wider records are stripped away. A small-bodied rheophile tied to a specific stretch of flowing water is inherently more exposed to localized pressures than a widespread generalist would be. The broader Upper Congo and Lake Mweru basin faces the familiar suite of regional threats to its fishes, including intensive artisanal fishing, deforestation and sedimentation of catchments, and the prospect of hydrological alteration of the rivers that rapids specialists depend on, though none of these is documented as a present, species-specific danger to this cichlid. The chief practical concern remains that 'Orthochromis polyacanthus' has been a taxonomic wastebasket, so any future reassessment will need to rest on the much narrower, redescription-based concept of the species.