Taxonomy & naming
Brienomyrus longianalis belongs to the family Mormyridae, the elephantnoses and mormyrids, within the order Osteoglossiformes — the bonytongues, one of the most ancient living lineages of ray-finned fish, whose members share a bony, toothed structure on the floor of the mouth that bites against a toothed patch on the roof. Bonytongues are the scattered survivors of a much older, once-worldwide radiation, found today in isolated pockets across South America, Africa, Southeast Asia and Australasia; the mormyrids are the African chapter of that story, a family of roughly 200 species restricted to the continent's rivers and lakes, every one of them weakly electric.
The species was described by the Belgian-British ichthyologist George Albert Boulenger in 1901. The parenthetical form of the authority, (Boulenger, 1901), signals that he did not originally place it in the genus Brienomyrus — that genus was erected later, when the mormyrids were revised and this species was reassigned to it along with its close relatives. The epithet longianalis is built from the Latin longus, "long", and analis, "of the anal [fin]", describing the trait that distinguishes it from its closest relative, Brienomyrus brachyistius: a visibly longer anal fin, paired with a proportionally lower caudal-peduncle height (FishBase).
Morphology
Like other Brienomyrus, B. longianalis has the typical mormyrid build: an elongated, laterally compressed body, a small, nearly toothless mouth set low on a blunt head, and small eyes that do comparatively little work for a fish that is mostly active after dark. Reported maximum size is about 6.3 in standard length (FishBase), a modest size within the elephantnose family, which ranges from a few centimetres to the half-metre giants of genus Mormyrus.
The defining feature, as the name promises, is the anal fin: longer and set further forward than in the closely related B. brachyistius, with which it can otherwise be confused, and a caudal peduncle that runs proportionally shallower. As in mormyrids generally, an electric organ near the base of the tail generates a weak, pulse-type discharge that is unique enough in waveform to serve as a kind of acoustic signature between individuals and species, and the fish is thought to carry the disproportionately large cerebellum characteristic of the family, an adaptation for processing the constant stream of electrosensory information its own field returns.
Habitat
Brienomyrus longianalis has a disjunct West African distribution: it is known from the Niger river basin in Guinea and Sierra Leone, and separately from the Kribi River of Cameroon, several hundred kilometres away with no confirmed populations bridging the gap (FishBase). Within these systems it appears to be benthopelagic and potamodromous, moving within the river network rather than to the sea, and records suggest it may be largely confined to deltas and lower river reaches rather than turning up throughout the whole basin.
Wild water temperatures are reported at 77–86 °F (FishBase), consistent with the warm lowland rivers of the West African coast. Specific pH and hardness data for this species have not been published — data sparse — but lowland rivers of this kind in Guinea, Sierra Leone and coastal Cameroon are typically soft, mineral-poor and often turbid or tannin-stained, the kind of low-visibility water in which a weak electric sense, rather than eyesight, does most of the navigational and foraging work.
Feeding
Dedicated dietary studies of B. longianalis have not been published, but like other small mormyrids it is presumed to be a benthic invertebrate feeder, probing soft substrate and cover for insect larvae, worms and small crustaceans, most actively around dusk and after dark. Its electric field, rather than sight, is thought to do most of the work of locating prey buried in sediment or hidden among roots and debris.
In the aquarium, mormyrids of this kind are fed live and frozen invertebrates — mosquito larvae, bloodworm, daphnia and tubifex are the standard offerings — and generally will not take flake or dry food readily. Because the fish forages mainly at night, food is best offered after the lights go down so it can find its own meal without competing with more visually oriented tankmates.
Mating
As with mormyrids generally, courtship in B. longianalis is presumed to be tied to its electric discharge: the pulse waveform is individually and sexually distinctive enough that it likely plays a role in species recognition and mate assessment in the wild, the fish equivalent of a mating call read through the water rather than the air. Beyond that, almost nothing has been documented for this species specifically.
Wild mormyrids in West Africa are generally thought to be seasonal spawners, with reproductive activity keyed to the rains and the flood pulse of their rivers, but the courtship behaviour, mate choice and spawning triggers of B. longianalis itself have not been described in the literature — this remains one of the many mormyrid species whose reproductive biology is essentially a blank page.
Breeding
Breeding B. longianalis in captivity is, as far as is documented, essentially unachieved. Aquarium sources covering the elephantnoses as a group are consistent on this point: captive spawning of mormyrids is rare to the point of being effectively unknown in the hobby, and this species has no reported exception.
Wild breeding details are similarly sparse. What can be said is generic to the family rather than specific to this fish: seasonal, flood-associated spawning is typical of West African mormyrids, and some species in the family are thought to guard eggs briefly among vegetation, but no such account exists for B. longianalis itself. Anyone attempting to breed it should expect to be working from first principles rather than an established protocol.
In the aquarium
This is not a beginner's fish, and not because it is delicate to feed or aggressive — it is because mormyrids as a group are demanding in ways that are easy to overlook. A tank of at least 75 gallons (roughly 74 US gal) and 39.4 in or more in length gives a fish that patrols constantly at night room to do so, over a soft sand or fine gravel floor that will not abrade its sensitive skin as it forages. Aquarium sources for the genus recommend a pH of roughly 6–7.5 and a temperature of 72–82 °F, dim lighting, and plenty of plants, wood and rockwork offering shaded retreats for a fish that is genuinely nocturnal and will spend much of the day tucked out of sight.
Socially, B. longianalis and its relatives are not straightforwardly aggressive, but they are quarrelsome when crowded: a single specimen does well, and a proper-sized group can work, but two or three fish confined together tend to squabble over space and hierarchy. Like other scaleless-skinned, electroreceptive fish, mormyrids as a family are known to be sensitive to poor water quality and to some medications, so stable, well-filtered water and conservative dosing matter more here than in most community setups. Tankmates should be chosen to avoid direct competition at night and should not be so small as to be mistaken for food; captive breeding should not be expected.
Conservation
The IUCN Red List assesses Brienomyrus longianalis as Least Concern, an assessment dated 3 September 2019 (IUCN species ID 182982). It is not currently considered threatened, and no major population decline has been reported.
That said, the species' known range is unusually patchy for an LC assessment — two separated West African river systems, the Niger basin and the Kribi River, rather than one broad, continuous distribution — and it may be further restricted to deltas and lower reaches within those systems. Disjunct, narrow-range freshwater fish of this kind are typically more exposed to localised pressures such as damming, sedimentation and water-quality decline than the Least Concern label alone conveys, even where no specific threat has yet been documented for this species.