Taxonomy & naming
Mormyrus hasselquistii was described by the French zoologist Achille Valenciennes in 1847, one of dozens of mormyrid species he and Georges Cuvier treated in the Histoire naturelle des poissons. It belongs to the family Mormyridae within Osteoglossiformes, the bonytongue fishes — a lineage that branched off early in teleost evolution and now survives as scattered relict groups on four continents: the electric mormyrids of Africa, the unrelated Notopteridae featherback knifefishes also of the Old World, the arowanas and arapaima of South America, Asia and Australasia, and a handful of others. Mormyridae is entirely African and holds close to 200 species, most of them, including this one, placed in the large genus Mormyrus.
The genus name comes from the Greek mormyros, an old name Aristotle used for an unrelated marine bream; Linnaeus reused the word in the eighteenth century for these freshwater African fish and it has served as the genus and family name ever since. The species epithet honors Fredric Hasselquist, a Swedish naturalist and student of Linnaeus who traveled through Egypt and the eastern Mediterranean in the early 1750s and whose collected specimens underpinned several fish descriptions published after his early death. Mormyrus species are notoriously hard to separate on external appearance alone, since snout length and body proportions shift with age and population, and in the field individuals are often reliably told apart only by their electric-organ discharge rather than by anatomy.
Morphology
Mormyrus hasselquistii has the elongate, laterally compressed body typical of the genus: a small, low-set, nearly toothless mouth on a moderately drawn-out snout, a single long-based dorsal fin set well back on the body, and a deeply forked tail on a slender caudal peduncle. FishBase records 63 to 82 soft rays in that long dorsal fin and 16 to 20 in the anal fin, giving a low, ribbon-like dorsal rather than a tall, sail-shaped one. Colouration is a plain silvery grey to olive-brown, unremarkable next to the electric signal these fish actually use to identify one another.
The defining mormyrid feature sits at the base of the tail: an electric organ, built from modified muscle tissue, that fires a brief, species-specific pulse discharge. Mormyrus hasselquistii uses this weak electric field much as a bat uses echolocation, building a moving picture of its surroundings in dark, silt-laden water and signalling to other mormyrids nearby. Processing that constant stream of electrosensory information takes disproportionate brain tissue: mormyrids have among the largest brain-to-body ratios of any fish, built around a greatly enlarged cerebellum, which is why elephantnose-type mormyrids are sometimes called "thinking fish." Unlike the elephantnose Gnathonemus petersii most familiar in the aquarium trade, M. hasselquistii does not carry a soft, mobile chin appendage — the schnauzenorgan is a trait of Gnathonemus and Campylomormyrus, not a feature shared across all mormyrids, and this species' snout is simply elongated and fixed.
Habitat
This is a genuinely wide-ranging species by mormyrid standards. FishBase's distribution data place it across a broad swath of West Africa — the Senegal, Gambia, Volta, Mono, Gêba, Bandama and Comoé rivers, along with the Ébrié and Aguien coastal lagoons of Côte d'Ivoire — as well as the Nile and Lake Chad basins farther east. Most mormyrids are confined to a single river system or lake; few span this much of the continent.
Across that range it is a freshwater, bottom-associated fish of river channels and adjoining floodplain habitat, and it is potamodromous, making seasonal migrations entirely within fresh water, typically timed to the rains and the resulting rise in river and floodplain levels. Mormyrid rivers of this kind are generally warm, in the roughly 75–82 °F range, soft and only mildly acidic to neutral, and often turbid with suspended sediment or stained with tannins — exactly the low-visibility water an electric sense is built to read.
Feeding
Mormyrus hasselquistii forages over the river bottom for aquatic invertebrates — insect larvae, worms and small crustaceans — probing sand and silt rather than hunting by sight. Its eyes are small and its vision is thought to be poor; the electric-organ discharge and the array of electroreceptors covering its skin do the work of finding buried or hidden prey in turbid water and after dark, the same sense it also uses for navigation and communication.
In the aquarium it needs live or frozen invertebrate foods — bloodworm, mosquito larvae, brine shrimp and similar items — and generally refuses dry flake or pellet food, at least as a staple diet. Feeding after the lights go out matches its natural rhythm and cuts down on competition from more sight-driven tankmates.
Mating
Courtship and spawning in Mormyrus hasselquistii have not been well studied. As in other mormyrids, the electric-organ discharge is thought to carry information beyond simple electrolocation — its waveform varies between individuals and, in several better-studied species, between the sexes, pointing to a role in mate recognition and courtship — but the specifics for this species have not been worked out.
What is better established is the general seasonal pattern: potamodromous mormyrids such as this one are believed to move and spawn in step with the rains and the resulting rise in river and floodplain water, a widespread strategy among African riverine fish that times reproduction to a pulse of food and cover for the young. Beyond that broad picture, courtship behaviour, spawning sites and any pair formation in Mormyrus hasselquistii remain essentially unrecorded.
Breeding
Mormyrus hasselquistii is, for practical purposes, not bred in the aquarium. Mormyrids as a group are notoriously resistant to captive reproduction: sexing individuals by eye is unreliable, the seasonal and environmental cues that appear to trigger wild spawning are hard to replicate in a tank, and no consistent aquarium-breeding protocol has been published for this species.
What little is known of its reproduction is sparse and largely inferred from the wider genus rather than observed directly in this species: eggs are presumed to be scattered rather than mouthbrooded or nested, and parental care, if any, has not been documented. A confirmed captive spawn of Mormyrus hasselquistii would be a genuinely rare event.
In the aquarium
This is a specialist's fish, not a community staple. FishBase records a maximum around 19.7 in standard length, and even allowing for smaller wild-caught imports, an adult of this size needs a long, spacious aquarium — aquarium literature on mormyrids generally suggests something on the order of 59.1 in or more in length as a practical minimum — with a fine sand substrate the fish can graze over without abrading its skin, dim lighting, and ample cover such as driftwood, root tangles or wide-bore pipe to retreat into by day. Like most mormyrids it is most active at dusk and after dark.
Water quality matters more here than for many aquarium fish: mormyrids are widely reported as sensitive to poor water conditions and to some medications, so stable, well-filtered, mature water and conservative dosing are the rule rather than the exception. Individuals can also be aggressive toward their own kind, particularly in a tank too small to let a subordinate fish escape a dominant one, so a single specimen, or a genuinely large tank with careful stocking if more are kept, is the safer approach. It tolerates other African species too large to be seen as prey and too robust to be bullied, but it is not an easy community fish and not one for a beginner's tank.
Conservation
The IUCN Red List assesses Mormyrus hasselquistii as Least Concern; FishBase, citing that assessment (made in 2019), gives the same status. The species' wide range across West African river systems and the Nile and Chad basins supports that reading — this is not a narrow-range endemic riding on a single threatened water body.
That said, many of the rivers within its range face damming, pollution, sand mining and other pressures common across West African freshwater systems, and a wide range does not guarantee every population within it is stable. No population-specific conservation concern has been raised for this species in the sources reviewed here.