Taxonomy & naming
Mormyrops microstoma was described by the Belgian-British ichthyologist George Albert Boulenger in 1898. It sits in the family Mormyridae within the order Osteoglossiformes, the bonytongues — an ancient lineage of freshwater fishes present today on four continents (South America, Africa, Southeast Asia and Australasia) that diverged from other teleosts very early in fish evolutionary history. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the standing authority for the valid name.
The genus name Mormyrops combines Mormyrus, the mormyrid type genus (itself derived from mormyros, an ancient Greek name for a marine bream the fish were thought to resemble), with the Greek suffix -ops, "face" or "appearance" — so Mormyrops means roughly "Mormyrus-like". The species epithet microstoma is built from the Greek mikros, "small", and stoma, "mouth", describing the comparatively small, low-slung mouth that distinguishes this species and several relatives from larger-mouthed Mormyrops such as M. anguilloides. Mormyrops is one of about twenty genera in the Mormyridae, a family confined to Africa and the Nile system.
Morphology
Like other mormyrids, Mormyrops microstoma has an elongated, laterally compressed body, a single soft dorsal fin set well back over an equally long anal fin, and a slender caudal peduncle carrying the electric organ near the tail. FishBase records a maximum size of about 6.5 in total length, a modest figure for the genus — Mormyrops anguilloides, the best-known member, grows past a metre, so M. microstoma is one of the small-bodied species within it.
As the epithet indicates, the mouth is small and set low on the head, an adaptation for picking food items off or out of the substrate rather than taking large prey. The eyes are small, typical of a fish that navigates mainly by electrolocation rather than sight. Mormyrids as a group carry an unusually large cerebellum relative to body size — among the highest brain-to-body mass ratios recorded in fishes — which processes the constant stream of information returning from the electric organ discharge (EOD). The EOD itself is species-specific in waveform and is generally the most reliable way to distinguish similar-looking mormyrids in the field, since coloration and shape alone can be ambiguous between species.
Habitat
Mormyrops microstoma is known from the middle Congo River and the Uélé River, a northern tributary that joins the Congo system in the Democratic Republic of the Congo. Both are part of the vast Congo basin, the second-largest river system in the world and a centre of freshwater fish diversity. The species is described as a freshwater, demersal, tropical fish — living near the bottom rather than in open water.
Detailed water-chemistry data for this species has not been published, but it lives in conditions typical of the middle Congo system: warm, tropical rivers running through dense forest, often turbid or tannin-stained and low in dissolved minerals. That kind of dim, sediment-laden water is exactly the setting in which a weak electric sense is most useful, since vision alone is unreliable at more than a short distance. Nothing in the available sources describes population density or habitat preference within the river in more detail, so the picture beyond "middle Congo and Uélé rivers, demersal" remains sparse.
Feeding
FishBase assigns Mormyrops microstoma a trophic level around 3.2, consistent with a carnivore feeding low on the food web — mainly aquatic invertebrates such as insect larvae and other small benthic animals rather than fish. The small mouth points to picking rather than engulfing prey, foraging over sand, mud or leaf litter on the riverbed.
Like other mormyrids it most likely forages primarily after dark, using its electric organ discharge to detect prey items and obstacles the eyes cannot resolve in dim or turbid water — a sense mormyrids rely on generally, though no feeding-behaviour study specific to this species has been located. In captivity, hobbyist guides to Mormyrops as a genus recommend sinking pellets and live foods such as blackworms, offered after the lights go down, mirroring the fish's natural rhythm.
Mating
Courtship and spawning behaviour in Mormyrops microstoma specifically has not been documented. What is known about mormyrids as a family is that the electric organ discharge carries a role in courtship and species recognition — individuals can in principle distinguish potential mates and rivals of the right species by EOD waveform alone, in water too dark or turbid for visual signals to work reliably. Beyond that general pattern, nothing about pair formation, seasonal timing or courtship displays in this species is published.
In the wild, mormyrids as a group are typically seasonal spawners tied to the rains and the resulting rise in river or floodplain water levels, which likely applies to M. microstoma given its Congo basin range, but this has not been confirmed for the species itself.
Breeding
Mormyrops microstoma has essentially never been bred in captivity, and this is consistent with mormyrids generally: they are among the least-understood breeders in the freshwater aquarium hobby. Wild spawning is believed to be seasonal and flood-triggered, and the electric organ discharge likely plays a courtship role, but the detailed mechanics — nest building, egg-laying substrate, fertilisation, and whether either parent guards the clutch — are barely documented for the family and entirely undocumented for this particular species.
Aquarists should not expect to breed this fish. It is kept, when available at all, purely as a display animal; any claim of a reliable captive-breeding protocol for Mormyrops microstoma would be unsupported by the available literature.
In the aquarium
Mormyrops microstoma is a narrow Congo basin endemic assessed as Data Deficient, and it is rarely if ever seen in the aquarium trade; most of what applies to it in captivity is drawn from general Mormyrops and mormyrid husbandry rather than species-specific accounts. A soft sand or fine gravel substrate is recommended for the genus, since these fish probe the bottom with a sensitive, easily damaged snout and mouth. A well-planted tank with caves, wood and other hiding spots suits their preference for cover during the day and activity after dark, and dim lighting is generally better tolerated than bright.
Mormyrops species can be aggressive toward their own kind, particularly in pairs, so tank mates and stocking need care; a single specimen or a larger group in a spacious tank tends to work better than two fish forced together. Feed sinking pellets and live foods such as blackworms, ideally after lights-out. As with mormyrids generally, this fish is scaleless-skinned in the sense that it lacks true fish scales over parts of the body and is sensitive to copper-based medications and to poor water quality generally, so any treatment should be dosed cautiously and water changes kept regular. Given its Data Deficient status and lack of an established trade history, prospective keepers should treat any offered specimen with the caution due an uncommon wild-caught fish rather than an established aquarium staple.
Conservation
The IUCN Red List assessed Mormyrops microstoma as Data Deficient on 16 February 2009, meaning there is not enough information on its population size, trend or the scale of threats it faces to assign a status of Least Concern, threatened, or otherwise. This is common among small, range-restricted mormyrids that have received little targeted study since their original description.
The species is known only from the middle Congo River and Uélé River in the Democratic Republic of the Congo, a large and in parts remote stretch of the Congo basin. General pressures on Congo basin fish faunas — artisanal fishing, deforestation-driven sedimentation, and localised habitat disturbance — could plausibly affect it, but no assessment specific to Mormyrops microstoma has quantified any of this. Its actual conservation status is genuinely unknown rather than reassuring, and it should be reported as such.