Taxonomy & naming
Stomatepia mongo was described by the British ichthyologist Ethelwynn Trewavas in 1972, in the monographic paper 'Ecological studies on crater lakes in West Cameroon: fishes of Barombi Mbo' (Trewavas, Green & Corbet, Journal of Zoology, London, volume 167, where the species appears on page 73 with figures 24-25). The holotype, a specimen from Lake Barombi Mbo, is held at the Natural History Museum, London under register BMNH 1971.10.20.23, with ten paratypes catalogued as BMNH 1971.10.20.24-33. Eschmeyer's Catalog of Fishes lists the name as valid in its original combination, with no synonyms. The genus Stomatepia had itself been erected by Trewavas a decade earlier, in 1962, and contains just three species, all of them endemic to Barombi Mbo: the relatively common Stomatepia pindu and Stomatepia mariae, and the scarce Stomatepia mongo. Stomatepia is one of four cichlid genera (alongside Konia, Myaka and Pungu) found only in this single lake; a fifth lineage in the flock belongs to the otherwise widespread, and here paraphyletic, genus Sarotherodon. Molecular work by Schliewen and colleagues (Nature, 1994; Frontiers in Zoology, 2004) established that the whole Barombi Mbo assemblage forms one monophyletic group of oreochromine cichlids most closely related to the riverine Sarotherodon galilaeus of the surrounding region, and is the textbook example of sympatric speciation within a lake. Within Stomatepia, Stomatepia mongo is distinguished by its strikingly elongate neurocranium and downturned snout compared with its two congeners.
Morphology
The mongo is a small, slender cichlid. FishBase records a maximum standard length of about 4 in, and the type series and most field-caught specimens fall in this range (Trewavas et al. recorded a holotype female and small samples of similar size); freshly caught males photographed by recent workers measured around 3 in SL. Some secondary sources cite a larger figure near 6.5 in, but the original description and the catalogued material do not support a fish of that size, and the smaller measurement should be treated as the reliable one. Trewavas diagnosed the species as elongate and gracile, with a body depth of only 25.0-30.5 percent of standard length and a relatively small mouth (upper jaw 26.8-28.8 percent of head length). The fin counts are typical of the genus: 14-16 dorsal spines and 10-12 soft rays, with 3 anal spines and 8-9 anal soft rays. The most distinctive external feature is the long, somewhat downturned snout and the array of enlarged cephalic sensory pores, which the fish appears to use to detect prey in sand and detritus. Coloration is remarkably plastic. Modern observers (Musilová et al., 2014) describe three modes: a silvery body with rose hues, typical over sand; a 'halftone' form with the lower body or ventral region blackened or blotched, associated with rocky sites; and a wholly velvety-black form, linked to territorial and breeding males. Non-black individuals often show light-orange margins on the dorsal, anal and ventral fins.
Habitat
Stomatepia mongo is endemic to Lake Barombi Mbo, a roughly circular volcanic crater lake about 2 mi across and over 361 ft deep, lying at around 984 ft elevation about 1 mi from the rapidly growing town of Kumba in southwestern Cameroon. The lake is part of the Cameroon Volcanic Line, is radiocarbon-dated to roughly one million years old, and was designated a Ramsar wetland of international importance in 2006. Like many deep tropical crater lakes it is permanently stratified: studies from the 1970s (Green, Corbet & Betney, 1973) found no detectable dissolved oxygen below about 66 ft, so the cichlid flock is effectively confined to the oxygenated upper layer and littoral. Surface water is warm and near-neutral to mildly alkaline; a 2014 survey (Tabot, Che & Fonge, 2016) measured open-lake temperatures of about 24-28 degrees C and pH from roughly 7.0 to 7.9, with very low conductivity (below 0.3 mS/cm) characteristic of soft freshwater, though magnesium- and calcium-bicarbonate inputs from inflow streams keep it from being strongly acidic. The mongo was long thought to be a deep-water specialist that explained its apparent rarity, but Musilová et al. (2014) repeatedly recorded it in shallow water no deeper than about 7 ft as well as below 10 ft; unlike the genuine deep-water specialist Konia dikume, it shows no known physiological adaptation to deep, low-oxygen water. It is most often seen close to the bottom over sand, debris and around rock, foraging with the head angled downward.
Feeding
The mongo is a predator and benthic forager. FishBase classes it as predatory with an estimated trophic level around 3.3, and the few life-history notes that exist suggest it feeds on macroinvertebrates and small fish (Musilová et al., 2014, personal observations). Recent in-lake observations describe it foraging close to the substrate with the head turned down at a slight angle, apparently using its conspicuously enlarged cephalic sensory pores to locate small animals hidden in sand and detritus, a sensory style that fits its elongate, fine-pored head. It thus occupies a different feeding niche from its two commoner congeners in the same lake: Stomatepia pindu is principally an insect-larva feeder and Stomatepia mariae a fish predator that also takes insect larvae. Intriguingly, recent workers several times watched small groups of mongo foraging together with no sign of the intraspecific aggression usual among cichlids, and on one occasion saw a group of six adults resting collectively on the bottom, raising the possibility that some form of cooperative or loosely social foraging occurs. The hypothesis that the species is partly nocturnal, hunting by night and perhaps moving shallower after dark to avoid large catfish predators (Clarias), is supported by one unusually productive night of fishing, though it has not been confirmed.
Mating
Detailed courtship in the mongo has not been formally documented, and what is known comes from scattered field observation rather than systematic study. The strongest signal is the dramatic black colour change of breeding fish. Observers have found completely black pairs in shallow water sheltering in narrow flat zones beneath stones, and watched a rocky terrace dominated by a single black territorial fish surrounded by several large non-black adults, which together suggest that the velvety-black livery is tied to territoriality and sexual behaviour in males. Outside these contexts aggression among mongo is consistently low, and the same fish over sand or open rock revert to the silvery or halftone colour modes. Like the rest of the Barombi Mbo flock, the species is believed to form temporary pair associations at spawning sites on or near the substrate, with the female taking over sole care of the brood by mouthbrooding. Beyond this the pre-spawning behaviour of the mongo remains essentially unstudied, a consequence of how seldom the fish is encountered and the absence of any sustained captive population in which to watch it.
Breeding
No spawning of Stomatepia mongo has ever been described in detail, in the wild or in captivity, so its reproduction is inferred from its relatives. FishBase records it as probably an ovophilic maternal mouthbrooder, the reproductive mode shared across the Barombi Mbo flock and across the oreochromine cichlids from which it descends: the female takes the fertilised eggs into her mouth and broods the developing embryos and larvae there until they are free-swimming. Clutch size has not been measured for the mongo specifically; congeneric and related maternal mouthbrooders of comparable size typically carry only modest broods, on the order of a few dozen to perhaps a hundred eggs, the small clutch being the trade-off for the protection oral incubation provides. Although small numbers of mongo have been exported for the aquarium trade in Europe and North America, there is, to the best of researchers' knowledge, no record of the species ever having been bred successfully in captivity (Musilová et al., 2014). This failure to reproduce in aquaria is one reason proposals to safeguard the species through a captive-bred 'ark' population have so far come to nothing, and it leaves the wild crater lake as the only place the fish is known to reproduce.
In the aquarium
Stomatepia mongo is essentially not an aquarium fish in any normal sense. It is critically endangered, exceptionally rare even in its home lake, and has only ever reached hobbyists as a trickle of wild-caught exports; it has never been reliably bred in captivity, so any specimen in a tank is taken directly from a fragile, single-lake population. For these reasons it cannot be recommended as a fish to seek out, and the realistic 'husbandry' note is a conservation note: this is a species to leave in the wild. For the handful of public aquaria and specialist breeders who have worked with the genus, the requirements that can be inferred from the lake are warm water around 24-28 degrees C, near-neutral to mildly alkaline pH (roughly 7.0-8.0) and soft, well-oxygenated conditions, with a sandy bottom, scattered rockwork and dim lighting to suit a shy, bottom-oriented, possibly nocturnal predator that forages over sand and around stones. It should be offered meaty live and frozen foods in keeping with its diet of small invertebrates and fish. The fish is reported to be relatively peaceful and not destructive toward plants. But the central, honest point stands: with no established captive breeding and a wild population that may number only a few thousand individuals, the mongo belongs to the category of fish whose best home is the lake it evolved in, and whose appearance in the trade is a warning sign rather than an opportunity.
Conservation
The IUCN Red List assesses Stomatepia mongo as Critically Endangered under criteria B1ab(iii)+2ab(iii), assessed on 16 February 2009 by T. Moelants, with the population trend listed as unknown. The listing reflects the species' confinement to a single tiny water body of well under 10 square kilometres and the ongoing degradation of that habitat. Its history is a cautionary tale: never recorded as abundant, it was represented by only a handful of specimens in the detailed surveys of the early 1970s, was 'not seen in catches for last two years' by 1988, and was repeatedly speculated to be the first West African cichlid to go extinct (Reid, 1990, 1991; Harrison & Stiassny, 1999). Musilová and colleagues (Copeia, 2014) reviewed more than 180 records and confirmed the fish still persists, with a relative abundance probably similar to that in 1972, but it remains by a wide margin the rarest of all the Barombi Mbo cichlids. The threats are those facing the whole lake, which sits beside the expanding town of Kumba: fishing, agriculture with its attendant deforestation and shoreline erosion, eutrophication from nutrient- and sewage-laden inflow streams (the lake is now classed as mildly eutrophic and supports macroalgae blooms), and damming of the outflow. A more insidious danger is the lake's volcanic setting: like the notorious lakes Nyos and Monoun elsewhere in Cameroon, Barombi Mbo can release dissolved gas or experience sudden upwelling of anoxic deep water, and fish kills tied to such events were reported around 2007 and 2009. In so small and stratified a lake, any one of these pressures could push an already scarce endemic over the edge, which is why the mongo continues to be treated as critically endangered.