Taxonomy & naming
Mormyrops caballus was described by the French ichthyologist Jacques Pellegrin in 1927. It belongs to the family Mormyridae — the elephantnoses and mormyrids — within the order Osteoglossiformes, the bonytongues, a deep-time lineage of teleost fishes that also includes the arowanas, arapaima, African butterflyfish and featherback knifefishes of Asia and Africa. Occurrence databases also list an older name, Mormyrus caballus Boulenger, 1898, associated with populations now placed in Mormyrops; the currently accepted combination and authorship for this species is credited to Pellegrin, 1927.
The genus name Mormyrops combines Mormyrus — a related genus of mormyrids, itself derived from the ancient Greek mormyros, the name of an unrelated marine sea bream — with the Greek suffix -ops, "face" or "appearance", so the genus name means, roughly, "Mormyrus-like". The species epithet caballus is Latin for "horse" or "nag", a reference to the fish's long, narrow, downturned chin, which early describers thought resembled a horse's face. Mormyrops is the largest-growing genus of mormyrids, sometimes called the bottlenose mormyrids in older literature, and several species are difficult to separate on appearance alone.
Morphology
Mormyrops caballus has the elongated, laterally compressed body typical of its genus, with a long, narrow, distinctly horse-like snout and chin that give the species its name. The body is uniformly dark — grey to blackish-brown — without the bold pattern or trunk-like proboscis seen in some other mormyrids such as Gnathonemus petersii; the mouth and chin are simply elongated rather than modified into a fleshy schnauzenorgan.
FishBase records a maximum standard length of 18.1 in for this species, making it one of the larger mormyrids in the hobby and well beyond the size of the small, popularly traded elephantnoses. As in all mormyrids, an electric organ sits in the caudal peduncle near the tail, generating a weak, species- and often individual-specific pulse-type electric-organ discharge (EOD) used for electrolocation and communication. Mormyrids also carry an unusually large, highly folded cerebellum relative to body size — among the highest brain-to-body ratios of any fish — which processes the constant stream of electrosensory information; visually similar mormyrid species are often reliably told apart only by their EOD waveform, not their outward appearance.
Habitat
Mormyrops caballus is known from river systems of West and Central Africa, including the Sanaga, Nyong and Bénoué drainages of Cameroon, the Konkouré basin of Guinea, the Sewa and Jong rivers of Sierra Leone, and the upper Niger basin. It is a bottom-associated (demersal) freshwater fish of tropical rivers, occupying the kind of turbid, sediment-laden, often tannin-stained water typical of West African lowland and floodplain rivers — precisely the low-visibility conditions that favour a fish navigating primarily by electric sense rather than sight.
Neither FishBase nor the other cached sources report species-specific temperature or pH data measured at M. caballus localities. General mormyrid husbandry experience points to warm, soft, mildly acidic water — roughly 79–82 °F and pH 6.8–7.2 — as appropriate for the family as a whole, and it is reasonable to keep this species under similar conditions, but that figure reflects general practice for mormyrids rather than a measurement taken specifically for M. caballus.
Feeding
Mormyrids forage mainly after dark, sweeping the electric field generated by the tail organ over the substrate and using it to detect buried or hidden prey well before it can be seen. Mormyrops caballus almost certainly follows this pattern, hunting aquatic insect larvae, worms and other soft-bodied invertebrates sifted from sand, mud and cover along the riverbed.
In the aquarium, mormyrids are usually weaned onto bloodworm, brine shrimp, blackworms and other small live or frozen invertebrates, along with sinking pellet or wafer foods once settled. Feeding after the lights go out, matching the fish's natural nocturnal rhythm, generally gets a far better response than trying to feed it during the day.
Mating
Courtship and reproduction in mormyrids are built around the electric organ discharge: the species- and sex-specific EOD waveform is thought to function in mate recognition and courtship signalling as well as in general electrolocation, allowing individuals to identify appropriate partners in dark or turbid water where visual cues are of little use. Beyond this, very little is documented about courtship behaviour in Mormyrops caballus specifically.
In the wild, mormyrids as a group are believed to be seasonal spawners, with reproductive activity tied to the rains and rising floodwaters that create the still, vegetated margins many species use for spawning. The details of pair formation, spawning behaviour and egg deposition in M. caballus have not been documented in the literature available for this account.
Breeding
Mormyrids are essentially not bred in captivity, and Mormyrops caballus is no exception — reliable, repeatable captive breeding of this species has not been documented. What is known of mormyrid reproduction in the wild comes largely from general family-level observation rather than species-specific study: seasonal, flood-triggered spawning, with some species thought to scatter eggs among flooded vegetation, after which parental involvement — if any — is poorly understood.
For this species specifically, the honest state of knowledge is that breeding is rarely if ever achieved in the aquarium and the wild breeding biology remains sparsely documented. Any aquarium specimens in the trade are wild-collected rather than captive-bred.
In the aquarium
Mormyrops caballus is a large, sensitive, specialist fish, not a beginner's mormyrid. A reported maximum of 18.1 in places it well beyond the scale of the small elephantnose species commonly sold in the hobby, so a tank intended to house an adult needs to be sized accordingly, with plenty of open swimming room along the bottom rather than the modest footprint that suffices for a Gnathonemus petersii. Like other mormyrids it is shy and largely nocturnal, doing best in a dimly lit tank with a soft sand substrate for sifting and ample cover — driftwood, pipes and root tangles — to retreat into during the day.
Water quality matters more for mormyrids than for most community fish: they are widely regarded as intolerant of poor or fluctuating water conditions and are unusually sensitive to many medications, so any treatment should be researched and dosed conservatively rather than assumed safe. Mormyrids can also be territorial or aggressive toward their own kind, particularly in confined quarters, so keeping this species — like most Mormyrops — as a lone specimen or in a suitably large group with careful monitoring is safer than casually mixing multiple individuals. Because reliable visual identification of mormyrid species is difficult, keepers should treat the exact identity of any imported "Mormyrops" with some caution.
Conservation
The IUCN Red List assesses Mormyrops caballus as Least Concern, an assessment made on 4 September 2019. The species is known from multiple, geographically separate river systems across West and Central Africa — from Sierra Leone and Guinea through to Cameroon — which spreads its overall extinction risk across a fairly wide range rather than concentrating it in a single vulnerable population.
As with many West African river fish, the longer-term concerns for this species are the general pressures facing its habitat: damming, sedimentation, pollution and other forms of freshwater degradation across its range. No population-specific threats or trends beyond the current Least Concern assessment are documented in the cited sources.