Taxonomy & naming
Petrocephalus binotatus was described by the French ichthyologist Jacques Pellegrin in 1924. It belongs to the family Mormyridae, the elephantfishes or mormyrids, within the order Osteoglossiformes — the bonytongues, one of the oldest branches of the living teleost fishes, named for a bone on the floor of the mouth armed with teeth that bite against a toothed pad on the roof of the mouth. The order's living members are scattered across four continents: arowanas and the arapaima in South America, mormyrids and featherback knifefishes in Africa, the Asian arowana in Southeast Asia, and the saratoga in Australasia, each a relic of a radiation far older than most familiar aquarium fish groups.
The genus Petrocephalus, meaning "stone head" in Greek, is separated from other African mormyrid genera such as Gnathonemus and Mormyrus by a short, blunt snout with no elongated chin appendage; it is a large genus, with species scattered across river systems from West Africa to the Nile and the Zambezi. The species epithet binotatus is Latin for "twice marked," most likely a reference to a pair of pale spots near the gill cover used to tell this species apart from its many close relatives — identification within Petrocephalus often comes down to exactly this kind of small marking, since most species share a similar plain silvery-grey body. No English common name is in wide use for Petrocephalus binotatus; it is essentially unknown outside ichthyological literature and the occasional mixed shipment of imported African mormyrids.
Morphology
Petrocephalus binotatus is a small mormyrid, reaching about 3.9 in (FishBase records the figure as standard length), among the smaller members of a family that includes species over 23.6 in. The body is compact and moderately compressed, the head short and blunt as the genus name implies, with a small mouth set low and a slender caudal peduncle leading into a deeply forked tail — the streamlined rear end typical of mormyrids, built for the slow, steady swimming that keeps the electric sense working cleanly.
As in all mormyrids, an electric organ sits in the caudal peduncle, just ahead of the tail, built from modified muscle cells (electrocytes) that fire in a species-specific pattern. The discharge is weak, well under a volt outside the fish, and is not used to stun prey; it sets up an electric field around the body that the fish reads through skin receptors to detect objects, obstacles and other mormyrids in the dark, turbid water of its Congo habitat. Mormyrids also carry an unusually large cerebellum relative to body size, among the highest brain-to-body ratios recorded in fish, which processes the constant stream of electrosensory information.
Habitat
Petrocephalus binotatus is known from the middle stretch of the Congo River, from Pool Malebo (the wide, lake-like reach of the river near Kinshasa) downstream to Wagenia Falls near Kisangani, within the Democratic Republic of the Congo. It is a freshwater, bottom-associated (demersal) fish of the main river channel and its margins rather than a lake or floodplain specialist.
Water in this stretch of the Congo runs warm and only moderately mineralised: reported conditions are around 77–81 °F, pH 7.0–7.5 (essentially neutral to just alkaline) and 6–10 °dH. The river here carries a heavy sediment load and limited light penetration, the kind of visually murky water in which a weak electric sense is more useful than eyesight. Middle Congo populations of Petrocephalus binotatus share this stretch with a wide range of other mormyrids and Congo-basin fish, and habitat needs beyond this basic water chemistry are not well documented for the species.
Feeding
Wild diet has not been specifically studied for Petrocephalus binotatus, but the genus and family are well characterised: mormyrids are benthic invertivores that use their electric sense, together with touch and smell, to locate small invertebrates buried in sand or hiding among debris, feeding mostly around dusk and after dark when much of that prey is active.
In the aquarium, mormyrid keepers rely on live or frozen bloodworm, mosquito larvae, Tubifex and Daphnia, all readily eaten and closer to a natural diet than dried foods, which most mormyrids largely ignore. Feeding after the lights go off matches the fish's natural rhythm and cuts down on competition from more forward tankmates.
Mating
Courtship and mating in Petrocephalus binotatus have not been documented in detail. Across the Mormyridae, the species-specific pattern of the electric organ discharge is known to play a role in courtship and species recognition — fish appear to identify their own species, and potential mates, by the shape of the pulse alone — but exactly how this translates into pairing behaviour in Petrocephalus is not established.
Most mormyrids studied in the wild are seasonal spawners, breeding around the rains when rivers rise and flood adjacent vegetation, which is thought to provide both spawning substrate and cover for eggs and larvae. There is no specific published account of Petrocephalus binotatus spawning behaviour to confirm whether this general pattern holds for the species.
Breeding
Breeding biology in Petrocephalus binotatus is poorly known, and captive breeding of this species has essentially not been reported. That is true of the great majority of mormyrids: the group is notoriously difficult to reproduce in aquaria, and only a handful of species have ever been bred under controlled conditions, usually by accident rather than by design.
What little is understood about mormyrid reproduction generally comes from a few better-studied relatives rather than from Petrocephalus itself: flood-triggered spawning, adhesive eggs scattered or laid among submerged plants and root mats, and no sustained parental care beyond, in some species, loosely guarding the immediate spawning site. None of this has been confirmed for Petrocephalus binotatus specifically, and anyone attempting to breed it would be working from genus-level inference rather than a documented protocol.
In the aquarium
Petrocephalus binotatus is rarely, if ever, seen in the aquarium trade; most imported mormyrids belong to the more familiar trunk-nosed genera, and small, plain-bodied Petrocephalus species mostly arrive as incidental bycatch in mixed African shipments, if they arrive at all. Anyone keeping it should treat the following as general mormyrid care extrapolated to this species rather than species-specific guidance.
Mormyrids do best in a dimly lit tank with soft sand, plenty of driftwood, roots and dense cover, and stable, mature water — they are notably sensitive to poor water quality and to many common medications, and should be dosed conservatively if treatment is unavoidable. Petrocephalus binotatus, at around 3.9 in, is one of the more modestly sized mormyrids and does not carry the space demands of the larger elephantnoses, but conspecifics can be territorial toward one another, so a single specimen or a well-spaced group in a suitably large, well-furnished tank is the safer approach. It feeds and moves mostly after dark, so daytime activity is limited even in a settled fish.
Conservation
The IUCN Red List assesses Petrocephalus binotatus as Least Concern. It is known from a defined stretch of the middle Congo River rather than the whole basin, but there is no evidence of a population decline or a specific threat driving conservation concern.
As with most Congo-basin fish, the longer-term picture depends on pressures across the river system as a whole — sedimentation, deforestation along tributaries, and any future hydropower or mining development — rather than on anything specific to this species. Because Petrocephalus binotatus has essentially no aquarium trade presence, collection for the hobby is not a relevant threat.