Taxonomy & naming
Stomatorhinus microps was described by the Belgian-British ichthyologist George Albert Boulenger in 1898, in a paper on new Congo fishes published in the first volume of the Annales du Musée du Congo. It belongs to the family Mormyridae — the elephantnoses and mormyrids — within the order Osteoglossiformes, the bonytongue fishes, one of the oldest surviving lineages of teleosts, named for a bony, tooth-bearing structure on the tongue that bites against toothed plates on the roof of the mouth. Eschmeyer's Catalog of Fishes lists Stomatorhinus microps as a currently valid species, with no synonyms recorded.
The type locality is the rapids of the Lower Congo River near Bulu, close to 5°1'S, 14°1'E, and the syntypes are held at the Royal Museum for Central Africa in Tervuren, Belgium (MRAC 68 and MRAC 94). Boulenger placed the species in Stomatorhinus, a genus he had erected for a group of small, blunt-snouted Central African mormyrids set apart by a small mouth positioned low and near the tip of the snout rather than the elongated chin barbel — the schnauzenorgan — of the related genus Gnathonemus.
Morphology
No maximum-length measurement specific to Stomatorhinus microps has been published in the sources reviewed for this article. Related species in the genus range from roughly 2 in standard length in Stomatorhinus ivindoensis to considerably larger forms, but no measured figure for S. microps itself has been located; like most of its genus it is presumed to be a small fish, under about 4 in.
The species is best known for its eye — reduced to about 6 percent of head length and roughly 17 percent of the distance between the eyes, and partly covered by overlying skin. That is far smaller than the eye of any other described Stomatorhinus and is the character the name microps ('small-eyed') refers to directly. As in every mormyrid, an electric organ near the base of the tail produces a weak, pulsed discharge that the fish reads through electroreceptors spread across its skin; research on the genus has further found that Stomatorhinus carries an electrosensory lateral line lobe with only two processing zones, where every other mormyrid studied has three, a difference linked to a class of sensory afferent that is present but not connected into the circuit. No discharge recording or other electrophysiological measurement specific to S. microps has been published, and it is not established whether the reduced eye and the altered electrosensory anatomy reported for the genus are directly connected in this species.
Habitat
Stomatorhinus microps is known only from the Lower Congo River in the Democratic Republic of the Congo, recorded from rapids on both banks below Pool Malebo (Stanley Pool), including the type locality near Bulu. No occurrence outside this stretch of river has been documented.
The species lives deep within the river's whitewater rapids, a fast, turbulent reach of the Congo that drops through a maze of channels, boulders and submerged rock in near-total darkness — a setting researchers studying the Lower Congo's fish fauna have likened to a subterranean, cave-like environment despite lying entirely within open river. That permanent darkness, rather than any actual cave, appears to be the pressure behind the eye reduction that gives the species its name. Water temperature and chemistry specific to S. microps have not been published.
Feeding
No diet study specific to Stomatorhinus microps has been located. Mormyrids as a group are bottom-foraging invertebrate feeders, using electrolocation to find insect larvae, worms and small crustaceans hidden in mud, silt and rock crevices before taking them with a quick strike of the small, downturned mouth, and S. microps is presumed to feed the same way — though that has not been confirmed for this species, and the turbulence and darkness of its rapids habitat make direct observation difficult.
Mating
Courtship and mate-recognition behavior have not been described for Stomatorhinus microps. In mormyrids generally, the electric organ discharge is thought to carry more than navigational information — its pulse pattern is often species-specific, and in some genera sex-specific, and is believed to play a part in mate recognition and shoal cohesion. Whether that holds for S. microps, whose genus is already known to differ from other mormyrids in how it processes electrosensory information, is unknown; no discharge recording or courtship observation for the species has been published.
Breeding
There is no published record of Stomatorhinus microps breeding, in the wild or in captivity. Spawning behavior, egg or larval description, and parental care are all unrecorded for this species. Breeding biology is poorly documented across Mormyridae even in well-studied genera, and for a fish confined to the interior of Congo River rapids, a species-specific account would be invention rather than fact.
In the aquarium
Stomatorhinus microps has no documented presence in the aquarium trade. Its habitat argues against it ever developing one: the species lives deep inside turbulent Lower Congo rapids in permanent darkness, a niche with no practical route to collection or shipping, and nothing in the sources reviewed for this account describes a keeping report, import record, or husbandry note for this fish.
What follows is general practice for small mormyrids rather than any experience specific to this species. The family as a whole needs a soft, fine substrate to probe with the mouth, ample cover, dim lighting and stable, low-turbidity water, and mormyrids are known for unusual sensitivity to poor water quality and to a number of common aquarium medications, which argues for cautious acclimation and conservative dosing whenever one needs treatment. They typically refuse dry food and need live or frozen invertebrate foods such as bloodworm and brine shrimp, offered after dark to match a nocturnal foraging habit. None of that has been tested against Stomatorhinus microps, which remains a museum and research species rather than an aquarium one.
Conservation
The IUCN Red List assesses Stomatorhinus microps as Vulnerable, an assessment carried in the 2020 update of the Red List. The listing reflects a range restricted to a single stretch of one river system — the rapids of the Lower Congo below Pool Malebo — rather than any documented population decline.
Proposed hydropower development on the Lower Congo, including the long-planned Grand Inga project, is widely cited as a threat to the rapids' unusual and highly localized fish fauna in general, though the sources reviewed for this article do not detail the specific reasoning behind this species' assessment. A single-river endemic confined to a narrow and unusual habitat of this kind carries an inherent vulnerability to any change in flow, sediment, or water clarity along that stretch of river.