Taxonomy & naming
Marcusenius senegalensis was described by the Austrian ichthyologist Franz Steindachner in 1870, from specimens collected at Taoué and Dagana on the Senegal River. It belongs to the family Mormyridae — the elephantfishes, an exclusively African family of weakly electric fishes — within the order Osteoglossiformes, the bonytongues, a scattered, deep-time radiation of fishes found on four continents (South America, Africa, Southeast Asia and Australasia) and united by that toothed lingual bone rather than by any shared modern habitat. The Mormyridae are entirely convergent with, and unrelated to, the electric knifefishes of South America (Gymnotiformes); the two lineages evolved similar electric senses independently, on opposite sides of the Atlantic. Catalog of Fishes (Eschmeyer, California Academy of Sciences) — the standing authority for valid fish names — accepts Marcusenius senegalensis as the valid name.
The genus name Marcusenius honours Dr. Johann Andreas Marcusen (1817–1894), a Latvian physician and ichthyologist. The species epithet senegalensis is a straightforward Latin locality name, "of Senegal", matching the type locality on the Senegal River. Mormyrids as a group are notoriously difficult to tell apart by eye — many species, including several other Marcusenius sharing its West African range, are reliably distinguished only by their species-specific electric organ discharge (EOD) — so field identifications of this genus should be treated with some caution.
Morphology
Marcusenius senegalensis has the general mormyrid build: an elongate, laterally compressed body, a slender caudal peduncle carrying the tail's electric organ, and a small, forked caudal fin. Like its relatives it lacks true dorsal and anal spines — FishBase records 28 to 38 soft rays in a long-based anal fin — and carries a modest, fleshy chin lobe rather than the long, trunk-like Schnauzenorgan that makes some of its relatives in the genus Gnathonemus so distinctive; aquarium accounts describe it probing the substrate with this barbel-like structure while hunting for buried food.
FishBase records a maximum size of about 12.6 in standard length (SL), making it a mid-sized member of the genus; total length, including the tail fin, would run somewhat longer. Aquarium specimens are typically smaller and, as with most mormyrids, are usually imported as juveniles. Colouration follows the typical mormyrid pattern — a plain silvery-grey to brownish body without bold markings — and, as in all mormyrids, an electric organ near the base of the tail generates the weak, pulse-type discharge (EOD) that the fish relies on for electrolocation and communication far more than it relies on eyesight.
Habitat
Marcusenius senegalensis is widespread across the rivers of West Africa, recorded from the Gambia and Senegal basins in the west, east through the Niger and Volta systems, to the Sassandra basin and the Lake Chad basin — a range spanning the Sudanian and Guinean savanna zones. FishBase classifies it as a freshwater, demersal, potamodromous fish: a full migrant that moves within fresh water on a seasonal cycle, following river and floodplain conditions rather than migrating to sea.
Across this range it lives over soft riverbed substrate in warm, typically soft, slightly acidic to near-neutral water — hobbyist accounts describe conditions around 79–82 °F and pH 6.8–7.2 as suiting the species well in captivity, consistent with the dim, tannin- and sediment-stained floodplain water that favours an electric sense over eyesight for finding food and avoiding obstacles.
Feeding
In the wild, Marcusenius senegalensis is an invertivore. FishBase gut-content data show aquatic insects dominating the diet (around 74% by volume), supplemented by detritus, crustaceans and molluscs, with incidental phytoplankton and sand taken up while foraging over the bottom. Like other mormyrids it hunts mainly at dusk and after dark, sweeping soft substrate and probing with its chin for buried prey located by active electrolocation rather than sight.
In the aquarium it takes small live and frozen invertebrate foods readily — bloodworm, mosquito larvae and similar prey — offered over a soft, sandy substrate it can probe undisturbed. Because it forages mainly at night, feeding after the lights go out helps a shy elephantfish get enough to eat in a mixed tank without competing head-to-head with faster, bolder daytime feeders.
Mating
Courtship and mate recognition in mormyrids generally run through the electric organ discharge (EOD) rather than through visual display, since a species- and often sex-specific pulse pattern can serve much the same signalling role that colour or movement plays in other fishes. Detailed observations of courtship in Marcusenius senegalensis specifically were not found in the sources consulted for this account, and mormyrid breeding biology in general remains poorly studied even where the fish themselves are common and widespread.
Mormyrids as a family are thought to be seasonal, flood-triggered spawners, with reproductive activity keyed to the rains and rising water of the West African wet season. Whether M. senegalensis pairs, forms leks, or uses some other courtship structure has not been reliably documented, and the acoustic and electrical noise of a home aquarium likely interferes with EOD-based signalling — one plausible reason natural courtship is essentially never observed in captivity.
Breeding
Marcusenius senegalensis is essentially not bred in the aquarium hobby, and — as with most mormyrids — its reproductive biology is poorly known even in the scientific literature. Wild spawning is believed to follow the same seasonal, flood-triggered pattern thought typical of the family, with the EOD likely playing some role in courtship or species recognition, but details of nest site, egg-laying and any parental behaviour have not been reliably documented for this species. Some mormyrids are thought to guard eggs among vegetation, but this has not been confirmed for M. senegalensis.
Aquarists should not expect to breed this fish, and specimens that reach the trade — where it appears at all — are wild-caught rather than captive-bred. It is a species to watch and appreciate for its electric sense rather than a project fish to pair up for eggs.
In the aquarium
Marcusenius senegalensis is a shy, sensitive fish that rewards patient, careful keeping. Hobbyist accounts recommend warm, soft water around 79–82 °F and pH 6.8–7.2, in a heavily planted aquarium of 53 US gal or more, with subdued lighting, plenty of cover, and a fine sandy substrate the fish can probe with its chin for buried food. Water quality needs to be excellent and stable — mormyrids in general tolerate essentially no ammonia or nitrite — and, like other members of the family, this species is likely sensitive to copper-based medications, so any treatment should be dosed conservatively.
It is solitary to territorial toward its own species and toward other mormyrids, which can trigger aggression via clashing electric discharges, but it is reported as peaceful toward unrelated tankmates. A single specimen, or a properly sized group in a suitably large tank, tends to work better than pairing two individuals in cramped quarters. As with most elephantfishes, it is rarely offered in the trade under its own name and is easily confused with other Marcusenius species, so buyers should not assume a confirmed identification without checking further.
Conservation
The IUCN Red List assesses Marcusenius senegalensis as Least Concern, an assessment dated 2 June 2019. It is described as a full migrant with an unknown population trend, but with a wide distribution across multiple West African river basins that keeps it clear of any elevated threat category.
The species supports a minor commercial fishery across parts of its range and is considered harmless to humans. As with many broadly distributed, lightly studied West African river fishes, the main long-term pressures are generic rather than species-specific — damming, sedimentation and water-quality decline across the rivers and floodplains it depends on — rather than any documented population decline.