Taxonomy & naming
Marcusenius bentleyi belongs to the order Osteoglossiformes, the bonytongues — among the most ancient lineages of living teleost fish, named for a toothed bone on the floor of the mouth that bites against toothed plates on the roof, an arrangement found across a scattered, deep-time radiation on four continents. Within that order it sits in the family Mormyridae, the elephantnoses and mormyrids: an exclusively African, weakly electric group that is convergent with, but entirely unrelated to, the electric knifefishes of South America (order Gymnotiformes). The species was described in 1897 by the Belgian-British ichthyologist George Albert Boulenger as Mormyrus bentleyi, and is now placed in the genus Marcusenius, one of the larger mormyrid genera, with species spread across the Congo, Niger, Nile and other African river systems.
The genus Marcusenius was erected by the American zoologist Theodore Gill in 1862; it is conventionally understood to honour the 19th-century naturalist J. Marcusen, who published early anatomical work on the electric organs of mormyrids and related fish, though the dedication is not spelled out explicitly in Gill's original text. The species epithet bentleyi honours William Holman Bentley (1855–1905), a British Baptist missionary who spent decades based on the Congo River and collected natural-history specimens, including the type material for this fish, from the Stanley Falls area near present-day Kisangani, Democratic Republic of the Congo — still the only confirmed locality for the species.
Morphology
Like other mormyrids, Marcusenius bentleyi has a laterally compressed body, a small, low-set mouth on a fairly blunt head, and a slender caudal peduncle carrying the specialised electric organ near the base of the tail. FishBase records a maximum size of about 10.6 in total length for the species, a mid-sized mormyrid — larger than the popular dwarf elephantnoses of the hobby but well short of the largest members of the family.
Mormyrids, including species of Marcusenius, are notoriously difficult to tell apart on looks alone; many are visually near-identical and are confidently distinguished only by the waveform of their species-specific electric-organ discharge (EOD), recorded with an oscilloscope, rather than by colour pattern or fin shape. Marcusenius bentleyi is presumed to share the family's other hallmarks: small eyes, weak vision, and a disproportionately large, deeply folded cerebellum — part of the neural hardware, among the largest brain-to-body ratios of any fish, that processes the fish's constant stream of electrosensory information.
Habitat
Marcusenius bentleyi is known only from the type locality at Stanley Falls — also called the Wagenia Falls — a dramatic series of rapids on the Congo River near Kisangani in the Democratic Republic of the Congo. That single-site record makes it, on current evidence, an endemic of turbulent, fast-flowing, sediment-laden water tumbling over rock, rather than the slow, tannin-stained backwaters and floodplains more commonly associated with mormyrids.
No further occurrence records beyond the original collection appear in the sources available here, and GBIF's own mapped occurrences for the species are similarly confined to that one site. The gap most likely reflects how difficult the falls are to survey rather than genuine rarity, but that cannot be resolved without further fieldwork on the upper Congo.
Feeding
Mormyrids are nocturnal, bottom-oriented insectivores, and Marcusenius bentleyi almost certainly follows the same pattern seen across the family: foraging after dark over sand and rock substrate for aquatic insect larvae, small crustaceans and other benthic invertebrates.
The fish's weak electric organ generates a continuous, low-voltage field around the body; distortions in that field caused by nearby objects and prey are picked up by electroreceptors spread across the skin, letting it hunt effectively in darkness and in the churned, sediment-clouded water typical of a rapids habitat, where eyesight alone would be of little use.
Mating
Courtship and species recognition in mormyrids revolve around the electric-organ discharge (EOD): each species produces a distinctive pulse waveform, and individuals are thought to use it to identify and assess potential mates and rivals of their own kind — a plausible mechanism in a genus like Marcusenius where many species look almost identical to the eye. Beyond that general family pattern, nothing specific to the courtship behaviour of Marcusenius bentleyi has been documented in the sources available.
Wild mormyrids more broadly are believed to be seasonal spawners, with breeding activity often linked to seasonal flooding, but how the EOD signal translates into pairing and spawning for this particular species has not been described in the literature surveyed here.
Breeding
Breeding in Marcusenius bentleyi, as in mormyrids generally, is essentially unknown territory. No aquarium spawning of the species has been documented, and even basic reproductive details — size or age at maturity, fecundity, egg placement — remain unrecorded; FishBase lists maturity as unknown.
The wider family offers only a rough outline to go on: mormyrids are thought to be seasonal, flood-triggered spawners, and some species are believed to scatter or guard eggs among aquatic vegetation, but the details are sparse across the group and captive breeding of mormyrids is rarely if ever achieved. There is no basis here for describing a specific breeding mode for this species beyond "poorly known."
In the aquarium
Marcusenius bentleyi is not an established aquarium fish — it does not appear to be part of the regular ornamental trade, and the sources available describe its biology rather than its keeping. What follows is drawn from the general care requirements of Mormyridae as a family, which would apply if a specimen were ever obtained, not from species-specific aquarium experience.
Mormyrids as a group want soft, gently moving water over a fine, soft substrate that will not abrade their sensitive, electroreceptor-rich skin, and a dim or well-shaded tank suits their nocturnal habits and their reliance on electrolocation over vision. They are social fish that tend to fare poorly kept singly and are best kept in a same-species group where space allows, with plenty of cover such as roots, pipes and dense planting to retreat into by day. Mormyrids are also notably sensitive to copper-based and some formalin-based medications, so any treatment in a mormyrid tank has to be chosen and dosed with particular care.
Conservation
The IUCN Red List assesses Marcusenius bentleyi as Data Deficient, last evaluated in 2009. That reflects how little has been documented about the species rather than evidence of a healthy or a declining population: it is known from a single locality, Stanley Falls on the Congo River, and no subsequent surveys establishing its current range, abundance or trend appear to have been published.
A single-locality Data Deficient status is not the same as a clean bill of health — the true range could extend well beyond the historical record, or the fish could genuinely be a narrow-range species vulnerable to disturbance at that one site. Either way, resolving the picture needs targeted field surveys of the Stanley Falls / Wagenia Falls area and the wider upper Congo, which so far has not been a research priority.