Taxonomy & naming
The French ichthyologist Jacques Pellegrin described this fish in 1928 as Leptotilapia rouxi, in a paper on the fishes of the Kasai (then Belgian Congo) that erected one new genus and four new species. The single type specimen had been entrusted to him from the collection of the Natural History Museum of Basel, and the holotype (NMBA 4000) still resides there; the type locality is Luluabourg, the colonial name for what is now Kananga, on the Lulua River in Kasai Province. Pellegrin's genus Leptotilapia was later sunk, and the species was transferred to Steatocranus Boulenger, 1899, the buffalohead genus, where Snoeks and De Boeck confirmed its placement in their 1991 CLOFFA treatment. In the process they recognized that Steatocranus elongatus, described by Nichols and La Monte in 1934 from material also collected near Luluabourg, was the same fish, and it now stands as a junior synonym. Within the African cichlid subfamily Pseudocrenilabrinae, Steatocranus anchors its own tribe, the Steatocranini, a lineage of small rheophilic cichlids specialized for life in the rapids and rocky channels of the Congo system. Steatocranus rouxi sits among the more elongate, less crested members of that group, distinguished in the literature by a count of 15 to 16 scales around the caudal peduncle.
Morphology
Steatocranus rouxi is one of the smaller buffaloheads: FishBase records a maximum of 2.5 in standard length, with the largest measured fish reaching about 3 in in total length, so this is a fish that fits comfortably in the palm of a hand. The body is markedly elongated and low-slung — the genus name advertises the fatty cranial cushion that swells on the foreheads of dominant males in some congeners, but rouxi is among the more streamlined, weakly crested species rather than a heavily domed one. Preserved specimens are a fairly uniform brownish to greyish, the drab, current-blending coloration typical of rock-dwelling rheophiles; living fish have not been documented in any detail, and no reliable colour photographs of confirmed wild material circulate. The published diagnostic character is the circumpeduncular scale count of 15 to 16. As in all Steatocranus, the swim bladder is reduced, an adaptation that sacrifices buoyancy so the fish can hold station on the bottom of fast water rather than be lifted into the current. Sexual dimorphism has not been described from this species specifically; across the genus males grow larger and develop a more pronounced nuchal hump than females, and the same pattern is the reasonable expectation here, though it remains unconfirmed for rouxi.
Habitat
The species is a narrow endemic, recorded only from the Lulua River, a large right-bank tributary that drains north off the Angolan highlands into the Kasai, itself one of the great southern affluents of the Congo. The Lulua is a long, structurally varied river: surveys of its lower and middle reaches describe a mosaic of falls, rapids, pools, braided channels, floodplains and swamps, with long series of cataracts that fragment the river and drive a high level of fish endemism — the recent ichthyofaunal inventory of the lower Lulua lists Steatocranus rouxi explicitly among the river's endemic species. As a member of a rheophilic genus, rouxi belongs to the rocky, fast-flowing, well-oxygenated stretches rather than the slack floodplain lakes, very likely tucking into crevices and the lee of boulders in the manner of its relatives. No in-situ water chemistry has been published for this particular fish; the central Congo tributaries it inhabits are generally warm, soft to moderately mineralized and near-neutral, and the genus as a whole occupies water in roughly the pH 6 to 8 band at tropical temperatures around 24 to 28 degrees Celsius. Because it lives in shallow, wadeable rapids and rocky margins rather than deep water, its depth band is effectively the top metre or two of the river.
Feeding
No gut-content study has been published for Steatocranus rouxi, so its diet is inferred from its anatomy and its close relatives rather than directly observed. FishBase places it at a trophic level near 3.2, the value expected for a small benthic omnivore. Buffaloheads in general are bottom-grazers that work over rock and gravel surfaces, rasping the film of algae (aufwuchs) and picking out the small invertebrates — insect larvae, micro-crustaceans — that shelter in it; the best-known congener, Steatocranus casuarius, feeds primarily on algae scraped from rocks in the Congo rapids. A reduced swim bladder and a low, mobile body are exactly the equipment for this kind of close-quarters foraging in moving water, and rouxi is almost certainly a generalist grazer-picker of the rocky substrate it lives on. In its community it would function as one of the small benthic invertivore-detritivores that exploit the rapids biofilm, a guild that is unusually rich in the rocky reaches of the Congo and its tributaries.
Mating
Reproductive behaviour has not been observed in wild Steatocranus rouxi, and the account here draws on the consistent pattern across the genus. Buffaloheads are pair-bonding, strongly territorial cichlids that form durable monogamous bonds — in some species so durable that a bereaved fish will not readily re-pair. A pair holds and defends a small rocky territory centred on a cave or crevice, with both partners driving off intruders, and courtship is comparatively understated by cichlid standards: there is no open-water display in fast current, and pairing instead revolves around joint defence of, and preparation of, the chosen cavity. The reduced buoyancy that keeps these fish on the bottom shapes the whole social style — interactions play out as substrate-level posturing, lateral displays and short hopping charges rather than the mid-water dances of open-water cichlids. For rouxi specifically the details are unrecorded, but a small, cavity-defending, monogamous social system is the firm expectation.
Breeding
FishBase classifies Steatocranus rouxi, on the basis of its genus, as a pair-bonding cave spawner, and no wild or captive breeding account specific to this species has been published. The genus-typical mode is well documented in relatives such as Steatocranus casuarius: a monogamous pair selects or excavates a cave under a rock, the female lays a modest clutch — on the order of 20 to 150 eggs in Steatocranus casuarius — on the cave wall or roof, and both parents tend the brood. Eggs hatch in roughly five to seven days and the fry become free-swimming a week or so later, at which point the parents lead and shepherd them over the substrate, herding strays back and continuing to guard the young for an extended period. This biparental cave-brooding strategy, with its small clutch of well-guarded eggs sealed away from the current, is a sensible reproductive bet in turbulent rapids, and it is the pattern that should be assumed for rouxi until it is actually bred and described. The figures cited here are genus-typical, drawn from its better-studied congeners rather than from rouxi itself.
In the aquarium
Steatocranus rouxi is essentially absent from the aquarium hobby. It is not collected for the ornamental trade in any volume, no reliable photographs of living aquarium specimens circulate, and it has no established common name — the buffalohead, lionhead and blockhead labels in the trade belong to its larger, widely kept relative Steatocranus casuarius and should not be transferred to it. Anyone who did obtain genuine rouxi would treat it as a small rheophilic Steatocranus, and the husbandry of the genus is well understood. The key requirements are warm, well-oxygenated water with brisk flow in part of the tank (a powerhead reproduces the rapids current) balanced by sheltered pockets of slack water; a sand or fine-gravel bottom the fish can dig in; and an abundance of secure rockwork forming caves and crevices, since these are cave-spawning, bottom-hugging fish that spend most of their time resting on or hopping across the substrate rather than swimming in open water. Genus-typical parameters are temperature around 24 to 28 degrees Celsius, pH from about 6.0 to 7.5, and soft to moderately hard water; rock piles should be seated on the tank base or siliconed so a burrowing fish cannot undermine and topple them. Diet is undemanding — quality dried foods supplemented with live and frozen bloodworm and brine shrimp suit the genus — and a single pair in a modest tank, with peaceful rheophilic dither fish such as Congo tetras, is the standard arrangement. None of this has been tested on rouxi specifically; it is the reasonable extrapolation from a genus that keeps and breeds readily, applied to a fish that, in practice, almost no one keeps.
Conservation
The IUCN Red List assesses Steatocranus rouxi as Data Deficient, in an evaluation by T. Moelants assessed on 16 February 2009 and still current. The category reflects exactly the problem this article runs into throughout: the species is known from very little material, its population size and trend are unknown, and there is not enough information to judge its risk of extinction. It carries no CITES listing. What can be said is that it is a narrow-range endemic confined to a single river system, and narrow endemics are inherently more exposed than widespread fish to localized pressures. The Lulua and the wider Kasai ecoregion are recognized as a continental hotspot of fish diversity, but the lower Lulua inventory that re-recorded rouxi was published under the heading of an ichthyofauna 'under threat,' flagging habitat degradation, deforestation, sedimentation and intensifying artisanal fishing across the basin. None of these is documented as a specific danger to this particular cichlid, but for a fish restricted to one river and assessed only as Data Deficient, the sensible reading is that its status is genuinely unknown and that basic survey and life-history work is overdue.