Taxonomy & naming
Hippopotamyrus castor was described by the German ichthyologist Ernst Pappenheim in 1906. It belongs to the family Mormyridae — the elephantnose fishes and their weakly electric relatives — within the order Osteoglossiformes, the bonytongues: a scattered, deep-time radiation of fishes found across four continents (South America, Africa, Southeast Asia and Australasia) and united by that toothed lingual bone rather than by any shared modern habitat. Mormyridae is an exclusively African family, and its weak electric sense is entirely convergent with — not related to — the electric knifefishes of South America (Gymnotiformes); the two groups evolved similar electrosensory systems independently on opposite sides of the Atlantic.
The genus Hippopotamyrus holds roughly a dozen species scattered across West, Central and southern African river systems, of which H. castor is one of the lesser-known. As with most mormyrids, closely related species can look very similar to the eye and are reliably distinguished only by the waveform of their species-specific electric organ discharge (EOD), so identifications based on appearance alone deserve appropriate caution.
Morphology
Like other members of its genus, Hippopotamyrus castor has the classic mormyrid silhouette: a laterally compressed, moderately deep-bodied form, a single soft dorsal fin set well back on the body opposite a long-based anal fin, and a deeply forked caudal fin on a narrow peduncle. It has an ordinary small mouth rather than the elongated, trunk-like chin appendage (the fleshy "Schnauzenorgan") carried by some relatives, notably Gnathonemus — that specialised probing organ is not a feature of Hippopotamyrus.
Near the base of the tail sits the electric organ, modified muscle tissue that generates a weak, pulse-type discharge unique in waveform to the species. Mormyrids are well known for having, size for size, some of the largest brains and highest brain-to-body ratios of any fish, with an enlarged cerebellum dedicated to processing the constant stream of electrosensory information the EOD returns. FishBase records a maximum size of about 10.2 in, making H. castor a moderately built mormyrid — nowhere near the true giants of the wider bonytongue order such as the arapaima, but larger than the smallest elephantnoses kept in aquaria. Detailed colour pattern and fin-ray counts for this particular species are not documented in accessible sources.
Habitat
Hippopotamyrus castor is a narrow-range endemic of a small cluster of river systems on the Cameroon–Nigeria border: FishBase records it from the Sanaga and Lokoundjé Rivers in Cameroon and the Cross River in Nigeria, with the type locality on the Lokoundjé (recorded as the "Lokundi River" near Bipindi, Cameroon). This places the species among the coastal forest drainages of West-Central Africa rather than in any of the continent's great lake or basin systems.
FishBase classifies it as a freshwater, demersal — bottom-dwelling — species of tropical climate. Like other mormyrids from this part of Africa, it is presumed to inhabit the soft, slow-moving, often tannin-stained water typical of the region's forest rivers, where poor visibility favours electrolocation over eyesight; precise water-chemistry measurements specific to H. castor are not documented in accessible sources.
Feeding
As in other mormyrids, Hippopotamyrus castor almost certainly forages along the riverbed for small invertebrates — aquatic insect larvae, worms and tiny crustaceans hidden in sediment — using its weak electric field to detect prey and obstacles it cannot see in dim, tannin-stained water. Because it lacks an elongated chin appendage, food is taken with an ordinary small mouth rather than an extended probing organ, so this genus reads its surroundings electrically but forages in a comparatively unspecialised way compared with trunk-nosed relatives such as Gnathonemus.
No aquarium-specific diet has been documented for this species — it is essentially unknown in the hobby — but, as with other small mormyrids, live and frozen invertebrate foods such as bloodworm, brine shrimp and blackworm would be the expected staples for any specimen kept in captivity.
Mating
Mating in mormyrids generally is built around the electric organ discharge: each species produces a distinctive pulse waveform that serves as a species-recognition signal and is thought to play a courtship role, letting individuals identify suitable, same-species partners in murky water where vision is of little use. Beyond that broad family pattern, nothing specific has been documented about courtship or pair formation in Hippopotamyrus castor.
Wild mormyrids as a group are believed to be seasonal spawners, with reproduction cued to the flood pulse of the rainy season as rivers rise and inundate marginal vegetation — conditions that presumably apply to H. castor within its Sanaga, Lokoundjé and Cross River range, though this has not been directly studied in the species.
Breeding
Breeding biology in Hippopotamyrus castor is essentially undocumented, and that is true of most mormyrids: reproduction is one of the most poorly understood aspects of the family's natural history. Successful captive breeding of mormyrids in general is rare to the point of being noteworthy when it happens, and there is no record of H. castor having been bred in an aquarium.
What little is known of mormyrid reproduction more broadly suggests some species scatter eggs among flooded marginal vegetation during the rainy season, with at least some element of egg-guarding suspected in a few better-studied elephantnoses — but the details vary by species and remain sparse even for those. For H. castor specifically, the honest position is that its breeding mode, egg description, incubation period and any parental care are simply not documented in accessible sources.
In the aquarium
Hippopotamyrus castor is not an established aquarium fish. It is a narrow-range West-Central African river endemic that, unlike the widely traded Gnathonemus petersii, does not appear to have any real presence in the hobby, and no species-specific husbandry has been documented for it. Anyone encountering a fish offered under this name should treat general Mormyridae care as a starting point rather than as tested, species-specific guidance.
That general mormyrid care is worth stating plainly because it departs from most community fishkeeping: soft, dimly lit water with plenty of cover — driftwood, roots, fine leaf litter — to mimic a tannin-stained forest river, a fine sandy substrate the fish can probe without injury, and tankmates chosen with care, since mormyrids can be intolerant of their own species in a confined tank even though they are often social in the wild. They are notoriously sensitive to poor water quality and to many medications, so stable, clean water matters more here than for most fish, and any treatment should be researched as mormyrid-appropriate and dosed conservatively.
Conservation
The IUCN Red List assesses Hippopotamyrus castor as Least Concern, an assessment made in September 2019. Despite its narrow range across a handful of rivers in Cameroon and Nigeria, the species is not currently considered threatened.
As with many range-restricted West African river fish, though, a Least Concern rating at a single point in time is not a guarantee of long-term safety: deforestation, agricultural runoff and other habitat-scale pressures on the Sanaga, Lokoundjé and Cross River systems are the kind of threats that could affect a narrow-range species like this one, even without a formal reassessment yet reflecting it.