Taxonomy & naming
Mormyrus macrophthalmus was described by the English zoologist Albert Günther in 1866. It belongs to the family Mormyridae within Osteoglossiformes, the bonytongue fishes, an ancient order whose living members are scattered across South America, Africa, Southeast Asia and Australasia and that takes its name from a toothed basihyal bone at the base of the tongue, which bites against toothed plates on the roof of the mouth. Mormyrus is the largest genus in the family, and many of its species are difficult to separate on external appearance alone; ichthyologists increasingly rely on the shape of each species' electric organ discharge (EOD) as a reliable, largely non-invasive way to tell look-alike mormyrids apart.
The genus name Mormyrus is borrowed from mormyros, an ancient Greek name originally applied to an unrelated Mediterranean sea bream; early naturalists reused it for these African freshwater fish. The species epithet macrophthalmus is Greek for "large-eyed" (makros, large + ophthalmos, eye), describing the species' comparatively large eyes within the genus. FishBase and GBIF both carry Mormyrus macrophthalmus as the current valid combination.
Morphology
Mormyrus macrophthalmus has the classic mormyrid silhouette: a laterally compressed, elongated body, a small, near-terminal mouth set beneath a bluntly rounded snout, a single soft dorsal fin set well back on the body, and a deeply forked tail on a narrow caudal peduncle. FishBase lists a maximum length of about 11.8 in, among the larger species in the genus, though most individuals seen are smaller. As the name suggests, the eyes are proportionately large for a Mormyrus.
The feature that matters most is internal and invisible at a glance: a specialised electric organ near the base of the tail generates a weak, species-specific pulse-type electric organ discharge (EOD), sensed through electroreceptors spread across the skin. Mormyrids as a family have among the highest brain-to-body mass ratios of any fish, built around a greatly enlarged cerebellum that processes this electrosensory stream — the reason they are sometimes nicknamed "the thinking fish." The discharge lets the fish electrolocate objects, prey and obstacles in dark or turbid water, and it also carries information other mormyrids can read, making it a channel for communication as well as navigation.
Habitat
The species is native to West-Central Africa, recorded from the Niger, Volta and Bénoué (Benue) river basins. FishBase classifies it as a freshwater, demersal, potamodromous fish — one that stays within fresh water and migrates internally within river systems, likely tied to seasonal flood cycles rather than moving to the sea.
Detailed water-chemistry data for this species is sparse in the available literature. West African savanna rivers of this kind generally run warm year-round, in roughly the mid-20s to high-20s °C, over sand, mud and submerged wood, often turbid with sediment or stained by dissolved organics — exactly the low-visibility conditions a fish that navigates by electrolocation is built for.
Feeding
Mormyrus macrophthalmus is a nocturnal to crepuscular forager, most active around dusk and after dark, which is typical for the genus. It hunts along the bottom for aquatic invertebrates, using its electrosense to detect prey hidden in sediment or among cover rather than relying on vision in the dim water it lives in.
In captivity, mormyrids in general are reluctant to take dry foods and are best offered live or frozen invertebrate foods — bloodworm, other insect larvae and small crustaceans — matching the diet reported for the species in the wild. Feeding after the lights go down suits its natural rhythm and reduces competition from more visually oriented tankmates.
Mating
Courtship and spawning behaviour in Mormyrus macrophthalmus has not been documented in the sources available for this account. Across the mormyrid family, reproduction is generally tied to the flood season, when rising water opens up flooded margins and vegetation; the pulse-type EOD is thought to play a role in species recognition and courtship signalling between potential mates, as it does in the better-studied mormyrids, but the specific courtship sequence for this species is not recorded.
Data is sparse enough that no reliable account of pairing behaviour, courtship displays or spawning triggers specific to this species can be given here.
Breeding
Mormyrus macrophthalmus is essentially not bred in aquaria, and its reproductive biology remains poorly known even in the wild. Mormyrids as a group are thought to be seasonal, flood-triggered spawners, but published detail on egg deposition, incubation and any parental care is scarce for most species, this one included.
Given how rarely the species reaches the aquarium trade at all, captive spawning reports do not exist in the literature reviewed here. Anyone keeping it should expect to enjoy the fish for what it is rather than plan for breeding.
In the aquarium
This is not a species most aquarists will ever be offered; it is rarely exported and seldom appears in the trade. Where it is available, care follows general mormyrid practice: a large, mature aquarium (a minimum footprint on the order of 59.1 in has been suggested), dim lighting, soft substrate the fish can probe without injury, and plenty of caves, wood or dense planting for daytime cover, since it is most active after dark.
Mormyrids are scaleless-skinned and notably sensitive to medications containing copper salts, so treatments need to be chosen and dosed with care. They are also intelligent, socially complex fish that can be aggressive toward their own kind, particularly in the confines of an aquarium, so stocking more than one individual needs caution and a large tank; a single specimen kept with calm, similarly sized dither fish is the more reliable approach. Stable, clean, well-filtered water suits the species better than any specific hard numbers, given how little is published on its exact requirements.
Conservation
The IUCN Red List assesses Mormyrus macrophthalmus as Least Concern, an assessment dated 2019. It is known from multiple river basins across West-Central Africa rather than a single restricted locality, which supports the non-threatened rating.
As with many wide-ranging West African river fish, there is no specific, well-documented threat singled out for this species in the sources reviewed; general pressures on West African freshwater systems — damming, sedimentation and water abstraction — are relevant to the region's fish communities as a whole, but data tying them to this particular species' population trend is sparse.