Taxonomy & naming
Stomatepia mariae was first described in 1930 by the Austrian zoologist Maximilian Holly, in his "Synopsis der Süsswasserfische Kameruns," under the name Paratilapia mariae — placed, as so many African cichlids then were, in a broad catch-all genus. The species was reassigned its own genus only in 1962, when Ethelwynn Trewavas of the British Museum erected Stomatepia for it and treated mariae as the type species; the binomial Stomatepia mariae (Holly, 1930) carries Holly's name in parentheses to record the original combination. Trewavas, Green and Corbet's 1972 monograph on the fishes of Barombi Mbo remains the foundational treatment of the genus, formally recognizing three species: Stomatepia mariae, the insect-larva specialist Stomatepia pindu, and the rare, enigmatic Stomatepia mongo, described in the same work. Within the family Cichlidae (subfamily Pseudocrenilabrinae) Stomatepia belongs to the tribe Oreochromini; molecular work by Schliewen and colleagues places the entire Barombi Mbo flock as a single monophyletic lineage of oreochromine cichlids, derived from a riverine ancestor close to Sarotherodon galilaeus that colonized the crater lake roughly a million years ago. The genus is very close to Sarotherodon, and the boundaries among the three Stomatepia species are subtle, resting largely on mouth size, dentition and coloration. The type locality is Lake Barombi Mbo itself, near Kumba in Cameroon's Southwest Region — the only place the species has ever been found.
Morphology
Stomatepia mariae is a small cichlid, reaching about 4.5 in in standard length (FishBase); aquarium accounts of larger fish use total length and a generous tape. It is the most robustly built of the three Stomatepia, with a body depth of roughly 29.5–35.4% of standard length and, most diagnostically, a notably large mouth — the upper jaw runs to about 28.5–35.2% of head length, the feature that gave the genus its name. The fin counts are typical of the group: 14–15 dorsal spines with 10–12 soft rays, three anal spines with 9–10 soft rays. A continuous dark mid-lateral band runs the length of the flank, from the gill cover to the middle of the caudal peduncle, over a ground colour that hobbyists describe as a muted greenish-brown. The lateral-line sensory system on the head is not enlarged (a point that separates it from some lake-mates), and microbranchiospines are present on the outer faces of the second through fourth gill arches. Like several Barombi Mbo cichlids, mariae can switch its colour mode dramatically with mood and status: a settled, displaying or dominant fish turns from drab brown to a deep velvety jet-black, the kind of rapid melanic shift used in territorial and breeding contexts across the flock. Sexual dimorphism is modest — males grow somewhat larger than females, but the sexes are otherwise similar, and reliable sexing usually waits on breeding behaviour.
Habitat
The entire world range of Stomatepia mariae is Lake Barombi Mbo, a crater lake about 2 mi across and over 361 ft deep, lying roughly 1 mi northwest of the town of Kumba in southwestern Cameroon. The lake sits in the Cameroon Volcanic Line, formed in a collapsed volcanic caldera, and is among the oldest radiocarbon-dated lakes in Africa, with its youngest underlying lava flow dated to around a million years ago — long enough for its eleven endemic cichlids to evolve in place. Within the lake mariae is reported to favour the more open water, ranging widely as a fast-swimming, vagabonding hunter rather than holding to one zone. The water is warm and soft: FishBase cites a preferred range of about 77–81 °F, while in-situ surveys record surface temperatures near 75–82 °F, a circumneutral pH around 7.0, very low conductivity, and dark, tannin-stained "black" water clouded with suspended leaf litter and sediment over a sand-and-pebble bottom strewn with submerged wood. The lake is strongly and permanently stratified — classic studies found no detectable dissolved oxygen below about 66 ft — so although the basin is deep, the cichlid-bearing, oxygenated habitat is effectively confined to the upper water column. That stratification, and the chronic risk of dissolved volcanic gas trapped at depth, are defining features of life in this crater.
Feeding
Among the Barombi Mbo flock, Stomatepia mariae is the principal piscivore — a predator on small fish, supplemented by shrimp and insect larvae — and FishBase estimates its trophic level at about 3.4, the highest tier within its genus. Trewavas, Green and Corbet's ecological work characterized mariae as a fish predator and insect-larva feeder, contrasting it with its sibling Stomatepia pindu, a more dedicated insect-larva specialist. Its large mouth is the obvious adaptation: it is the grasping, gape-limited gear of an active hunter rather than the fine pickax of a substrate-sifter. The species also has one of the more remarkable feeding habits documented in the lake — it has been recorded kleptoparasitizing the freshwater crab Potamon africanus, stealing food items the crab has gathered, a behaviour noted in the same Dominey and Snyder study that described this trick in several Barombi Mbo cichlids. In the aquarium the species sheds any specialist pretensions and feeds as an opportunistic omnivore, taking prepared and frozen foods readily. As the flock's roving predator it occupies a trophic role that, in a larger lake, would belong to a quite different lineage; here it is simply one more way the single ancestral colonist's descendants have carved up the resources of one small crater.
Mating
Stomatepia mariae forms loose, temporary pairs in the days before spawning, a pattern shared with several of its Barombi Mbo relatives. During this brief courtship the prospective pair stays close together and selects a spawning site, typically a cleared patch of substrate or a flat stone. As in much of the flock, intensifying black coloration accompanies sexual and territorial display: a ready male darkens markedly, advertising condition and defending the chosen site. The species is, by the standards of African cichlids, not strongly aggressive — keepers describe it as relatively shy and inclined to keep to itself — so the pre-spawning territoriality is comparatively mild and the pair bond is fleeting, dissolving once the eggs are laid and taken up. Because the male contributes nothing after fertilization, the "pairing" is better thought of as a short reproductive partnership than a lasting bond; the female alone carries the parental investment from the moment she collects the eggs into her mouth.
Breeding
Stomatepia mariae is an ovophilic maternal mouthbrooder: the female collects the fertilized eggs into her mouth and incubates them there, with no parental role for the male after spawning. Clutches are small — hobbyist accounts report roughly 25 to 80 small eggs per spawn — and brooding females hold for on the order of three weeks (about twenty days) before releasing free-swimming fry. The pattern is broadly that of a haplochromine-style mouthbrooder, which is how experienced keepers describe its reproduction in practice. The species is not difficult to spawn once settled, but it is a famously unreliable holder: females frequently swallow or spit a brood prematurely, and prematurely abandoned clutches often prove to carry many infertile eggs, problems reported repeatedly by the small circle of aquarists who maintain the fish. Disturbance is a particular trigger — a female will commonly spit her eggs at the approach of a net — so stripping or moving a holding female risks losing the brood and damaging the larvae. When a spawn does carry through, the fry are comparatively easy to rear and grow reasonably quickly. There is no evidence of successful captive breeding having stabilized the species in the wild, and reproduction in nature has been little studied beyond the general mouthbrooding biology.
In the aquarium
Stomatepia mariae is strictly a specialist's and conservation-keeper's fish, not a commercial aquarium-trade staple; it reaches hobbyists almost entirely through dedicated breeders and programs such as C.A.R.E.S. that maintain Barombi Mbo endemics. Those who keep it describe a relatively shy, mild-mannered cichlid that does best in a calm tank and is easily intimidated by boisterous tankmates. A breeding group of about five can be housed in a tank of roughly 40 US gallons (a standard "40 breeder" footprint), aquascaped with rockwork and a few flat stones to serve as spawning sites and to break up sightlines. Suitable companions are other quiet, similarly sized Barombi Mbo endemics — the species is routinely kept in single-lake community tanks alongside Stomatepia pindu, Konia, Pungu and Sarotherodon caroli — or gentle dither fish; aggressive or fin-nipping cichlids are a poor match and will keep mariae perpetually hiding.
Water should be warm and well filtered. The lake itself runs warm, soft and roughly neutral (in-situ pH near 7.0, around 75–82 °F), and although the genus tolerates a broad chemistry, keepers generally maintain it warm — high-20s to around 86 °F — over a sand substrate with stone cover. The fish is reported to be "not easy to raise in tanks," a reputation that owes less to any delicacy in the adults than to the difficulty of getting reliable broods from a poor-holding mouthbrooder. The two practical hurdles are getting the fish settled and confident enough to spawn at all, and then carrying a brood to term: females spook easily and abandon eggs, and infertile clutches are common. Patience, stability and minimal disturbance of holding females are the keys. Feeding presents no difficulty — wild fish are predatory, but captives accept prepared, frozen and live foods without fuss. Anyone taking on this species should understand they are participating in ex-situ conservation of a Critically Endangered fish as much as keeping an attractive display animal, since mariae is, by candid hobbyist admission, a plain greenish-brown fish whose appeal lies in its rarity and its story.
Conservation
The IUCN Red List assesses Stomatepia mariae as Critically Endangered, under criteria B1ab(iii)+2ab(iii), in an assessment by T. Moelants dated 16 February 2009. The listing turns almost entirely on the species' single-lake endemism: with its whole range a crater barely 2 mi across, both the extent of occurrence and area of occupancy fall below the thresholds for the category, and any degradation of that one habitat threatens the entire global population at once. The documented and potential pressures are local but serious — fishing by lakeside villagers, agriculture and the deforestation and shoreline erosion that come with it, effluent and the damming of the lake's outflow, and the looming, low-probability but catastrophic risk inherent to a Cameroonian crater lake: a limnic gas release. Lake Barombi Mbo holds dissolved gas at depth and lies in the same volcanic line as Lakes Nyos and Monoun, whose sudden CO2 outbursts in the 1980s killed people and livestock; observers have reported turbid water and fish die-offs in Barombi Mbo consistent with smaller upwellings. Against that backdrop the lake is also, encouragingly, regarded as sacred by the local Barombi community, which has historically limited overfishing. The species has no CITES listing. As with every endemic of this remarkable flock, its long-term survival is tied to the integrity of one small, fragile crater lake, and captive populations maintained by hobbyist conservation networks are widely seen as an important insurance against loss in the wild.