Taxonomy & naming
Petrocephalus magnoculis was described in 2012 by Bernd Kramer, Roger Bills, Paul Skelton and Michael Wink as part of a critical revision of the genus Petrocephalus in southern and eastern Africa, published in the Journal of Natural History. That revision resurrected the species Petrocephalus tanensis and described five species new to science, of which P. magnoculis was one. The genus sits in the family Mormyridae — the elephantnoses and mormyrids — within Osteoglossiformes, the bonytongue fishes, one of the older surviving lineages of teleosts.
The holotype, SAIAB 78788, was collected in the Cunene River, Namibia, just below Ruacana Falls near the Hippo Pool Campsite, at roughly 2625 ft elevation. Five paratypes are held between the South African Institute for Aquatic Biodiversity (SAIAB) and the Zoologische Staatssammlung München (ZSM). No synonyms have since been documented for the name.
Morphology
Petrocephalus magnoculis is a small mormyrid, reaching about 4.5 in in standard length. Like other members of the genus it has a compact, laterally compressed body, a rounded, blunt head, and a small downturned mouth rather than the elongate chin appendage seen in the related genus Gnathonemus. In the original description the species carries 20–24 dorsal fin rays, 26–31 anal fin rays, a body depth of 28.3–34.0% of standard length, a head length of 25.0–27.7% of standard length, and 38–42 scales along the lateral line.
The character that gives the species its name is eye size: the eye diameter runs 25.9–32.9% of head length, larger relative to the head than in the other Petrocephalus species treated in the same 2012 revision. As in all mormyrids, an electric organ near the base of the tail generates a weak, pulse-type discharge that the fish reads through electroreceptors across its skin; each species in the genus tends to carry its own discharge waveform, though no discharge data specific to P. magnoculis has been published.
Habitat
Petrocephalus magnoculis is known from the lower Cunene River on the Namibia–Angola border, downstream of Ruacana Falls, at around 2625 ft elevation. FishBase describes the habitat as freshwater and demersal, with rather strong current, rocky or sandy banks lined by dicotyledonous trees, and extensive reed beds and other semiaquatic vegetation along the margins — a fast-flowing stretch of river rather than a still backwater. The climate at the type locality is tropical.
Specific temperature and pH figures have not been published for this species; the fast-flowing, structurally complex habitat described for the type locality is the extent of what the record supports.
Feeding
Petrocephalus magnoculis is recorded as a benthic invertebrate feeder, with an estimated trophic level of 3.2 (± 0.4 SE) on FishBase, consistent with the genus as a whole. Mormyrids typically locate small invertebrates in mud, sand and vegetation using their electric sense before taking them with the small, downturned mouth; the specific prey items eaten by this species have not been documented.
Mating
No courtship or mating behavior has been published for Petrocephalus magnoculis. In mormyrids generally, the electric organ discharge is thought to carry information used in species and mate recognition alongside navigation, but nothing has been recorded to confirm that role for this particular species.
Breeding
Breeding in Petrocephalus magnoculis is unrecorded. No account of spawning, egg deposition, or parental care exists for this species in the wild or in captivity, and — as with most Petrocephalus described in the last two decades — it has essentially no history in the aquarium trade from which such observations might otherwise come.
In the aquarium
Petrocephalus magnoculis has no established presence in the aquarium hobby; it is a range-restricted species known only from a single river system and described too recently and too locally to have built any keeping record. General mormyrid husbandry would apply if it were ever offered: a soft sand substrate for probing, dim lighting, ample cover, and stable, clean water, since mormyrids as a group tolerate poor water quality and many medications poorly. Live or frozen invertebrate foods rather than dry foods would be the expected diet, following the pattern documented for the wider genus.
At under 4.5 in it would not be a large fish to house, but none of this is drawn from experience with the species itself — it should be read as general genus practice, not a track record for P. magnoculis.
Conservation
Petrocephalus magnoculis has not been evaluated by the IUCN Red List. FishBase lists it as harmless to humans and assigns it a low vulnerability score of 13 out of 100, but neither figure substitutes for a formal conservation assessment. Its known range — the Cunene River below Ruacana Falls — is narrow, and no population trend or threat data has been published for the species.