Taxonomy & naming
Hippopotamyrus paugyi was described in 1985 by the French ichthyologists Christian Lévêque and Renaud Bigorne, both of whom worked extensively on the freshwater fish faunas of West Africa. It belongs to the family Mormyridae — the elephantfishes — within the order Osteoglossiformes, the bonytongues, an ancient lineage of teleosts that also includes the arowanas, arapaima, African butterflyfish and featherback knifefishes on other continents. Mormyrids and their close relatives the Gymnarchidae form the suborder Mormyroidei, an entirely African radiation defined by a shared trait unique among fishes: a large electric organ in the tail used for electrolocation and communication rather than defence.
The genus Hippopotamyrus is one of several West and Central African mormyrid genera, alongside Marcusenius, Mormyrus and Petrocephalus, whose species are frequently christened "stonebashers" by aquarists and ichthyologists alike, a nod to their taste for foraging low over rocky and sandy river bottoms. The species epithet paugyi honours the French ichthyologist Didier Paugy, a prolific researcher on West African fish taxonomy and a frequent collaborator on the very river surveys that turned up this fish. Because many Hippopotamyrus species are visually similar, confident field identification within the genus often relies on the shape of the electric organ discharge rather than appearance alone.
Morphology
Hippopotamyrus paugyi has the elongated, laterally compressed body typical of the smaller mormyrid "stonebashers": a fairly slender profile, a small terminal or slightly subterminal mouth, and a short-based dorsal fin set well back on the body opposite an equally short anal fin, with a forked tail. FishBase gives a maximum total length of about 9.8 in, though most individuals recorded and any that reach the aquarium hobby are considerably smaller.
As in all mormyrids, the standout feature is not visible from the outside: an electric organ near the base of the tail generates a weak, species-specific pulse-type electric organ discharge (EOD), picked up by electroreceptors scattered over the skin. This electrosense compensates for poor eyesight in dark or turbid water, letting the fish electrolocate obstacles, substrate texture and prey, and — because the discharge waveform differs between species and even between individuals — recognise others of its own kind in murky conditions where vision is useless. Mormyrids are also famous for having one of the highest brain-to-body-mass ratios of any fish group, with a greatly enlarged cerebellum devoted to processing this constant stream of electrosensory information; they are sometimes called "thinking fish" for this reason. Unlike the trunk-nosed elephantnose Gnathonemus petersii, Hippopotamyrus lacks an elongated chin appendage (schnauzenorgan) — its head is comparatively plain, a useful field mark separating it from its more famous, proboscis-bearing relatives.
Habitat
This species is known from West African coastal river systems, associated with the Kolenté drainage that runs through Guinea and into Sierra Leone (where its lower reaches are known as the Great Scarcies). As with many small mormyrids from this region, published distribution data are sparse, and it is best treated as a narrow-range West African river endemic rather than a widely surveyed species.
Like other mormyrids of the region it is a fish of soft, typically slightly acidic, warm running water — the kind of tannin-stained, dim, sediment-laden river habitat in which an electric sense is more useful than eyesight. It shelters by day among submerged roots, rock crevices and overhanging cover, emerging to forage over the bottom mainly after dark.
Feeding
Hippopotamyrus paugyi is a carnivorous invertebrate feeder. In the wild it forages over sand and rubble substrates for small aquatic invertebrates — insect larvae, worms and similar benthic prey — locating them in the dark or turbid water primarily by electrolocation rather than sight, a foraging style shared across the stonebasher mormyrids.
In the aquarium it takes live and frozen foods readily — bloodworm (mosquito larvae) and small worms are specifically noted as accepted foods — and, like most mormyrids, feeds most actively once the lights are down. It is not a fish that thrives on flake or a purely dry diet; a varied, meaty, sinking diet offered in low light suits its natural foraging behaviour far better.
Mating
Courtship in mormyrids runs largely on the same channel the fish uses to sense its world: the electric organ discharge. In well-studied relatives, males and females modulate the timing and pattern of their EOD pulses during encounters, and discharge characteristics play a role in species recognition and mate assessment in the dark, tannin-stained water these fish inhabit. Whether Hippopotamyrus paugyi follows this pattern in detail has not been documented — like much of this species' basic biology, its courtship behaviour is essentially unrecorded in the literature.
What is known from mormyrids generally is that spawning tends to be seasonal, cued by rising water levels and rains rather than occurring year-round, which fits the flood-pulse rhythm of the West African rivers this genus inhabits. No locality-specific spawning behaviour has been reported for this species.
Breeding
Breeding in Hippopotamyrus paugyi is poorly known and essentially unrecorded in captivity. Mormyrids as a group are seasonal, flood-triggered spawners in the wild, and while the electric organ discharge is thought to play a courtship and species-recognition role, the fine details of pair formation, spawning site, and egg-laying have not been worked out for most species, this one included. Some mormyrids are believed to scatter or deposit eggs among vegetation and to offer some form of guarding, but this has not been confirmed for Hippopotamyrus paugyi specifically, and it would be misleading to describe a definite breeding mode for this fish.
Deliberate captive breeding of this species has not been documented; the handful of mormyrids bred in aquaria at all (mostly in the related genus Marcusenius and a few Mormyrus) required carefully managed, large, mature systems, and even those successes are occasional rather than routine. Anyone keeping this species should expect it to be, for practical purposes, a wild-collected fish with unknown captive-breeding prospects.
In the aquarium
Hippopotamyrus paugyi is rarely seen in the aquarium trade and is not a fish for a beginner or a mixed community tank. Aquarium-care notes call for soft to moderately hard, slightly acidic water at 75–82 °F, a sand substrate, and plenty of caves, roots and shaded cover — dim, structured surroundings that mirror the tannin-stained rivers it comes from and let a nocturnal, poor-sighted fish feel secure. A tank of at least 53 US gal is recommended even for a modestly sized specimen. It can be kept in small groups, though mormyrids in general are sensitive to crowding and to being combined with boisterous tankmates, and aggression between conspecifics is a real risk in an undersized or poorly furnished tank.
Water quality matters more than usual with this fish: mormyrids are notably sensitive to poor water conditions and to many common aquarium medications, so stable, well-filtered water and cautious, conservative dosing (avoiding copper-based treatments in particular) are essential. Feed after the lights go out to match its nocturnal, electrolocation-driven foraging habit rather than expecting daytime activity. Given how little is documented about this species' needs and how rarely it is offered for sale, it is best approached as a project for an experienced keeper prepared to accept some trial and error, not as a routine community fish.
Conservation
The IUCN Red List assesses Hippopotamyrus paugyi as Least Concern, with no conservation threats currently documented for the species. This reflects the general pattern among West Africa's smaller mormyrids — most are not considered globally threatened — rather than any detailed population assessment specific to this narrow-range river fish.
As with many small-range West African freshwater species, the main long-term concerns are the broad ones facing the region's rivers generally: deforestation and sedimentation, water abstraction, and habitat degradation in the coastal drainages of Guinea and Sierra Leone. No population trend data are available, and its restricted, only partially surveyed range means it is worth keeping an eye on even though it is not currently flagged as at risk.