Taxonomy & naming
The kululu was described by the British ichthyologist Ethelwynn Trewavas in 1962, in her account of the fishes of the crater lakes of the northwestern Cameroons, as Tilapia steinbachi. The holotype (BMNH 1961.10.18.40, with one paratype) was taken from Lake Barombi-ma-Mbu — also called Elephant Lake, and today written Barombi Mbo — and material had been collected during the expedition led by the zoologist Martin Eisentraut. When Trewavas split the mouthbrooding tilapias from the substrate-spawning ones in her 1983 monograph, steinbachi moved into Sarotherodon, the genus of paternal and biparental mouthbrooders, where it has remained; the Catalog of Fishes and the CLOFFA checklist (Trewavas & Teugels 1991) both list it as valid as Sarotherodon steinbachi. Its real interest is its place in the Barombi Mbo flock. Molecular work by Schliewen and colleagues showed that all eleven endemic cichlids of the lake form a single monophyletic lineage descended from one riverine colonist close to the widespread Sarotherodon galilaeus, which entered the lake and diversified in place; because the crater basin is barely 1 mi across, has no internal barriers, and is cut off from outside rivers, Barombi Mbo became one of the most-cited cases for sympatric speciation. Sarotherodon steinbachi belongs to a tight cluster of three ecologically and morphologically similar congeners — alongside Sarotherodon caroli and Sarotherodon lohbergeri — that are among the hardest of the flock to tell apart.
Morphology
Sarotherodon steinbachi is a small, short and rather deep-bodied cichlid: FishBase gives a maximum of about 4.5 in standard length, so even large adults are hand-sized. The fin counts run to 16-17 dorsal spines and 10-11 soft rays, with 3 anal spines and 8-10 soft rays, and 29-31 vertebrae. In life the fish is a plain pearly yellow with no markings on the body; preserved specimens darken to grey on the back, some retaining a faint horizontal band, and the snout is distinctly decurved. The features that separate it from its near-twins Sarotherodon caroli and Sarotherodon lohbergeri are internal and subtle — Trewavas distinguished them on gill-raker counts (19-23 rakers on the lower limb of the first arch in steinbachi) and on the proportions of the lower pharyngeal jaw, whose length runs 40-50% of head length. Sexual dimorphism is weak: the species lacks marked colour difference between the sexes even when sexually active, so ripe males and females look much alike, separable mainly by the genital papilla and the fuller body of a gravid female. There is none of the bright nuptial colour that flashes through the Rift Lake mouthbrooders.
Habitat
The entire world range of this fish is Lake Barombi Mbo, a maar (explosion-crater) lake near Kumba in Cameroon's Southwest Region, roughly seven square kilometres of surface in a near-circular basin about 2 mi wide. The lake is deep and steep-sided — average depth around 226 ft, maximum around 361–364 ft — and strongly stratified: the upper water is warm and oxygenated while the deep water below roughly 131 ft is essentially anoxic and rich in organic matter and dissolved gases. The endemic cichlids, kululu included, therefore live in a comparatively thin shell of habitable water near the surface and shore. Within that, Sarotherodon steinbachi keeps to shallow areas near the edges of the lake and is seen shoaling at the surface. Surface water is warm and close to neutral or mildly alkaline; field and aquarium accounts of the lake put temperatures around 75–82 °F and pH near neutral to about 8, with low conductivity and soft water typical of a rain-fed crater. FishBase records a narrower preferred band of about 75–79 °F. It is the shallowness and the surrounding forested crater rim — not chemistry — that define this fish's world.
Feeding
Sarotherodon steinbachi is a low-trophic-level grazer and sand-sifter; FishBase estimates a trophic level of about 2.2, near the bottom of the lake's food web. Stomach examinations found organic debris together with benthic and epilithic diatoms, sponge spicules nearly always present, and frequent inorganic particles — the signature of a fish working algal films and sediment rather than hunting. Trewavas described individuals shovelling sand into the mouth, working it about in the buccopharynx, then spitting the grit back out and swallowing the edible fraction, a winnowing routine shared with many tilapiine grazers. A modern quantitative diet study of the Barombi Mbo flock (which sampled steinbachi but treated its very similar relatives caroli and lohbergeri separately) found that Sarotherodon steinbachi carried an unusually high proportion of unidentified 'egg-like' material in its gut and foraged largely in the littoral zone. In the radiation as a whole it sits among the benthic diatom-and-detritus feeders of the shallow margins, a niche subtly partitioned from the shrimp-eaters, the pelagic plankton-feeder Myaka, and the deep-water midge specialist Konia dikume.
Mating
Like other Sarotherodon, the kululu is a mouthbrooder rather than a nest-tending substrate spawner, and its social system reflects that. FishBase characterises it as forming little or no, and only temporary, pair bonds — courtship and spawning happen, but the lasting pair territory of a substrate-spawning cichlid is absent. Because the sexes are alike even in breeding condition, there is no flashy nuptial dress to mediate courtship; mate choice and synchronisation rely on behaviour and proximity in the shallow shoals rather than on colour signalling. Aquarium keepers describe steinbachi as one of the more easy-going members of the flock, a fish that 'minds its own business and goes with the flow' rather than holding and defending ground aggressively. The picture, then, is of loose, transient mating associations within a shoaling species — a contrast to the territorial, strongly dimorphic courtship of the haplochromine mouthbrooders of the East African lakes.
Breeding
Sarotherodon steinbachi is a mouthbrooder; FishBase records biparental mouthbrooding for the species, and describes it as probably a biparental ovophilic mouthbrooder, meaning the eggs are picked up and incubated orally after spawning. This is the ancestral reproductive style of the genus — Sarotherodon spans paternal and biparental mouthbrooding, unlike the strictly maternal Oreochromis — and in steinbachi both parents may share incubation, consistent with the weak, transient pairing seen in the shallows. Specific clutch sizes and fry-development data for this species have not been published; as a small Sarotherodon it can be expected to produce a modest clutch (congeneric and related Barombi Mbo mouthbrooders carry on the order of a few dozen eggs and brood them to the free-swimming stage), but precise wild figures are genuinely undocumented. In captivity the species has proved difficult to bring through: keepers report spawnings and mouthfuls of eggs but, as of published hobby accounts, no viable surviving fry — a recurring problem with the Barombi Mbo cichlids generally. Warmth helps; experienced breeders find these lake species spawn far more readily once the temperature is pushed above about 84 °F (85 °F).
In the aquarium
Sarotherodon steinbachi is a specialist's fish, almost never seen in general trade and kept mainly by hobbyists deliberately working to preserve the Barombi Mbo flock (it is a CARES Preservation Program species). It is small and undemanding in temperament — among the calmer fish in a mixed Barombi Mbo tank, content to shoal and graze rather than fight — which makes the practical challenge one of conservation breeding rather than aggression management.
The environment to recreate is the warm, near-neutral to mildly alkaline shallow margin of a crater lake. Keepers maintain Barombi Mbo communities at pH between about 7 and 8 and a temperature of 82 °F (82 °F) or higher, nudging it north of 84–86 °F (85 °F) to trigger spawning. A fine sand substrate suits the species' sand-shovelling feeding, and a few scattered rocks, tangles of wood and a scatter of dried leaf litter (catappa/almond leaves) provide cover that these fish appreciate; live plants are optional. Filtration should be generous and water changes frequent — a stocked species tank benefits from heavy biological and mechanical filtration plus weekly changes around 40% — because the fish come from a small, clean body of water and have limited tolerance for accumulated waste. Keep flow gentle.
Feeding is straightforward: as a diatom-and-detritus grazer it does well on spirulina and vegetable-based flake and pellets, supplemented with frozen or freeze-dried bloodworm, daphnia and krill. A quirk worth knowing is that these fish readily grab and hold pellets in the mouth for days, producing convincing 'false' mouthfuls that can be mistaken for a brood. Because all eleven Barombi Mbo cichlids are close relatives, hybridisation is a real risk in mixed tanks and a single-species group is the responsible choice for anyone hoping to produce stock; even then, the species is reported to spawn but rarely to yield surviving fry, so this is a project for a patient, experienced keeper rather than a community-tank candidate.
Conservation
The IUCN Red List assesses Sarotherodon steinbachi as Critically Endangered (criteria B1ab(iii)+2ab(iii)), assessed 16 February 2009 (published 2010, assessor T. Moelants). The reasoning is starkly geographic: the entire species lives in one crater lake of about seven square kilometres, so its extent and area of occupancy fall well below the thresholds for the category, and the quality of that single habitat is judged to be declining. The threats the assessment lists are all aimed at that one lake — sedimentation and pollution from oil-palm plantations and slash-and-burn agriculture on the crater slopes, water abstraction to supply the growing town of Kumba, and proposed tourist development. Two further dangers are peculiar to a deep, gas-laden crater lake: a limnic eruption or 'burp' of carbon dioxide of the kind that killed thousands at nearby Lake Nyos in 1986, and the risk that deforestation of the rim increases wind, mixing the lake and bringing the anoxic, organic-rich deep water up into the thin surface layer where the fish live — an overturn that could asphyxiate the whole endemic flock at once. The species has no CITES listing. Its best insurance beyond protecting the lake itself is the small network of aquarists and the CARES program maintaining captive lines, which is part of why even a fish this obscure is worth keeping correctly.