Taxonomy & naming
Campylomormyrus cassaicus was described by the Belgian ichthyologist Max Poll in 1967. It belongs to the family Mormyridae, the elephantfishes, within the order Osteoglossiformes — the bonytongues, a scattered, deep-time radiation of fishes found on four continents (South America, Africa, Southeast Asia and Australasia) and united by that toothed lingual bone rather than by any shared modern habitat. The Mormyridae are an exclusively African family of weakly electric fishes, entirely convergent with — not related to — the electric knifefishes of South America (order Gymnotiformes, covered separately at /knifefishes); the two lineages evolved similar electric senses independently on opposite sides of the Atlantic. GBIF records the authority as Poll, 1967.
The genus name Campylomormyrus combines Greek kampylos, "bent" or "curved", with mormyros, an old fish name borrowed for these African elephantfishes, together describing the down-curved, tubular snout typical of the genus. The species epithet cassaicus refers to the Cassai — now more commonly spelled Kasai — River, the drainage from which the type series was collected. Like other mormyrids, Campylomormyrus species are notoriously difficult to separate on external appearance alone; researchers generally rely on the species-specific electric organ discharge (EOD) to confirm identity, so any visual identification, including of aquarium-trade fish, deserves appropriate caution.
Morphology
Campylomormyrus cassaicus is a small mormyrid, reaching a reported maximum of about 7.9 in total length (FishBase), modest even within a genus that runs to considerably larger fish. The body is elongate and laterally compressed in the family's typical fashion, uniformly dark, with a small mouth set toward the tip of a down-curved, somewhat elongated snout — the trait behind the common name "donkey-faced elephantnose". FishBase records 31 soft rays in the dorsal fin and 35 in the anal fin, a long-based anal fin set opposite a shorter dorsal that is characteristic of the family, with a small, deeply forked tail.
As in all mormyrids, an electric organ near the base of the tail generates a weak, pulse-type electric organ discharge (EOD) used for electrolocation and, likely, communication with other elephantnoses. The discharge waveform is species-specific and, in this genus especially, is often the only reliable way to distinguish superficially similar Campylomormyrus species from one another. Mormyrids as a family are also known for an outsized cerebellum and one of the highest brain-to-body ratios recorded in any fish, the physiological cost of running such a sensitive electrosensory system — though this has been documented at the family level rather than specifically measured in C. cassaicus.
Habitat
Campylomormyrus cassaicus is known from a small number of localities straddling the Angola–DRC border: the Luachimo River in Angola, and the Sankuru and Kasai River drainages of the middle Congo basin in the Democratic Republic of the Congo. FishBase lists it as a freshwater, demersal species — a bottom-dweller rather than a fish of open water. Given how few localities have been recorded, it may prove to be a genuine narrow-range endemic of this corner of the Congo basin, though the data are too sparse to say so with confidence; further survey work would be needed to establish the true extent of its range.
The rivers of this region are typical of the middle Congo basin: warm, slow-flowing and often turbid or tannin-stained, running soft and only mildly acidic to neutral. An aquarium care source cites a pH of roughly 6.5–7.0, a temperature range of about 73–82 °F, and moderate hardness of 4–16 dGH — conditions consistent with soft, dim African floodplain water rather than anything hard or mineral-rich, and precisely the low-visibility habitat in which a weak electric sense earns its keep.
Feeding
Like other mormyrids, C. cassaicus is a benthic forager, using its electric sense together with its downturned snout to locate small invertebrates buried in sand and silt on the river bottom, most actively after dark or in dim light. Detailed dietary studies specific to this species are not available, so its wild diet is best inferred from the family's general pattern of worms, insect larvae and other soft-bodied invertebrates.
In the aquarium, care sources report it accepting live, frozen, flake and pellet foods once settled, which suggests reasonable dietary flexibility for a mormyrid — a group whose members are frequently reluctant feeders when first introduced. As with related elephantnoses, offering food after lights-out and providing a soft substrate that won't injure a fish that spends much of its time probing the bottom are sensible precautions.
Mating
Wild reproductive behaviour in Campylomormyrus cassaicus has not been well documented. Mormyrids in general are believed to be seasonal, flood-triggered spawners, with breeding activity linked to the rains and rising water of the African wet season, and the species-specific electric organ discharge is thought to play a role in courtship and mate or species recognition — but the courtship sequence, spawning site and any pair behaviour in this particular species have not been reliably described in the available literature.
One aquarium care source reports sexual maturity at around 5.9 in and describes a roughly 30–50 day interval between recorded spawning attempts in conditioned groups kept at a ratio of about two males to three females in large tanks. That is more specific detail than exists for many mormyrids, but it comes from a single hobby source rather than a peer-reviewed account, and the same source is explicit that captive breeding remains rare — so it should be read as a lead worth noting, not an established breeding protocol.
Breeding
Campylomormyrus cassaicus is essentially not bred in the aquarium hobby, and — as with the great majority of mormyrids — its reproductive biology remains poorly understood even in the scientific literature. The one aquarium source with breeding notes for this species describes it as breeding rarely in captivity even under the group housing and large-tank conditions it recommends, which is consistent with the family's general reputation as very difficult to spawn intentionally. No documented account of eggs, larvae or raised fry for this species appears in the sources available here.
Some mormyrids are thought to guard eggs among plants or roots, but this has not been confirmed for C. cassaicus. Aquarists should not expect to breed this fish on purpose and should treat any claim of reliable captive spawning with appropriate scepticism; virtually all specimens reaching the trade are believed to be wild-caught.
In the aquarium
This is a specialist's fish rather than a beginner's. It wants soft, dim, tannin-stained water that echoes its native Kasai and Sankuru habitat — roughly pH 6.5–7.0, 73–82 °F and moderate hardness per aquarium care sources — with a soft sand substrate, subdued lighting and plenty of cover from driftwood, roots and leaf litter to shelter in during the day. One care source recommends a substantial tank of 78.7 in or more and keeping fish in a small group rather than singly, which fits the general mormyrid pattern of needing enough space that individuals don't constantly clash over territory and electric "airspace".
Water quality has to be kept excellent and stable: mormyrids as a family are notably sensitive to poor water conditions and to many aquarium medications, copper-based treatments in particular, so any dosing needs real care. Like its relatives it can be territorial toward other mormyrids, especially in a tank that is too small or too sparsely furnished, and its electrosensory communication is easily disrupted by the electrical and acoustic noise of typical aquarium equipment. Given how little is documented about this species specifically, most of this guidance is extrapolated from the wider genus and family, and keepers should expect to proceed cautiously and adjust based on how an individual fish settles.
Conservation
The IUCN Red List assesses Campylomormyrus cassaicus as Data Deficient, per its 2022.2 assessment. That status reflects how little is known about the species — a small number of recorded localities, no detailed population data, and no clear picture of population trend — rather than any confirmed decline.
As with many poorly surveyed Congo basin fishes, the plausible long-term pressures are generic rather than species-specific: habitat degradation, sedimentation and water-quality decline across the Kasai and Sankuru drainages and the Luachimo. There is no evidence in the sources available here of a documented decline for this species, and a Data Deficient rating is best read as a call for more field survey work rather than as evidence of imminent risk.