Taxonomy & naming
Marcusenius greshoffii was described by the Dutch zoologist Louis Schilthuis in 1891. It belongs to the family Mormyridae, within 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 that toothed lingual bone, and numbered among the most ancient lineages of living teleosts. The Mormyridae are an exclusively African family, every member weakly electric, and entirely convergent with — not related to — the electric knifefishes of South America (order Gymnotiformes), which evolved an equivalent electric sense independently on the opposite side of the Atlantic. The genus Marcusenius holds a large number of similar-looking Congo, Niger and Zambezi basin species, and as in other mormyrids reliable species-level identification often rests on the fish's electric organ discharge (EOD) rather than on external anatomy alone.
The species epithet greshoffii is a Latinised patronym honouring Anton Greshoff (1856–1905), a Dutch trader and collector active in the Congo who presented specimens to the Zoological Museum of Utrecht University, material that fed directly into Schilthuis's original description. The type locality lies within the Congo River system, and the species has since been recorded in the lower Congo and up into the upper Lualaba.
Morphology
Marcusenius greshoffii shows the general mormyrid build: a laterally compressed body, a small, close-set mouth, and a caudal peduncle housing the tail-mounted electric organ that defines the family. FishBase records a maximum size of about 4.3 in total length, making this one of the smaller Marcusenius species — modest even by the standards of a family whose West African members are sometimes traded as "baby whale" mormyrids for their diminutive size.
Detailed published description of colouration and fin proportion for this species is thin in the sources consulted here, and — as with other mormyrids — visual identification among sympatric Marcusenius is notoriously unreliable; the Malebo Pool alone holds seven co-occurring Marcusenius species that are best told apart by their electric organ discharge rather than by eye. As in other mormyrids, an outsized cerebellum drives one of the highest brain-to-body ratios recorded in any vertebrate, the physiological cost of running a sensitive electrolocation and communication system built around that tail organ.
Habitat
Marcusenius greshoffii is a Congo River basin endemic, recorded from the lower Congo up into the upper Lualaba, with confirmed occurrences in the Central African Republic, the Republic of the Congo and the Democratic Republic of the Congo. It is well documented from the Malebo Pool, the wide, slow-flowing stretch of the lower Congo near Kinshasa and Brazzaville, where it is a freshwater demersal (bottom-dwelling) species living over sand, clay, mud and accumulated plant debris, favouring open bottom away from dense mats of floating vegetation such as water hyacinth (Eichhornia) and floodplain grasses (Echinochloa).
Congo basin water in this stretch runs warm, soft and only mildly acidic to neutral — reported aquarium-care parameters for the species and its close relatives put temperature around 79–82 °F and pH roughly 6.8–7.2 — precisely the dim, sediment-laden conditions in which an electric sense outperforms eyesight. This is soft, tannin-influenced river water, not the hard, mineral-rich water of Africa's rift lakes.
Feeding
Like other small mormyrids, Marcusenius greshoffii is a carnivorous, nocturnal bottom-forager, sweeping sand, mud and leaf litter for small aquatic invertebrates using electrolocation together with tactile probing. No species-specific dietary study is available in the sources consulted here, so the diet described follows the general pattern documented for the genus rather than a study of this species alone.
In the aquarium, small Congo basin mormyrids of this kind are typically kept on live and frozen foods — bloodworm and brine shrimp in particular — and are best fed after lights-out to match their nocturnal foraging rhythm; a soft sand substrate rather than gravel is important both for natural feeding behaviour and to avoid injury to sensitive skin and barbel-like structures near the mouth.
Mating
As in other mormyrids, courtship and species or sex recognition in Marcusenius greshoffii is thought to run substantially through the electric organ discharge rather than through visual display, since the species-specific EOD is one of the few reliable ways individuals of similar-looking, co-occurring Marcusenius tell each other apart in dark water. Beyond that general pattern, wild courtship behaviour for this species specifically has not been documented in the sources consulted here.
Mormyrids as a family are believed to be seasonal, flood-triggered spawners, with breeding activity tied to the rains and rising water levels that mark the Congo basin's wet season, but the courtship sequence and any pair-bonding in M. greshoffii have not been described in the available literature. Data sparse.
Breeding
Marcusenius greshoffii is essentially not bred in the aquarium hobby, and its reproductive biology remains poorly understood, in keeping with mormyrids generally. Wild populations are thought to be seasonal, flood-triggered spawners, and the species-specific electric organ discharge likely plays a courtship or mate-recognition role, but the 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. greshoffii.
Aquarists should not expect to breed this fish, and virtually any specimens reaching the trade would be wild-caught rather than captive-produced. This is a fish to keep and observe rather than a project fish to pair up for eggs.
In the aquarium
Marcusenius greshoffii is rarely seen in the hobby, but where kept it follows the well-established care template for small Congo basin mormyrids: warm water around 79–82 °F, a mildly acidic to neutral pH of roughly 6.8–7.2, soft substrate (sand or fine mud, never sharp gravel), and dim lighting — bright light stresses mormyrids and can cause a slow decline. Its modest maximum size of about 4.3 in total length makes it far more tank-friendly than the larger mormyrids and the size-trap giants elsewhere in the bonytongue order, but its husbandry needs are still specialised rather than casual.
Water quality has to be stable, and like other mormyrids this species is a scaleless-skinned fish that is notably sensitive to copper-based medications and to many standard aquarium treatments, so any dosing should be approached conservatively. Small mormyrids of this kind can show aggression toward their own species in close quarters, so tankmates and grouping decisions matter; robust, calm, non-nippy companions that will not out-compete a shy, nocturnal forager for food are the safest choice, and feeding after dark suits its natural rhythm best.
Conservation
The IUCN Red List assesses Marcusenius greshoffii as Least Concern, an assessment made on 16 February 2009. Its wide distribution across the lower Congo and upper Lualaba, spanning the Central African Republic, the Republic of the Congo and the Democratic Republic of the Congo, supports a stable population status at the species level.
As with many broadly distributed Congo basin fishes, the main long-term pressures are generic rather than species-specific: habitat degradation, sedimentation and water-quality decline across the river system. There is no evidence in the sources consulted here of population decline driving reassessment, and the picture for this widespread, small Congo basin mormyrid remains reassuring for now.