Taxonomy & naming
Mormyrops masuianus was described by the Belgian-British ichthyologist George Albert Boulenger in 1898. It belongs to the family Mormyridae within the order Osteoglossiformes, the bonytongues — an ancient teleost lineage that also includes the arowanas, the arapaima, the African butterflyfish and the featherback knifefishes of Asia and Africa, and that takes its name from a toothed bone on the tongue that opposes teeth on the roof of the mouth. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats Mormyrops masuianus as a valid species.
The genus name Mormyrops combines the Greek mormyros, an old name for an unrelated marine fish, with ops, "face" or "appearance" — a genus name built by analogy rather than by any direct resemblance. The species epithet masuianus is a Latinized honorific for Jean-Baptiste Théodore Masui, a Belgian army officer active in the Congo Free State around the time the species was described. Mormyrops is a large genus of superficially similar African mormyrids; because many species share the same blunt-headed body plan, they are often reliably told apart only by the waveform of their electric-organ discharge rather than by external appearance.
Morphology
Mormyrops masuianus reaches a recorded maximum of about 16.1 in total length, making it one of the larger species in a genus that already runs bigger than most other African mormyrids. The body is elongate and moderately compressed, with a blunt, rounded head that lacks the extended chin appendage seen in Gnathonemus; the mouth sits low and small, typical of a fish that does not rely on a probing organ to find food. As in other mormyrids the skin is smooth, and the dorsal and anal fins sit far back on the body, close to a slender caudal peduncle.
The defining feature is not visible at all: a specialized electric organ near the base of the tail generates a continuous, low-voltage pulse-type electric-organ discharge, and the skin is studded with electroreceptor organs that read the resulting field as it is distorted by nearby objects, prey and other fish. This electrolocation sense lets the fish navigate and forage in the dark, turbid water of the Congo by night. Mormyrids are also known for carrying one of the largest brain-to-body size ratios of any fish, with an enlarged cerebellum devoted to processing the constant stream of electrosensory information — a genuine "thinking fish" in terms of dedicated neural tissue, whatever its plain outward appearance.
Habitat
Mormyrops masuianus is native to the lower and middle Congo River basin, recorded from the Democratic Republic of the Congo, the Republic of the Congo and the Central African Republic. It is a freshwater, bottom-associated (demersal) fish of tropical river and floodplain habitat, part of the Congo basin's exceptionally diverse mormyrid fauna.
Water-quality studies from the Congo basin's floodplain rivers report temperatures around 79–84 °F, a pH of roughly 6.1–7.3, and very soft, dilute water with conductivity in the range of about 30–90 µS/cm, alongside dissolved oxygen that can drop to roughly half of saturation. This is the dim, tannin-stained, mineral-poor water typical of the region — the kind of low-visibility environment in which an electric sense is more useful than eyesight.
Feeding
Specific dietary data for Mormyrops masuianus is sparse, but the genus Mormyrops includes some of the larger, more predatory mormyrids in the family, several of which are known to take other fish alongside aquatic invertebrates, unlike the smaller, strictly invertivorous genera such as Marcusenius or Campylomormyrus. As a nocturnal forager it most likely hunts using its electric sense to detect prey hidden in the substrate or moving nearby after dark, rather than by sight.
In the aquarium, mormyrids are fed live or frozen meaty foods — bloodworm, blackworm, brine shrimp and tubifex are all reported to be readily taken — offered two to three times a day, with an emphasis on the evening feeding to match natural nocturnal activity. Prepared or flake foods are generally ignored.
Mating
Mating in Mormyrops masuianus, as in mormyrids generally, is poorly documented. The electric-organ discharge is known to carry information used in species and sex recognition, and it is thought to play a role in courtship, but the details of how a discharge pattern translates into pairing behavior have not been worked out for this species in the literature consulted here.
Wild mormyrids are believed to be seasonal spawners cued to the flood pulse of their rivers, timing reproduction to rising water levels and the drop in conductivity that follows a flood. Some mormyrids are thought to deposit eggs among flooded vegetation and to guard them for a period, but there is no specific confirmed account of nesting or parental behavior for Mormyrops masuianus itself.
Breeding
Mormyrops masuianus has no confirmed record of captive breeding. Aquarium sources describe breeding in mormyrids generally as extremely difficult, requiring keepers to simulate a seasonal flood — raising the water level and diluting the tank to lower conductivity — to have any chance of triggering spawning, and even then success is rare and reported mostly for smaller, better-studied genera rather than for Mormyrops.
Part of the difficulty is that mormyrids appear to recognize sex through their electric-organ discharge rather than through visual cues, which makes deliberately pairing fish close to impossible for a keeper working from appearance alone. For practical purposes, Mormyrops masuianus in the trade is wild-collected, and captive reproduction should be treated as essentially unachieved rather than merely difficult.
In the aquarium
Mormyrops masuianus is a large-growing mormyrid and needs to be sized for accordingly — a tank of at least 39.4 in in length is recommended, with total-volume guidance for mormyrids in this size class running from roughly 50–74 US gal as a minimum for one fish. Water quality has to be kept pristine: mormyrids are notably intolerant of ammonia, nitrite and accumulated organic waste, and they do best in soft, slightly acidic to near-neutral water (a pH of roughly 6.5–7.4 is typical guidance) at warm, stable temperatures matching the Congo basin's roughly 79–84 °F.
Provide dark refuges — caves, driftwood tangles, dense planting or shadowed corners — since this is a nocturnal fish that shelters by day and becomes active after the lights go down; a dim tank with a soft substrate suits it better than an open, brightly lit one. Medications need particular care: mormyrids are reported to be sensitive to copper-based and other caustic treatments, so any medication should be dosed conservatively and only when necessary.
Because sex cannot be told by eye, pairing is unreliable, and mormyrids can be aggressive toward their own kind in the confines of a tank, Mormyrops masuianus is usually best kept singly or in a very large tank with plenty of visual breaks between individuals rather than as an assumed peaceful group. It is not a fish for the general community: tankmates should be robust, similarly water-tolerant species that will not compete with it for cover or be small enough to be mistaken for food given its adult size.
Conservation
The IUCN Red List has assessed Mormyrops masuianus as Least Concern, an assessment reported as dating to 2009. It is known from a reasonably wide stretch of the lower and middle Congo basin across three countries, and there is no evidence of a population decline steep enough to warrant a higher threat category.
As with much of the Congo basin's ichthyofauna, the longer-term outlook depends on the health of the river system itself — water quality, sediment loads and habitat connectivity across a basin under increasing pressure from deforestation, mining and dam development. Least Concern reflects the fish's current status, not a guarantee against future habitat change.