Taxonomy & naming
Marcusenius gracilis was described by the German mormyrid specialist Bernd Kramer in 2013, in the journal Marine and Freshwater Behaviour and Physiology (46(2):105–133), from the Comoé River near Kafolo in Côte d'Ivoire — the type locality recorded as "Mouches Tsété" on that river. It sits in the family Mormyridae, within the subfamily Mormyrinae, and in the order Osteoglossiformes, the bonytongues: a scattered, deep-time radiation of fishes found on four continents (South America, Africa, Southeast Asia and Australasia), united not by any shared modern habitat but by a toothed bone on the tongue that bites against the roof of the mouth, and counted among the most ancient lineages of living teleosts. Mormyridae is an exclusively African family, and every member is weakly electric; that electrosensory system evolved entirely independently of, and is unrelated to, the electric knifefishes of South America (order Gymnotiformes) — the two lineages arrived at similar solutions to living in dark water on opposite sides of the Atlantic, a case of convergence rather than kinship.
The name has not stood unchallenged. In 2022, Ronald Fricke and Bernd Kramer himself found that Marcusenius gracilis Kramer, 2013 is a junior secondary homonym of an older name, Marcusenius senegalensis gracilis Pellegrin, 1922 — a subspecies name that, under the rules of zoological nomenclature, blocks the later species-rank name once both are treated within the same genus. They proposed a replacement name, Marcusenius elegans Fricke & Kramer, 2022, which is now the currently accepted valid name for this fish. This account is filed under the original combination, Marcusenius gracilis, but readers should understand that the homonymy is a nomenclatural bookkeeping correction rather than any change in the fish's actual identity or biology.
The genus Marcusenius holds around 35 recognized species spread across most of tropical Africa, from West Africa to the Nile and southern Africa, and commemorates the nineteenth-century naturalist Johann Marcusen. The species epithet gracilis is Latin for "slender", describing the fish's slim body relative to other Mormyrinae.
Morphology
FishBase describes Marcusenius gracilis as a slender-bodied mormyrid with notched teeth, reaching a recorded maximum of about 8.7 in measured as standard length — a modest size within a family that ranges from a few centimetres up to the metre-plus giants of the genus Mormyrus. As in other members of the subfamily Mormyrinae, the body is laterally compressed, with a single soft dorsal fin set opposite a long-based anal fin and a deeply forked tail on a slender caudal peduncle.
Near the base of the tail sits the electric organ, modified muscle tissue that produces a weak, pulse-type discharge unique in waveform to the species — used both to electrolocate prey and obstacles in dark water and to signal identity to other mormyrids. Mormyrids are noted, size for size, for some of the highest brain-to-body ratios of any fish, the cost of running the enlarged cerebellum that processes this constant stream of electrosensory information. Beyond the original description, detailed morphological study of M. gracilis is thin, and its resemblance to other small West African Marcusenius means it is best confirmed, where possible, by its electric organ discharge rather than by appearance alone.
Habitat
Marcusenius gracilis is known from the Comoé River in Côte d'Ivoire, a West African river system that rises in Burkina Faso and drains south through Comoé National Park to the Gulf of Guinea. FishBase records it as a freshwater, benthopelagic fish of this drainage; data on its range beyond the type-locality area is sparse, and the species may prove to be a narrow West African endemic.
West African mormyrids typically inhabit dim, tannin-stained rivers and floodplains — muddy, vegetated, low-visibility water where an electric sense outperforms eyesight — and M. gracilis is presumed to share that general habitat, though it has not been directly documented for this species. General care literature for the family describes warm water, around 75–82 °F, that is soft and ranges from neutral to slightly acidic, consistent with lowland West African river conditions.
Feeding
No species-specific feeding study exists for Marcusenius gracilis, but as in other mormyrids it almost certainly forages along the riverbed for small aquatic invertebrates — insect larvae, worms and tiny crustaceans — locating them by electrolocation rather than sight in the dim water of its habitat.
General care guides for Mormyridae recommend live or frozen foods — mosquito larvae, bloodworm and Daphnia — as the staple diet for any elephantnose kept in captivity, offered mainly after dark to match the family's nocturnal foraging habit. No aquarium-specific feeding record exists for M. gracilis itself, given how rarely, if ever, it has reached the hobby under either its original name or its current valid name.
Mating
As in other mormyrids, the species-specific electric organ discharge is presumed to play a central role in mate recognition and courtship in Marcusenius gracilis, letting individuals identify suitable partners of their own kind in water too dim or turbid for visual cues to work reliably; in crowded conditions the discharge is also thought to help fish avoid jamming one another's signals. Beyond that broad family pattern, no courtship behaviour has been directly documented for this species.
Nothing has been recorded about the seasonal timing of reproduction in M. gracilis specifically. Mormyrids in general are thought to be seasonal, flood-cued spawners tied to the West African rainy season, but this has not been confirmed for this species.
Breeding
Breeding in Marcusenius gracilis is essentially undocumented, which is true of mormyrids generally: captive breeding is rarely if ever achieved for the family, and reproductive biology in the wild remains poorly understood even for well-known species. No aquarium spawning of M. gracilis has been recorded.
What little is known of mormyrid reproduction more broadly suggests some species scatter eggs among flooded marginal vegetation during seasonal high water, with a degree of egg-guarding suspected in a few better-studied elephantnoses — but none of this has been confirmed for M. gracilis, and the honest position is that its breeding mode, egg description and any parental care remain unrecorded.
In the aquarium
Marcusenius gracilis has no established presence in the aquarium trade under either its original name or its current valid name, Marcusenius elegans, and no confirmed English common name exists for it. Anyone encountering a fish offered under this name should treat general Mormyridae care as an untested starting point rather than as species-verified guidance.
That general care is worth stating plainly: a dimly lit tank with plenty of cover — roots, driftwood, caves — a soft substrate the fish can probe without injuring its sensory organs, and warm, stable water around 75–82 °F that is soft and neutral to slightly acidic. Mormyrids are notoriously sensitive to poor water quality and to many medications, so any treatment should be researched as mormyrid-appropriate and dosed conservatively; the electric organ discharge is also disrupted by electrical noise and reflective surfaces, and mormyrids can be intolerant of their own kind in close confinement, so a spacious setup with visual breaks matters if more than one is housed together.
Conservation
The IUCN Red List has not published an assessment for Marcusenius gracilis, and no listing was located for its replacement name, Marcusenius elegans, either. The status here is recorded as Not Evaluated (NE), which is common for narrow-range or little-studied African mormyrids rather than any indication the species is doing poorly.
There is no evidence in the available sources of a specific conservation concern for this fish or the Comoé River basin, but data on small, little-studied West African mormyrids like this one is generally thin. The general pressures on West African freshwater systems — habitat degradation, sedimentation and water-quality decline from agriculture and development — remain the relevant long-term backdrop even absent a formal assessment.