Taxonomy & naming
Marcusenius macrophthalmus was described by the French ichthyologist Jacques Pellegrin in 1924, with the type locality given as the Kasaï River at Tshikapa, in the middle Congo River basin of the Democratic Republic of the Congo. It sits in the family Mormyridae — the elephantnoses and mormyrids — within the order Osteoglossiformes, the bonytongues, one of the most ancient surviving lineages of teleost fish, named for a toothed bone on the floor of the mouth that bites against toothed plates on the roof of the mouth. The genus Marcusenius is itself one of the largest mormyrid genera in Africa, holding several dozen species scattered across river systems from West Africa to the Zambezi, many of which look almost identical and are confidently distinguished only by their species-specific electric-organ discharge (EOD).
The parenthetical authority "(Pellegrin, 1924)" signals that the species has since been moved from its original genus combination, a common history among mormyrids as the group's taxonomy has been repeatedly revised. The epithet macrophthalmus is built from the Greek makros, "large", and ophthalmos, "eye" — "large-eyed" — a fitting name for a mormyrid whose eyes appear proportionally large for the genus, even though electrolocation, not eyesight, does most of the real work of navigating its dim home water.
Morphology
FishBase records a maximum size of 4.9 in standard length for Marcusenius macrophthalmus, making it a small mormyrid even within a genus that rarely produces giants. Like other Marcusenius it almost certainly follows the family's typical build: a compressed, elongate body, a long, low dorsal fin set well back over the body, a similarly positioned anal fin, and a forked tail leading into a slender caudal peduncle that houses the weak electric organ used for electrolocation and communication.
As its epithet suggests, the eyes are notably large relative to the head — a trait shared by several dim-water mormyrids that still rely on some vision alongside their electric sense. Colouration in Marcusenius is generally understated, silvery-grey to brownish tones with little in the way of bold pattern, which fits an animal that identifies conspecifics and reads its surroundings mainly through its electric-organ discharge rather than through sight. Beyond size and eye proportion, detailed fin-count and colour-pattern data specific to M. macrophthalmus are not documented in the sources consulted for this account, so the description here stays close to what is established rather than filling gaps with assumption.
Habitat
Marcusenius macrophthalmus is known from the Kasaï River at Tshikapa, in the middle Congo River basin of the Democratic Republic of the Congo — a freshwater, demersal, tropical fish living near the river bottom rather than in open water. The Kasaï is a major southern tributary of the Congo, and Tshikapa sits well upstream in a stretch of river shaped by the seasonal floodplain inundation typical of the wider basin.
Congo basin rivers of this kind typically run soft and low in dissolved minerals, generally acidic to neutral, and often tannin-stained and dim — the classic "blackwater" character found across much of central Africa. That dim, sediment-laden water is exactly the setting in which a weakly electric fish's sense pays off: electrolocation reads the riverbed and finds prey where eyesight alone would struggle. Precise pH and temperature figures specific to this species have not been documented in the sources available here, so those numbers should be read as regional context rather than a measured species profile.
Feeding
No species-specific feeding study exists for Marcusenius macrophthalmus in the sources consulted, but the general pattern for Marcusenius and Mormyridae more broadly is well established: these are benthic invertebrate feeders, taking insect larvae, small worms and other soft prey gleaned from the substrate rather than crushed with strong jaws.
A small, weakly toothed mouth — typical of the genus — suits picking at soft-bodied prey rather than tackling anything hard-shelled, and foraging is guided as much by electrolocation as by sight, particularly in the dim, turbid water of its home river and especially after dark. Aquarium accounts of the genus point to sinking foods such as bloodworm and other small frozen invertebrates offered where a weak-jawed, electrically-oriented fish can find them without competition, though no husbandry record exists for this species specifically.
Mating
Courtship and mate recognition in mormyrids like Marcusenius macrophthalmus is understood to run substantially through the electric-organ discharge: each species produces its own characteristic EOD waveform, and where several similar-looking Marcusenius share a river, that signal — more than colour or shape — is thought to matter more for finding and recognising appropriate mates than visual cues do.
Beyond that general, family-level pattern, the courtship behaviour of this particular species has not been documented. African mormyrids as a group are believed to be seasonal, rain-triggered spawners tied to rising water and floodplain inundation, but applying that broad pattern to M. macrophthalmus specifically would be guesswork rather than fact, so this account stops at what is actually known: essentially nothing species-specific.
Breeding
FishBase notes that breeding in Marcusenius macrophthalmus is poorly documented and that the species has not been bred in captivity — consistent with mormyrids as a whole, among which reliable, repeatable aquarium spawning is the exception rather than the rule across the entire family.
What little is known or suspected about mormyrid reproduction in general — seasonal spawning tied to the rains, an electric-organ discharge that likely plays some courtship role, and scattered reports of eggs being laid or loosely guarded among vegetation — remains poorly studied at the family level and should not be presented as established fact for this particular species. The honest summary is that M. macrophthalmus's breeding biology is essentially unknown and unlikely to be resolved outside a dedicated field study, since the species barely if ever reaches the aquarium trade.
In the aquarium
Marcusenius macrophthalmus is not a species with any real presence in the ornamental trade, and no species-specific aquarium account exists in the sources consulted — what can be said here draws on general Mormyridae and Marcusenius husbandry rather than direct experience with this fish. At a modest 4.9 in, it would not need an enormous tank, but general mormyrid guidance still applies: soft, dim, tannin-influenced water rather than bright, hard-water conditions, plenty of cover from roots, driftwood and leaf litter, and — importantly — being kept in a group rather than alone, since a solitary mormyrid confined with few other fish it can "see" electrically is prone to stress and aggression.
Mormyrids as a family have a reputation for sensitivity: they generally cope poorly with sudden swings in water quality, are intolerant of nitrogenous waste, and are commonly reported to react badly to some medications, so any dosing should be conservative and used only when necessary. Given how often Marcusenius species are confidently distinguished only by electric-organ discharge, a fish offered under this name in the trade should be treated with some caution about its true identity unless the source is reliable. This is, in short, a specialist's fish rather than a community staple, and one that would arrive in a tank with almost no prior husbandry record to draw on.
Conservation
The IUCN Red List assesses Marcusenius macrophthalmus as Data Deficient (DD), an assessment FishBase dates to 2009 — meaning there is not currently enough information on its population, distribution or trends to support a full conservation status. This is a common outcome for narrow-range African mormyrids, whose EOD-dependent identification and remote river habitats make them difficult to survey.
The species' only documented locality is the Kasaï River at Tshikapa in the Democratic Republic of the Congo, and no broader distribution has been established in the sources consulted, so it may be a genuine narrow-range endemic or simply under-recorded — the Data Deficient rating reflects that uncertainty rather than any confirmed absence of threat. There is no CITES listing for this species. Congo basin rivers generally face pressures from sedimentation, deforestation, and in places mining and overfishing, but without population data specific to M. macrophthalmus it is not possible to say how directly those pressures bear on this particular fish.