Taxonomy & naming
Petrocephalus haullevillii was described by the Belgian-British ichthyologist George Albert Boulenger in 1912, in his paper on fishes collected in the lower Congo region by W. J. Ansorge, published in the Annales du Musée du Congo Belge. The type material came from three localities in the Lower Congo: the Chiloango River at Mayili, the Luali River at Lundo, and the Luculla River. The species belongs to the family Mormyridae within the order Osteoglossiformes, the bonytongue fishes, one of the most ancient living lineages of teleosts.
For much of the twentieth century the name was treated as a subspecies rather than a full species: David and Poll (1937) placed it as Petrocephalus stuhlmanni haullevillii, and Whitehead and Greenwood (1959) later reassigned it as Petrocephalus catostoma haullevillii. Modern treatments, including Eschmeyer's Catalog of Fishes, recognize Petrocephalus haullevillii as a valid species in its own right. The genus name comes from the Greek petros ("rock, stone") and kephale ("head"), a reference to the hard, blunt skull typical of the genus; the species epithet honors Alphonse de Haulleville, director of the Congo Museum in Tervuren, Belgium, at the time of the description.
Morphology
Petrocephalus haullevillii is a small mormyrid, reaching only about 3 in, near the lower end of the size range for the genus. Boulenger's original description characterizes the body as strongly compressed, with body height fitting 3 to 3.5 times into the total length and head length fitting roughly 3.7 to 4.25 times into the total length — a deep-bodied, laterally flattened build typical of Petrocephalus.
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 spread across its skin, using the field to navigate and locate prey in murky or low-light water. Petrocephalus haullevillii is notable within its own genus for lacking the electroreceptive rosettes — small clusters of specialized sense organs — found on the head of most other Petrocephalus species; it is one of only five species in the genus known to lack them, a character used to help distinguish it from superficially similar relatives.
Habitat
The species is known from the Lower Congo region of the Democratic Republic of the Congo, with its type localities in the Chiloango and Kouilou-Niari river basins near the Congo River estuary. FishBase records it from swamps and minor tributaries in this area — a freshwater, bottom-dwelling species rather than one found in open river channels. No temperature, pH, or other water-chemistry data has been published for the species.
Feeding
Specific dietary data for Petrocephalus haullevillii has not been published, but the genus as a whole is made up of benthic invertebrate feeders, taking dipteran and ephemeropteran larvae, caddisfly (trichopteran) larvae, and dragonfly and damselfly (odonate) nymphs from mud, silt, and submerged vegetation. Feeding is presumed to rely on electrolocation, as in other mormyrids, though this has not been documented specifically for this species.
Mating
No published account describes courtship or mate selection in Petrocephalus haullevillii. In mormyrids generally, the electric organ discharge is thought to carry information relevant to species and sex recognition, but nothing has been recorded to confirm this for the present species, and extending that idea here would be speculation rather than documented fact.
Breeding
Breeding is unrecorded for Petrocephalus haullevillii, in the wild or in captivity. No spawning event, egg description, or larval account has been published for this species, and its reproductive biology remains essentially unknown.
In the aquarium
Petrocephalus haullevillii is essentially absent from the aquarium trade; the available literature offers no account of it being kept, bred, or even reliably identified alive in a hobbyist tank. What is documented for the species — its small size and Lower Congo swamp and tributary habitat — makes it a plausible subject for a dedicated mormyrid setup, but nothing in the sources supports treating it as an established aquarium fish.
Any attempt to keep this species would need to follow general practice for small Petrocephalus and other mormyrids: a soft substrate for probing, ample cover and dim lighting to offset a life spent reading electric fields rather than daylight, and live or frozen invertebrate foods rather than dry feeds. Mormyrids as a group are known to be sensitive to poor water quality and to some common medications, so cautious acclimation and conservative dosing would be essential rather than optional.
Conservation
Petrocephalus haullevillii has not been assessed by the IUCN Red List, and its conservation status is formally unknown. It is documented from only a handful of historical localities in the Lower Congo, and no data on population trend, abundance, or specific threats appears in the available literature. The absence of an assessment should not be read as evidence of safety — it reflects how little modern survey work has covered this species, a common situation among Petrocephalus described from limited nineteenth- and early twentieth-century collections.