Taxonomy & naming
Petrocephalus magnitrunci was described in 2012 by Bernd Kramer, Roger Bills, Paul Skelton, and Michael Wink, in the Journal of Natural History (volume 46, issue 35-36, page 2240). It belongs to the family Mormyridae, the elephantnoses and mormyrids, within the order Osteoglossiformes — the bonytongue fishes, one of the most ancient living lineages of teleosts. The Catalog of Fishes lists no synonyms, and Petrocephalus magnitrunci is the currently valid name for the species.
The holotype (SAIAB 67069) and ten paratypes (SAIAB 186057) were collected from the Boro River in the central Okavango Delta, southeast of Chief's Island, Botswana, at 19°31'57"S, 23°05'21"E — the species' type locality. The genus name Petrocephalus comes from the Greek petros (rock, stone) and kephale (head), for the hard, blunt cranium typical of the genus; the species epithet magnitrunci combines the Latin magnus (great) and truncus (trunk, torso), describing the deep, thickset body of the fish relative to its relatives.
Morphology
Petrocephalus magnitrunci is a small mormyrid, reaching about 3.5 in in standard length, with the type series measuring 3–3.5 in SL. The body is rounded and dorsolaterally compressed, giving the fish an egg-like outline rather than the elongated shape of some other mormyrids; body depth runs 32.7–36.8% of standard length. The head is rounded, the mouth small and subterminal, set low and just forward of the eye, and the eye itself is proportionally large at 21.8–25.0% of head length. The dorsal fin carries 19–22 rays, the anal fin is long relative to the body at 23.5–27.0% of standard length, and the lateral line runs 39–41 scales along the flank.
As in every mormyrid, Petrocephalus magnitrunci carries a weak electric organ in the caudal peduncle, which makes up about 19.2% of standard length (range 17.6–21.5%) and generates a species-specific pulse discharge. The electric organ discharge of this particular species has not been published, so it cannot be used here to separate it from its many similarly built relatives; the anatomical measurements above are, at present, the documented basis for recognizing the species.
Habitat
Petrocephalus magnitrunci is known from the Okavango River system in Botswana and Angola, with its type locality in the Boro River of the central Okavango Delta, southeast of Chief's Island. It occupies shallow floodplain lagoons with mud substrate and dense stands of emergent grasses and water lilies, in water up to about 7 ft deep.
The Okavango Delta floods seasonally on a cycle driven by rainfall far upstream in the Angolan highlands, which arrives as a winter flood pulse. The lagoons Petrocephalus magnitrunci inhabits expand with that flood and contract again during the low-water period. This boom-and-bust flood cycle, more than a fixed river channel, is the defining feature of the habitat.
Feeding
No diet study has been published for Petrocephalus magnitrunci specifically. Mormyrids as a group are benthic invertebrate feeders that use their electric sense to locate prey such as insect larvae, worms, and small crustaceans hidden in mud and vegetation. The species' habitat of muddy floodplain lagoons is consistent with that foraging style, but this is an inference drawn from the family rather than a documented observation for Petrocephalus magnitrunci itself.
Mating
No courtship or mating behaviour has been described for Petrocephalus magnitrunci. In mormyrids generally, the electric organ discharge is thought to carry information used in species and sex recognition and may factor into courtship, but nothing has been published connecting that general pattern to this species, and its own discharge waveform has not been recorded.
Breeding
Breeding has not been recorded for Petrocephalus magnitrunci, in the wild or in captivity. No spawning account, egg description, or parental-care observation exists for this species in the sources consulted for this profile, and none should be assumed from its better-studied relatives.
In the aquarium
Petrocephalus magnitrunci is essentially absent from the aquarium trade; no keeping or breeding record for the species was found. Small Petrocephalus species surface only occasionally in specialist shipments, and where similar-looking species are traded, individuals are frequently misidentified — the electric organ discharge, not external appearance, is the reliable way to separate look-alike species in this genus, and that is not something a hobbyist can check.
A keeper taking on any small mormyrid of this build should plan around the family's known needs rather than this species' own track record: a soft sand substrate for probing, dense cover, dim lighting, and stable, clean, low-turbidity water, since mormyrids as a group tolerate poor water quality and many common medications poorly. Live and frozen invertebrate foods offered after dark suit the natural foraging pattern of the genus far better than daytime flake feeding.
Conservation
Petrocephalus magnitrunci has not been evaluated by the IUCN Red List. No conservation assessment, and correspondingly no documented threats, currently exist for this species.
Its known range is confined to the Okavango Delta system spanning Botswana and Angola, a wetland shaped by a seasonal flood pulse that originates in the Angolan highlands. Upstream water use and land-use change in that catchment are the general kinds of pressures that could eventually affect the Delta's fish fauna, but no such link has been established for this species specifically.