Taxonomy & naming
George Albert Boulenger, working at the British Museum (Natural History) in London, described Pollimyrus nigricans in 1906 as part of the enormous run of African freshwater fishes he named across the first two decades of the twentieth century. The species belongs to the family Mormyridae, the elephantnoses and mormyrids, within the order Osteoglossiformes — the bonytongue fishes, one of the oldest surviving lineages of living teleosts.
The genus Pollimyrus did not exist in Boulenger's day. It was erected in 1971 by the Belgian ichthyologist Léon Taverne, who split a cluster of smaller-bodied mormyrids away from the much larger genus Marcusenius on osteological grounds, and P. nigricans was subsequently placed there — hence the parenthetical authorship. Which genus Boulenger originally used, the type locality, and any synonymy history are not stated in the sources reviewed for this account, so none of that is asserted here.
Morphology
No maximum-length figure specific to Pollimyrus nigricans was found in the sources reviewed for this account. Every species placed in Pollimyrus is a small-bodied mormyrid, with most congeners maturing well under 6 in, but that describes the genus as a whole rather than a measurement taken from this species.
As in all mormyrids, a weak electric organ near the base of the tail generates a pulse-type discharge, read by electroreceptors spread across the skin — a sense that substitutes for eyesight in dark or turbid water. Mormyrids also carry an unusually large brain for their body size, with an enlarged cerebellum devoted to processing this electrosensory input. No discharge waveform, colour pattern, or other morphological detail specific to P. nigricans has turned up in the sources consulted here; the epithet nigricans ("blackish") is the only descriptive clue on record, and Boulenger's original description does not elaborate on it further in the material reviewed.
Habitat
No confirmed locality record for Pollimyrus nigricans was located in the sources reviewed for this account, and none is stated here. The genus Pollimyrus as a whole ranges across tropical Africa, from coastal West African rivers through the Congo basin to parts of the Nile system and southern Africa, but that describes the genus, not a documented distribution for this particular species.
Temperature, pH, and other water-chemistry figures are likewise undocumented for this species. Mormyrids in general favour slow-moving, turbid water with heavy structure — flooded margins, backwaters, and river stretches thick with roots or submerged vegetation — the kind of low-visibility setting where an electric sense outperforms eyesight. That is a pattern true of the family broadly, not a habitat description confirmed for P. nigricans.
Feeding
No diet study specific to Pollimyrus nigricans is available in the sources reviewed for this account. Mormyrids as a family forage along the bottom for small invertebrates — insect larvae, worms, tiny crustaceans — locating them by electrolocation in mud, silt, and leaf litter before taking them with a quick strike of the mouth. This species almost certainly feeds the same way, but that has not been confirmed for it specifically; it is presented here as an inference from the family, not an established fact.
Mating
Nothing has been published on courtship or mate choice in Pollimyrus nigricans. Across mormyrids generally, the electric organ discharge does more than aid navigation: its pulse pattern varies by species and often by sex, and it is thought to help fish recognise potential mates and hold shoals together. One well-studied congener, Pollimyrus isidori, pairs its electric signalling with audible grunts and moans during courtship — but that finding belongs to isidori specifically, has not been demonstrated in Pollimyrus nigricans, and is not assumed to apply here.
Breeding
Breeding in Pollimyrus nigricans has not been recorded, in the wild or in an aquarium, in any source reviewed for this account. Nothing is known about its spawning behaviour, its eggs or larvae, or whether either parent provides care. As with most small African mormyrids, any specific claim about how this species reproduces would be invention rather than fact, so none is offered.
In the aquarium
Pollimyrus nigricans has essentially no track record in the aquarium hobby — no keeping reports, husbandry write-ups, or import records specific to this species turned up in the sources reviewed here. What follows is general mormyrid practice, not anything demonstrated for this particular fish.
Small mormyrids generally do best over a soft, fine substrate they can probe with the snout, with heavy cover and subdued lighting suited to a fish that is more active after dark. The family is also known for being unusually sensitive to poor water quality and to a number of common aquarium medications, so cautious, gradual dosing matters more here than with most fish. Live or frozen invertebrate foods such as bloodworm and brine shrimp are the standard diet, since mormyrids typically ignore dry foods outright. None of this has been verified for Pollimyrus nigricans specifically, but it reflects how the rest of its family is kept.
Conservation
No current IUCN Red List assessment for Pollimyrus nigricans was located in the sources reviewed for this account. No population trend, range extent, or specific threat has been published for this species.
A great many small, narrow-range African mormyrids have simply never been formally assessed, and this species may fall into that category rather than genuinely lacking any conservation concern. Broad pressures on African freshwater systems — habitat degradation, siltation, and declining water quality — are long-term risks facing the group as a whole, but none has been documented specifically for Pollimyrus nigricans, and none is claimed here.