Taxonomy & naming
Marcusenius monteiri was described by the German-British ichthyologist Albert Günther in 1873 as Mormyrus monteiri, from a specimen collected at Boma on the lower Congo River. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats Marcusenius monteiri as the currently valid name, placing it in the family Mormyridae — the elephantnoses and mormyrids — within the order Osteoglossiformes, the bonytongues, a lineage named for a toothed bone on the floor of the mouth that bites against a matching pad on the roof, and one scattered across South America, Africa, Southeast Asia and Australasia by deep evolutionary time.
The genus name Marcusenius honours Dr. Johann Andreas Marcusen, while the species epithet monteiri commemorates Joachim J. Monteiro, a Portuguese mining engineer and entomologist (1833–1878) who collected extensively in West-Central Africa and supplied specimens to European museums. Marcusenius is a large genus, and its species are frequently difficult to separate on external appearance alone; several are more reliably told apart by their electric-organ discharge (EOD) waveform than by colour or fin shape.
Morphology
Like other Marcusenius, M. monteiri has the mormyrid body plan: a laterally compressed trunk, a small, low-slung mouth suited to picking invertebrates off the bottom rather than the elongated, trunk-like schnauzenorgan of Gnathonemus, an elongated dorsal fin set opposite a similarly long anal fin, and a deeply forked tail. At the base of the tail sits the electric organ, a modified muscle mass producing the fish's signature pulse-type discharge — a signal used more by researchers and, presumably, by the fish itself than by an aquarist's eye to separate it from its many close relatives.
FishBase gives a maximum length of about 17.7 in total length, which is large for the genus — most Marcusenius species run well under 11.8 in, so this is one of the bigger members of the group. As in other mormyrids, the brain is proportionally enormous, dominated by an oversized, richly folded cerebellum devoted to processing a constant stream of electrosensory information, the anatomical basis for the family's reputation as unusually intelligent, "thinking" fish.
Habitat
Marcusenius monteiri is a Congo basin species, recorded from Cameroon, the Republic of Congo, the Democratic Republic of the Congo and Zambia (FishBase), with its type locality at Boma near the mouth of the Congo River. It has been specifically documented in Malebo Pool (Pool Malebo), the wide, lake-like stretch of the Congo River between Kinshasa and Brazzaville, where it was caught at the Kabongo, Mipongo and Molondo fishing stations as one of seven sympatric Marcusenius species partitioning the pool's habitats — a striking case of a single genus radiating into multiple coexisting species within one stretch of river.
As a demersal fish of Congo tributaries and floodplain water, it lives over soft substrate in dim, typically tannin-stained water — the dark, low-visibility conditions its electric sense evolved to read. This account reflects the Congo basin water this species actually inhabits rather than generalising from other African river systems.
Feeding
As a bottom-picking mormyrid, M. monteiri is presumed to take aquatic invertebrates from the substrate — insect larvae, small crustaceans and worms — largely at night, using electrolocation to detect prey it cannot see in dark or turbid water, consistent with feeding behaviour documented across the genus.
In the aquarium, sources describe it taking mosquito larvae, bloodworms, white worms, black worms, Daphnia, Tubifex and Cyclops, offered live or frozen. It is not a fish to keep on flake alone, and because it feeds mainly after dark, offering food once the lights are out helps ensure a shy mormyrid isn't out-competed by more active tankmates.
Mating
M. monteiri is unusual among Marcusenius in that its wild reproductive biology has actually been studied, alongside three co-occurring congeners (M. stanleyanus, M. schilthuisiae and M. macrolepidotus), in Malebo Pool on the Congo River. That work found it to be a seasonal, flood-triggered spawner: fish move into flooded sites during the rainy season to breed, and females carry a single well-developed left gonad whose large oocytes are expelled in one spawning event rather than released gradually over an extended season.
As in other mormyrids, the species-specific pulse EOD is thought to play a role in mate recognition and courtship in dark water, but the Malebo Pool study focused on gonad development and spawning timing rather than courtship behaviour itself, so this account does not claim more than what was documented: a migratory, flood-cued, single-spawning strategy.
Breeding
Despite the relatively detailed picture of its wild reproductive biology, M. monteiri is not documented as bred in captivity, and no farmed or commercially produced stock is known. This is consistent with mormyrids generally, which are notoriously difficult to spawn under aquarium conditions; virtually all mormyrids seen in the trade, on the rare occasions they appear, are wild-caught.
The wild pattern — a rainy-season migration to flooded habitat, a single well-developed left gonad, and one large-scale spawning event rather than repeated smaller ones — is unusually well characterised for this species compared with most Marcusenius. What is not documented is any form of nest-building, egg-guarding or parental care, and this account does not invent any.
In the aquarium
Where kept, M. monteiri and similar Marcusenius are given a soft substrate, dim lighting and ample hiding places — driftwood, roots and dense cover — reflecting their preference for the low-visibility water they navigate electrically rather than by sight. Reported parameters are a temperature of roughly 75–82 °F and a pH around 6–8, spanning slightly acidic to neutral water typical of the Congo basin.
At up to 17.7 in total length this is a substantially larger fish than most Marcusenius offered in the hobby, and it should be planned for accordingly — a long, mature tank rather than the smaller setups that suit the genus's more typical 5.9–9.8 in species. Mormyrids as a group are notably sensitive to poor water quality and to many medications, so stable, well-matured water and conservative, copper-free dosing matter more here than with most community fish. They can also show aggression toward their own kind in cramped quarters, so either a single specimen or a properly sized group with generous sightline-breaking cover is the safer approach.
Conservation
The IUCN Red List assessed Marcusenius monteiri as Least Concern as of 2020, reflecting its occurrence across a wide span of the Congo basin — Cameroon, the Republic of Congo, the DRC and Zambia — rather than a narrow, single-locality range. The Malebo Pool studies additionally suggest a diverse, apparently stable community of sympatric Marcusenius species sharing that stretch of the Congo River.
As with many Congo basin fish, the longer-term concerns are indirect rather than species-specific: habitat degradation, fishing pressure and hydropower development along the river system. Because the species is essentially absent from the aquarium trade, collection for the hobby is not considered a factor in its conservation status.