Taxonomy & naming
Mormyrus macrocephalus was described by the British ichthyologist Edgar Barton Worthington in 1929, one of a series of East African freshwater fish he named from survey material collected around the Ugandan lakes. It belongs to the family Mormyridae, the elephantnoses, within the order Osteoglossiformes — the bonytongues, an ancient lineage named for a toothed bone on the floor of the mouth that bites against a toothed pad on the roof. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats Mormyrus macrocephalus as a valid species in the genus Mormyrus.
The genus name Mormyrus is a Latinised borrowing of the ancient Greek mormyros, the name of an unrelated Mediterranean sea bream; Linnaeus reused the word in 1758 for the type species of this very different group of African freshwater fish. The epithet macrocephalus combines Greek makros ("large") and kephale ("head"), describing the fish's noticeably big head. Mormyrus is a large genus spread across Africa, and many of its species are superficially similar; identification usually relies on head and mouth proportions, fin-ray counts, and, where it has been recorded, the shape of the electric organ discharge.
Morphology
Like other Mormyridae, Mormyrus macrocephalus has an elongate body, a small, toothless mouth set below a blunt snout, and a slender caudal peduncle carrying the electric organ near the base of the tail. FishBase records a maximum of about 16.5 in total length, a mid-sized fish within its genus, and one distinguished from its relatives by the disproportionately large, blunt head referenced in its name.
Colouration follows the usual mormyrid pattern: a plain silvery-grey to brownish body without bold markings, an elongate dorsal fin set opposite a long-based anal fin, and a deeply forked tail. Eyes are small and vision plays a secondary role. Mormyrids navigate and hunt chiefly by electrolocation, sensing distortions in a weak electric field generated by the tail organ and picked up by electroreceptors across the skin — a sense that lets the fish work effectively in dark or silt-laden water where eyesight is of little use.
Habitat
Mormyrus macrocephalus is known from East Africa, restricted to Uganda: Lakes Kyoga and Kwania, the Victoria Nile, dams in the Teso district, and the Aswa River (FishBase). It is a demersal, tropical freshwater fish living over the bottom of rivers, lakes and connected floodplain waters within the upper Nile system.
As with most of its family, this is a fish of dim, often turbid water, where the electric sense rather than vision does the real work of finding food and avoiding obstacles. Specific temperature and pH figures for this species were not found in the sources reviewed; it should be treated as a typical warm East African lake-and-river fish, and its water should not be assumed to match the soft, acidic conditions of Congo-basin mormyrids without confirmation.
Feeding
Diet shifts with age. Juvenile Mormyrus macrocephalus feed mainly on insects, while adults take a broader mix of small crustaceans and aquatic insect larvae picked from the substrate (FishBase). Unlike Gnathonemus, whose long chin appendage probes deep into sediment, Mormyrus forages with a small, unadorned mouth close to the bottom, guided by electrolocation rather than sight.
In the aquarium, mormyrids generally take bloodworm, white worm, Daphnia and Tubifex readily, and will usually move on to sinking pellet and other frozen foods once settled. Live or frozen food is typically needed to get a newly acquired fish feeding at all.
Mating
Mating behaviour in Mormyrus macrocephalus is poorly documented. FishBase notes only that the species spawns in rocky riverine habitat, with limited data available on size or age at maturity; no detailed account of courtship has been published for this species specifically.
Across Mormyridae more broadly, the pulse-type electric organ discharge is thought to carry a courtship and species-recognition signal, with discharge characteristics shifting during social interaction in some studied species. Whether that applies to Mormyrus macrocephalus is not established — the fine detail of pairing and spawning behaviour in wild Mormyrus remains largely unstudied, and none of it has been confirmed for this species.
Breeding
Mormyrus macrocephalus is essentially not bred in the aquarium hobby. Mormyrids as a group are seasonal, flood-triggered spawners in the wild, and the mechanics of spawning, fertilisation and any parental care remain poorly known for nearly every species in the family, this one included.
What little is documented points to reproduction that is difficult to reproduce in captivity: confirmed captive breeding of Mormyrus species is rare and sparsely recorded. Keepers of this species should treat long-term husbandry and observation as the realistic goal rather than breeding.
In the aquarium
This is a fish for a serious, spacious set-up rather than a casual community tank. General mormyrid care calls for a tank of 59.1 in or more in length, a soft sand or fine-gravel substrate that will not abrade the fish's sensitive skin, dim lighting, and ample cover from wood and rockwork to break sightlines. At a recorded maximum of about 16.5 in, Mormyrus macrocephalus needs correspondingly large, stable, well-filtered water.
Mormyrids are frequently quarrelsome toward their own kind and are usually kept singly or in small groups given enough space and cover for individuals to hold separate areas. They are notably sensitive to copper and to some standard medications, including formalin — a trait shared across the family — so treatments should be dosed conservatively and copper-based products avoided outright. The species uses its electric organ for both electrolocation and communication with tankmates, and poor water quality or stress can affect that sense along with the fish's general condition.
Conservation
The IUCN Red List assesses Mormyrus macrocephalus as Vulnerable, an assessment dated 2021 (2022.2 update) under criteria A2bcd+3bcd, indicating an observed or projected population decline linked to habitat degradation, exploitation or similar pressures. That sets it apart from many wide-ranging Mormyrus species assessed as Least Concern or Data Deficient; confinement to a limited set of Ugandan lakes, rivers and dams leaves it more exposed to localised pressures than continent-spanning relatives.
The sources reviewed here do not break down a specific threat list for this species, but pressures consistent with the listing include habitat change in the Lake Kyoga–Victoria Nile system and general fishing pressure on East African lake and river fish stocks. Aquarium demand for this species appears minimal next to those broader pressures.