Taxonomy & naming
The species was described by the Belgian-British ichthyologist George Albert Boulenger in 1913 as Tilapia stigmatogenys, in a short note titled "Description d'un poisson nouveau recueilli à Luluabourg (Congo Belge)" published in the Revue de Zoologie Africaine (volume 2, fascicle 3, page 394). The type locality is Luluabourg — today the city of Kananga — on the Lulua River, a tributary of the Kasai in what is now the Democratic Republic of the Congo. The description rested on a good series of specimens: the surviving syntypes are split between the Natural History Museum in London (BMNH 1913.7.30.6-10, six specimens) and the Royal Museum for Central Africa at Tervuren (MRAC 2551–2575 and 2576–2592, forty-two specimens in all). Boulenger himself illustrated the fish two years later in his monumental Catalogue of the Fresh-water Fishes of Africa (1915). Placement of the species has shifted with the long unravelling of the catch-all genus Haplochromis. Peter Humphry Greenwood, in his 1979 reclassification of the haplochromine cichlids, erected the genus Thoracochromis for a group of largely riverine African species, and stigmatogenys has been treated as a member of it by recent workers including Munene and colleagues in their 2021 survey of the Lulua. Other authorities — the CLOFFA checklist (van Oijen et al., 1991) and Eschmeyer's Catalog of Fishes — retain it in Haplochromis pending a thorough modern revision, so the names Thoracochromis stigmatogenys and Haplochromis stigmatogenys both refer to this same fish in the current literature.
Morphology
Thoracochromis stigmatogenys is a small cichlid, reaching about 4 in in total length according to the standard reference (the CLOFFA treatment). In overall build it conforms to the generalised haplochromine body plan: a moderately elongate, laterally compressed fish with a single continuous dorsal fin combining spiny and soft portions, a rounded to slightly emarginate caudal fin, and the small, size-demarcated scaling of the chest region that gave the genus Thoracochromis its name (from the Greek thorax, a breastplate or cuirass). The most quoted diagnostic feature is the one embedded in the species epithet: a dark mark on the cheek and lower jaw — Boulenger described a black spot usually present on the chin — which is the "marked cheek" the name stigmatogenys translates to. Detailed live coloration and the extent of sexual dimorphism have not been well documented for this rarely collected fish, but its riverine congeners typically show the haplochromine pattern of brighter, more contrastingly marked breeding males and plainer, silvery females, the males often carrying egg-spot ocelli on the anal fin. In the absence of a modern colour description, those genus-level traits are the best available guide rather than confirmed observations of this species.
Habitat
This is a Congo-basin endemic, recorded only from the Democratic Republic of the Congo. FishBase summarises its range as the lower Kwango and the Kasai drainage of the middle Congo River basin, together with the middle Luapula in the upper Congo basin; its reported presence in Zambian waters has been flagged as needing confirmation. The type series came from the Lulua, a large tributary of the Kasai, and the species was recovered again from the lower Lulua during the extensive 2007–2014 survey published by Munene and colleagues in 2021, which recognised that river as a continental hotspot of fish diversity. These are the warm, soft, tannin-stained or sediment-laden waters typical of the central Congo lowlands — the Kasai system drains forested and savanna country within the Kasai freshwater ecoregion, while the Luapula population sits in the Bangweulu–Mweru ecoregion on the basin's southern rim. FishBase classes the fish as benthopelagic in fresh water and places its latitudinal band at roughly 5°N to 4°S. Specific in-situ depth and water-chemistry measurements for stigmatogenys have not been published; the figures used here for temperature and pH are the soft, warm, mildly acidic-to-neutral values that characterise its lowland Congo habitats rather than readings taken at the fish itself.
Feeding
No published gut-content study exists for Thoracochromis stigmatogenys, so its diet has to be inferred from its anatomy and its relatives. FishBase models place it at a trophic level of about 3.3, which puts it among the small generalist carnivores and omnivores rather than the strict herbivores or the high-level piscivores. Riverine haplochromines of this size and build are typically opportunistic micro-predators, working the bottom and the lower water column for aquatic insect larvae, small crustaceans, other invertebrates and organic detritus, and taking whatever the season provides in a fluctuating river. In the species-rich Congo tributaries it inhabits — the Lulua alone yields more than two hundred fish species — a small cichlid like this occupies a middle rung of the food web, both a consumer of invertebrates and prey for the basin's larger predatory fishes. Until a dietary analysis is done, that generalist invertivore-omnivore picture is the honest summary.
Mating
The courtship and pair-forming behaviour of Thoracochromis stigmatogenys has not been observed or described in the literature. What can be said rests on the reproductive biology of the genus and of riverine haplochromines generally. These fishes are not pair-bonding substrate spawners; they are polygamous maternal mouthbrooders, and in that system the male's role in reproduction is brief and centred on display. Breeding males of related species establish small temporary territories, intensify their coloration, and court passing females with lateral displays and quivering, frequently using the egg-spot markings on the anal fin to draw the female into the spawning sequence. Females are not held in a lasting bond and depart to brood alone. For this particular species none of this has been documented directly, so it should be read as the expected genus-typical pattern rather than as a confirmed account of stigmatogenys courtship.
Breeding
Thoracochromis stigmatogenys is a maternal mouthbrooder — the one piece of reproductive information FishBase records for it is "mouthbrooding by females." In the haplochromine version of this strategy, the female takes the freshly laid eggs into her mouth, often snapping at the male's anal-fin egg-spots so that fertilisation is completed there, and then carries the developing clutch in her buccal cavity through hatching and the early free-swimming stage, releasing the fry only when they can fend for themselves and taking them back into her mouth when danger threatens. No fecundity count has been published for this species; small riverine haplochromines of its size typically produce modest clutches on the order of a few dozen eggs (commonly reported genus-wide in the range of roughly 20 to 60), the trade-off being far fewer but much better-protected young than an open spawner. Specific spawning triggers, egg size, incubation period and fry development for stigmatogenys remain undocumented — the species has, as far as the record shows, never been bred or reared under observation.
In the aquarium
For all practical purposes, Thoracochromis stigmatogenys is not an aquarium fish. It is a small, plainly coloured Congo-basin haplochromine known almost entirely from scientific collections; it is not in the ornamental trade, is not bred by hobbyists, and there is no body of husbandry experience to draw on. Anything written about keeping it is therefore extrapolation from how comparable small riverine African mouthbrooders are maintained, not a record of this species in a tank. On that basis, a keeper who somehow obtained wild stock would want a tank of modest footprint for a 4 in fish — on the order of 35.5–47 in in length — furnished with a sand or fine-gravel bottom, rockwork and wood to break sight lines, and gentle to moderate flow. Soft to moderately hard, warm water around 75–82 °F with a pH in the roughly 6.0–7.5 range would mirror its lowland Congo habitat. As a maternal mouthbrooder it would likely do best in a group with several females per male to spread out male aggression, and would take standard small carnivore/omnivore fare. All of this is informed guesswork: the truthful statement is that this fish has essentially no aquarium history, and a hobbyist looking for a comparable but actually available Congo haplochromine would be better served by one of the established species in the trade.
Conservation
The IUCN Red List assesses Thoracochromis stigmatogenys (under the name Haplochromis stigmatogenys) as Least Concern. The assessment was carried out on 16 February 2009 by T. Moelants and published in 2010, with the population trend recorded as unknown. The Least Concern rating reflects a relatively wide distribution across several Congo-basin drainages rather than evidence of abundance, and there is no species-specific monitoring of its numbers. It carries no CITES listing and is of no commercial interest. The wider context is more sobering: the Kasai-system rivers it inhabits, including the Lulua from which it was first described, face intensifying pressure from deforestation, shifting agriculture, charcoal production, mining and destructive fishing — the very threats the 2021 Lulua survey by Munene and colleagues highlighted in calling that river a diversity hotspot under threat. None of these is documented as acting specifically on this cichlid, but they form the backdrop against which a poorly-studied, narrow-data Congo endemic like stigmatogenys should be viewed.