Taxonomy & naming
Petrocephalus wesselsi was described by Bernd Kramer and Antoinette van der Bank in 2000, in a paper titled "The Southern Churchill, Petrocephalus wesselsi, a New Species of Mormyrid from South Africa Defined by Electric Organ Discharges, Genetics, and Morphology," published in Environmental Biology of Fishes. It belongs to the family Mormyridae within the order Osteoglossiformes, the bonytongue fishes, and sits in the genus Petrocephalus Marcusen, 1854, a group of several dozen small African mormyrids often called churchills.
The holotype was collected from the Sabie River at the bridge near the Lower Sabie tourist camp in Kruger National Park, South Africa, at approximately 25°07'S, 31°55'E. As the species name suggests, its description leaned heavily on a then-newer approach for the genus: the authors used the fish's electric organ discharge waveform together with genetic data and standard morphometrics to separate it from superficially similar Petrocephalus populations in the region. No synonyms or prior name transfers are documented for this species.
Morphology
Petrocephalus wesselsi is a small mormyrid, reaching about 4.5 in standard length in males and unsexed individuals and about 4.5 in SL in females, according to FishBase. Like other Petrocephalus it has the compact, rounded head and small, underslung mouth typical of the genus, in contrast to the elongated chin appendage of its relative Gnathonemus. The original description notes a comparatively large eye, with head length running 3.0 to 3.71 times the eye diameter, and a fairly large mouth, with head length 2.0 to 3.7 times the mouth width. The dorsal fin carries 18 to 21 branched rays and the anal fin 25 to 28.
As in all mormyrids, a weak electric organ near the base of the tail generates a pulse-type discharge that the fish reads through electroreceptors across its skin, used for orientation and object detection in turbid or nighttime water. The species was defined in part by a distinctive electric organ discharge waveform, treated by its describers as a more reliable diagnostic character than external shape for telling Petrocephalus species apart — though the specific pulse duration and frequency values are not detailed in the sources available for this account. No difference in discharge waveform has been found between males and females of this species.
Habitat
Petrocephalus wesselsi occurs in southeastern Africa, recorded from South Africa, Mozambique, and Zambia. Within South Africa it is known from the Incomati River system (including the Sabie River, its type locality), the Blyde and Letaba Rivers, and the wider Limpopo system that these rivers feed, as well as the Pongola River in KwaZulu-Natal. In Zambia it has been documented from the Lufupa River, a tributary within the Zambezi drainage.
The species is found in fast-flowing, major, perennial rivers bordered by dense subtropical or tropical vegetation, and it is described as benthopelagic, living and feeding near the riverbed rather than in open water. Specific water temperature and pH data for the species have not been published; it should be understood as inhabiting warm, well-vegetated, moving-water river reaches typical of the region rather than still or heavily shaded backwaters.
Feeding
No species-specific diet study for Petrocephalus wesselsi is available. Mormyrids as a family are bottom-foraging invertebrate feeders, typically taking aquatic insect larvae — chironomid midges, blackfly (Simuliidae), and beetles — along with annelid worms, located by electrolocation in silt and among vegetation. Given its benthopelagic habit, this pattern is a reasonable expectation for P. wesselsi, but it has not been confirmed for the species itself and should be read as a family-level inference rather than a documented fact.
Mating
Mature males of Petrocephalus wesselsi show an indentation at the base of the anal fin that is not present in females, an external sexually dimorphic character noted in the original description. No corresponding difference has been found in the electric organ discharge waveform between the sexes, so whatever role the discharge plays in this species' social or reproductive signalling, it is not one that visibly separates males from females in the waveform itself.
Beyond that anatomical detail, courtship behaviour, pair formation, and any role of electric signalling in mate choice have not been studied in this species, and no account of wild spawning behaviour is published.
Breeding
No spawning has been recorded for Petrocephalus wesselsi in the wild or in captivity, and there is no account of this species being bred in an aquarium. A study of a Zambian population in the Lufupa River, based on 913 specimens collected in August 2017, documented three gonad maturity stages — active, immature, and inactive — and found a sex ratio skewed strongly toward females, at roughly one male to four females, a marked departure from an even split. That work speaks to the population's reproductive condition at the time of sampling, but larval development, spawning site choice, and courtship behaviour remain unrecorded.
In the aquarium
Petrocephalus wesselsi has no established presence in the aquarium hobby; the sources available for this account describe it only as a wild river fish studied in field surveys, with no aquarium-keeping record to draw on. General mormyrid husbandry practice — a soft substrate for probing, dim lighting, ample cover, and stable, clean, well-oxygenated water — would apply to any attempt at keeping this species, along with the well-documented sensitivity of mormyrids as a group to poor water quality and to many common fish medications.
At around 4.5 in this is a modest-sized fish rather than a size trap, but that says nothing about temperament or compatibility, which have not been reported for it specifically. Anyone encountering this species in the trade should treat it as effectively untested in captivity rather than assume the general mormyrid-care picture has been verified for it.
Conservation
The IUCN Red List assesses Petrocephalus wesselsi as Least Concern, based on an assessment dated 6 December 2016. It is recorded across a reasonably broad span of river systems in South Africa, Mozambique, and Zambia — the Incomati, Limpopo, Pongola, and Zambezi drainages among them — without evidence pointing to a significant population decline.
No species-specific threats are detailed in the sources available for this account. As with other river-dwelling mormyrids in the region, general pressures on southern African rivers such as damming, water abstraction, and siltation are the kind of long-term concerns that could affect the species, but the current Least Concern status should not be read as flagging any immediate risk.