Taxonomy & naming
Mormyrus cyaneus was described by the American ichthyologists Tyson R. Roberts and Donald J. Stewart in 1976. It belongs to the family Mormyridae 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 (Gymnotiformes); the two lineages evolved similar electric senses independently on opposite sides of the Atlantic, and neither should be confused with the unrelated Old World featherback knifefishes (Notopteridae), which share this order but have no electric sense at all.
The genus name Mormyrus was coined by Linnaeus in 1758 and borrows a much older Greek word, mormyros, which classical authors including Aristotle applied to an unrelated Mediterranean sea bream — the name was simply repurposed, centuries later, for this large genus of African river fishes, which now holds more than sixty species. The species epithet cyaneus is Latin, from Greek kyanos, "dark blue", a colour descriptor. Many mormyrids look alike to the eye and are told apart reliably only by their species-specific electric organ discharge (EOD), so external identification in this genus is best treated with some caution.
Morphology
Detailed published description of Mormyrus cyaneus is sparse, and no aquarium-facing account of its colour pattern or fin proportions could be confirmed for this article. As in other members of the genus, it likely carries the elongated, gently downturned snout typical of Mormyrus — a fixed extension of the head rather than the flexible chin barbel (the Schnauzenorgan) found in the related genus Gnathonemus — with a small, subterminal mouth set well back from the tip. An electric organ near the base of the tail generates a weak, pulse-type electric organ discharge used for electrolocation and communication, the sense this whole family runs on in place of, or alongside, vision.
FishBase records a maximum size of about 10.2 in total length, a modest figure for a genus whose larger members grow well past that. Because the species is known from very few specimens and is essentially absent from the aquarium trade, claims about colour, fin shape or sexual differences beyond the bare species description should be treated as unconfirmed rather than repeated as fact.
Habitat
Mormyrus cyaneus is endemic to the lower Congo River in the Democratic Republic of the Congo — the short, famously turbulent stretch of the river below Malebo Pool, cut by deep canyons and some of the most powerful rapids on Earth. That hydraulically fractured landscape isolates populations from one another and from the rest of the Congo basin, and it has produced an unusually high concentration of narrow-range endemic fishes found nowhere else, of which this mormyrid is one.
No species-specific temperature or pH data has been published for Mormyrus cyaneus. Water in the lower Congo rapids is fast-moving, turbid with suspended sediment and generally soft and circumneutral, consistent with tropical Central African river conditions, but any numbers given for this fish specifically would be an estimate rather than a measured range, and are presented here as such.
Feeding
Wild feeding behaviour has not been specifically documented for this species. Mormyrids in general are nocturnal, bottom-oriented foragers that use their electric sense to locate small invertebrates — aquatic insect larvae, worms and crustaceans — buried in sand or hidden among rock and debris, and there is no reason to expect Mormyrus cyaneus differs from that pattern.
General mormyrid husbandry calls for small live and frozen foods — bloodworm, Daphnia and similar aquatic invertebrates — offered after dark or in dim light to match the fish's natural rhythm, since these are not fish that readily compete for food in bright, busy conditions.
Mating
Courtship and mate recognition in mormyrids run substantially through the electric organ discharge rather than through visual display, since the EOD is often species- and sex-specific and functions almost like an acoustic signature. Beyond that general pattern, nothing specific to Mormyrus cyaneus has been documented. Mormyrids as a family are thought to be seasonal, flood-triggered spawners tied to the rains and rising water of the African wet season, but the courtship sequence and spawning behaviour of this particular species have not been described in the available literature.
Given how rarely this fish is seen at all — in the wild or in captivity — even the basic outline of its mating behaviour remains a gap in the record rather than a documented process.
Breeding
Mormyrus cyaneus is essentially unknown in captive breeding, and its reproductive biology is poorly understood even in the scientific literature. As with mormyrids generally, wild spawning is presumed to be seasonal and flood-triggered, and the species-specific EOD likely plays some role in courtship, but nest site, egg-laying and any parental behaviour have not been reliably documented for this species. Some mormyrids are thought to guard eggs among vegetation or root tangles, but this has not been confirmed here.
No aquarium breeding reports could be confirmed for this species, and none should be expected: it is a narrow-range wild endemic that essentially does not appear in the trade, not a fish anyone is currently working with as a breeding project.
In the aquarium
Mormyrus cyaneus is not an aquarium fish in any practical sense — it is a narrow endemic of a single stretch of the lower Congo River, collected rarely if at all for the trade, and any specimen offered for sale should be treated with scepticism about its identification given how similar many mormyrids look. General care for the genus, where it applies, follows the same rules as other elephantnoses: a soft sand substrate (never sharp gravel, which can injure a sensitive, electroreceptor-rich snout), dim lighting, dense cover from roots and wood, and excellent, stable water quality, since mormyrids tolerate essentially no ammonia or nitrite and are notably sensitive to copper and to many standard medications.
Mormyrids are best kept solitary or, where space allows, in a properly sized group; two or three specimens together tend to squabble over territory long-term, since an individual defends both physical space and its own electric "signature" against others. None of this is likely to be tested in practice for Mormyrus cyaneus specifically — the honest summary is that this is a scientifically documented species rather than a hobby fish.
Conservation
The IUCN Red List assesses Mormyrus cyaneus as Vulnerable, an assessment made on 16 February 2009 under criterion D2 — a very small area of occupancy that leaves the species exposed to any single damaging event. That narrow range is the entire lower Congo River stretch it is known from, and nothing wider.
The lower Congo rapids are recognised by conservation biologists as one of Africa's richest concentrations of narrow-range endemic fish, and the same stretch of river has long been the focus of major hydropower proposals, including the long-planned Inga Falls dam schemes. Large-scale changes to flow and habitat structure in this reach are the plausible long-term threat to a fish whose entire known distribution sits inside it, though no updated population trend specific to Mormyrus cyaneus is available beyond the 2009 assessment.