Taxonomy & naming
Marcusenius rheni was described by the American ichthyologist Henry Weed Fowler in 1936. It belongs to the family Mormyridae — the elephantnoses and mormyrids, an exclusively African family in which every member is weakly electric — 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 counted among the most ancient surviving lineages of teleost fish. The Mormyridae are entirely convergent with, and unrelated to, the electric knifefishes of South America (Gymnotiformes), which evolved an equivalent electric sense independently on the far side of the Atlantic.
The species epithet rheni honours James Rehn, an American entomologist; FishBase notes that Fowler's spelling appears to have misrendered the surname "Rehn" as "rheni." The genus name Marcusenius is a Latinized patronym whose exact honouree is not well documented in the sources consulted here.
The name's standing is itself unsettled. FishBase and the IUCN Red List both flag Marcusenius rheni as likely a junior synonym of Marcusenius victoriae, following Seegers et al. (2003), though formal confirmation is still pending. That kind of uncertainty is common among Lake Victoria's mormyrids: many are difficult to separate on preserved museum material alone and are more reliably distinguished by their species-specific electric organ discharge (EOD) than by external morphology, so a name resting on old type specimens can remain provisional for decades.
Morphology
Marcusenius rheni has the general mormyrid build: a laterally compressed body, a small, close-set mouth with weak teeth suited to picking rather than biting prey, and a caudal peduncle housing the tail-mounted electric organ that defines the family. FishBase records a maximum size of about 8.2 in total length, placing it among the mid-sized members of a family that spans a few centimetres up to the metre-plus giants in genera like Mormyrus and Gnathonemus.
Detailed species-level description beyond the type material is thin, and the resemblance among Lake Victoria's small mormyrids compounds the problem — external features alone are often not enough to separate this species confidently from its close relative Marcusenius victoriae, which is part of why the two names remain entangled. As in other mormyrids, the electric organ is paired with an outsized cerebellum and one of the highest brain-to-body mass ratios recorded in any vertebrate, the physiological cost of running a sensitive electrolocation and communication system.
Habitat
Marcusenius rheni is endemic to Lake Victoria, the vast freshwater lake shared by Tanzania and Uganda (with Kenya also bordering the wider lake) and, by surface area, the largest tropical lake on Earth. FishBase classifies it simply as a freshwater, tropical, demersal species — a bottom-associated fish of the lake rather than a surface or midwater dweller.
Beyond that basic classification, habitat detail specific to this species is sparse in the available sources. Like other Lake Victoria mormyrids it presumably favours the lake's shallower, vegetated or sediment-rich margins over open, exposed water, the kind of dim, low-visibility setting in which electrolocation outperforms eyesight, but this has not been independently documented for M. rheni itself.
Feeding
In the wild, Marcusenius rheni feeds on aquatic insects and their larvae, consistent with the small mouth and weak teeth typical of mormyrids built for picking small prey rather than crushing or biting it. Foraging in this family is guided substantially by electrolocation, letting the fish detect prey hidden in sediment or low-visibility water where vision offers little help.
Species-specific feeding data for the aquarium is limited, but hobby sources covering Marcusenius elephantnoses generally describe a diet of rotifers, daphnia (water fleas), copepods and other small live or frozen invertebrates — food sized to match the fish's modest mouth and picking feeding style.
Mating
As in other mormyrids, the electric organ discharge is understood to play a central role in courtship and mate recognition — the pulse itself is thought to carry information about species identity and readiness to spawn, functioning as a private communication channel in water where sight is unreliable. This general pattern has not been documented specifically for Marcusenius rheni, and detailed observations of its courtship behaviour are not available in the sources consulted.
The electrical noise of a typical home aquarium — filters, heaters, pumps and reflective glass — can interfere with the weak EOD signal mormyrids rely on for this kind of communication, one likely reason natural courtship is rarely, if ever, observed in captivity for fish in this family.
Breeding
No captive breeding has been documented for Marcusenius rheni. As with mormyrids generally, its reproductive biology is poorly understood even in the wild, and breeding in this family is essentially never achieved in aquaria.
Aquarists should not expect or attempt to breed this fish. Given the additional taxonomic uncertainty around its identity, any specimen offered as M. rheni may in practice be Marcusenius victoriae or another closely related Lake Victoria mormyrid.
In the aquarium
Marcusenius rheni is essentially unknown in the hobby, and what guidance exists comes from care information for the wider genus Marcusenius rather than this species specifically. Hobby sources recommend a tank at least 39.4 in long, and stress that these fish are social and should not be kept singly — a lone mormyrid in a small tank is a recipe for a stressed, hidden fish.
Offered food should suit the small mouth and picking feeding style: rotifers, daphnia, copepods and similar small invertebrates, live or frozen. As with mormyrids generally, good, stable water quality matters more than it does for many aquarium fish, and dosing medications — particularly copper-based treatments — calls for caution. Given the unresolved question of whether this name is distinct from Marcusenius victoriae, a keeper acquiring a Lake Victoria elephantnose under either name should treat the identification as provisional.
Conservation
The IUCN Red List assesses Marcusenius rheni as Data Deficient, an assessment made in 2014. That status reflects both the general scarcity of population data for small Lake Victoria mormyrids and the specific taxonomic uncertainty noted above — a species whose validity is itself in question is difficult to assess with confidence.
No species-specific threats are documented in the sources consulted. Lake Victoria as a whole has undergone major ecological upheaval over the past century — the Nile perch introduction, eutrophication, and heavy fishing pressure across the lake's native fish community are well documented at the lake-wide scale — but no source here ties those pressures specifically to this species, and its Data Deficient status should be read as an absence of adequate data rather than evidence of decline.