Taxonomy & naming
Petrocephalus arnegardi was described in 2014 by Sébastien Lavoué and John Sullivan as part of a revision of Petrocephalus species from the Congo Basin. It sits in the family Mormyridae, the elephantfishes, within the order Osteoglossiformes, the bonytongues — a deep-time lineage of freshwater fishes found today on four continents (South America, Africa, Southeast Asia and Australasia) that split apart long before most modern fish families existed. The Congo Basin alone holds an unusually diverse mormyrid fauna, and Petrocephalus is one of its larger genera; species are often difficult to tell apart on colour pattern alone and are more reliably separated by fin-ray counts, body proportions and, in many cases, the waveform of their electric-organ discharge.
The genus name Petrocephalus comes from the Greek petros, "rock" or "stone", and kephale, "head", a reference to the hard, bony head typical of these fish. The species epithet arnegardi honours Matthew E. Arnegard, a biologist whose research on mormyrid electric signalling and systematics has shaped modern understanding of the family. The holotype was collected from the Pandaka River inside Odzala-Kokoua National Park in the Republic of Congo, at roughly 0.62°N, 14.92°E, and is held with several paratypes ranging from about 2.47–3.55 in standard length.
Morphology
Petrocephalus arnegardi is a small mormyrid: the holotype measures 2.86 in standard length, with paratypes spanning roughly 2.47–3.55 in SL, and FishBase lists a maximum of about 3.5 in total length. Like other members of its genus it has the blunt, rounded snout typical of Petrocephalus, quite unlike the long, trunk-like schnauzenorgan of the more familiar aquarium elephantnose, Gnathonemus petersii.
The original description separates P. arnegardi from its close relatives by a distinctive colour pattern: three paired dark patches arranged bilaterally along the body, one below the dorsal fin, one over the pectoral-fin base, and one at the base of the caudal fin. Fin-ray counts are also diagnostic, with 20 to 22 dorsal-fin rays and 31 to 34 anal-fin rays. As in all mormyrids, an electric organ in the caudal peduncle produces a weak, pulse-type electric-organ discharge that the fish uses to sense its surroundings in dark water and, in many species, to signal to others of its kind; mormyrids are also known for carrying one of the largest brain-to-body-size ratios of any fish group, driven by an enlarged cerebellum that processes the constant stream of electrosensory information.
Habitat
The species is known from the Likouala drainage, a middle Congo River tributary system, in the Republic of Congo and the Democratic Republic of the Congo. The type locality, the Pandaka River, lies within Odzala-Kokoua National Park, where FishBase records the species as abundant in the Lékoli and Pandaka river systems.
Like most Congo Basin river habitats used by mormyrids, this is soft, tannin-tinted flowing water rather than the mineral-rich lake water suited to many aquarium cichlids — the kind of dim, sediment-laden environment in which a weak electric sense is more useful than eyesight. FishBase notes the species is riverine and mostly active around dusk, in keeping with the largely crepuscular and nocturnal habits typical of the family.
Feeding
No dedicated dietary study of Petrocephalus arnegardi has been published, so its wild diet is not directly documented. Mormyrids in general are invertivores that use their electric sense and touch, rather than sight, to detect prey buried in sand or hidden among debris, taking aquatic insect larvae, small crustaceans and other soft-bodied invertebrates, mostly around dusk and after dark.
Given the blunt snout of Petrocephalus, foraging is likely closer-range probing over the substrate rather than the fine, extended reach a longer-snouted mormyrid like Gnathonemus achieves with its schnauzenorgan, but this is inference from the genus rather than a confirmed observation for this species.
Mating
Courtship and spawning have not been described for Petrocephalus arnegardi. Across the Mormyridae, the electric-organ discharge is known to carry species-specific and often sex-specific information and is thought to play a role in courtship and mate recognition, but the fine detail of how any given species uses its EOD to pair up is documented for only a small number of well-studied species, and this is not one of them.
Breeding
Breeding biology in the wild is essentially unrecorded for this species. Mormyrids as a group are typically seasonal, flood-triggered spawners, with breeding activity linked to rising water at the start of the rains; some species are thought to scatter eggs among submerged plants or root mats with little or no further parental involvement, but the details vary by species and are poorly resolved even for far better-known mormyrids.
Captive breeding of Petrocephalus arnegardi has not been reported. Mormyrids as a whole are rarely if ever bred in aquaria — captive spawning is essentially undocumented across the family, and this narrowly distributed, recently described species has no aquarium-trade history to draw on.
In the aquarium
Petrocephalus arnegardi is not known to be present in the aquarium trade; it was described only in 2014 from a single national park drainage, and no husbandry record specific to the species exists. Anything offered here is extrapolated from general mormyrid care rather than direct experience with this fish.
Mormyrids as a group do best in dimly lit, well-planted tanks with soft sand substrate and plenty of cover such as pipes or hollow wood, kept at a warm, stable temperature (commonly cited around 79–82 °F) with a moderate, close-to-neutral pH. They are shy, mostly nocturnal fish that appreciate company of their own kind but can turn territorial toward one another and toward other species, so tankmates need care. Two points matter for any mormyrid: the skin lacks scales, so many medications and copper-based treatments must be dosed conservatively, and the fish relies on a weak electric field to navigate, so rough handling and heavy electrical interference in the water are best avoided.
Conservation
Petrocephalus arnegardi has not been formally assessed by the IUCN Red List and is listed as Not Evaluated. FishBase characterises the species as having high resilience and low vulnerability to fishing pressure, though this is a generic trait-based estimate rather than a population assessment.
The known range sits largely within Odzala-Kokoua National Park, a protected area, which offers the species some habitat security even without a formal conservation listing. Beyond that, there is too little published data on distribution and population size to say more than that the species is confined to a limited part of the Likouala drainage in the Congo Basin.