Taxonomy & naming
Petrocephalus steindachneri was described by the American ichthyologist Henry Weed Fowler in 1958, in Notulae Naturae, a short-paper series published by the Academy of Natural Sciences of Philadelphia. It belongs to the family Mormyridae, the elephantnoses and mormyrids, within Osteoglossiformes, the bonytongue fishes — an ancient teleost lineage named for a toothed structure on the tongue that bites against toothed plates on the roof of the mouth. Within the family it sits in Petrocephalus, a genus of small, round-headed African mormyrids often called churchills, distinguished from relatives such as Gnathonemus by the absence of an elongated chin appendage. The type locality is Msola Brook, near Kiperege, in the Ulanga River system of the Rufiji basin, Tanzania; the type series consists of six syntypes held at the Naturhistorisches Museum Wien (NMW 55117–18).
The name steindachneri is a replacement name, not an original coinage: Franz Steindachner had already described this fish in 1916 as Petrocephalus affinis, but that name was preoccupied by an earlier Petrocephalus affinis Sauvage, 1878, so Fowler supplied a new name for Steindachner's taxon. The species' validity has been questioned since — Seegers (1996) treated it as a synonym of Petrocephalus catostoma — but Gosse (1984) and a comprehensive revision of the genus by Kramer, Bills, Skelton and Wink (2012) both concluded it is a distinct species, and Eschmeyer's Catalog of Fishes currently lists it as valid.
Morphology
Petrocephalus steindachneri is a small mormyrid, reaching about 2.5 in in standard length according to FishBase — near the small end even for a genus of generally modest-sized fish. The dorsal fin carries 22 to 27 soft rays and the anal fin, longer-based and set further back, carries 26 to 31, the deep, laterally compressed body and blunt, rounded snout typical of Petrocephalus.
As in every mormyrid, an electric organ in the caudal peduncle generates a weak, pulse-type discharge that the fish reads through electroreceptors spread across its skin, using the resulting field to navigate and sense its surroundings in low-visibility water. The exact discharge waveform of Petrocephalus steindachneri has not been described in the available literature. In a genus where many species look nearly identical externally, the electric organ discharge is often the more reliable character for separating them, which is one reason taxonomic opinion on this species has shifted over time.
Habitat
Petrocephalus steindachneri is endemic to Tanzania, known from the Ulanga River system within the Rufiji basin, the largest river system draining East Africa's southern highlands into the Indian Ocean. Its type locality, Msola Brook near Kiperege, is a small tributary stream rather than the main river channel. FishBase lists the species as a freshwater, demersal fish of tropical waters, consistent with the bottom-oriented, electrolocating lifestyle typical of the family.
Specific water chemistry data — temperature, pH, or hardness — has not been documented for this species, and no broader survey of its distribution beyond the type locality area appears in the sources consulted for this account.
Feeding
No dedicated diet study has been published for Petrocephalus steindachneri. Other members of the genus are benthic invertebrate feeders, using electrolocation to detect small prey such as insect larvae and worms hidden in mud and sediment before taking them with a quick strike of the small mouth, and this species can reasonably be expected to feed in a similar way, though that has not been confirmed directly.
Mating
No courtship or mating behavior has been described for Petrocephalus steindachneri. In mormyrids generally, the electric organ discharge is thought to carry information used in social and sexual recognition among fish of the same species, but whether or how this applies to Petrocephalus steindachneri has not been studied, and its reproductive behavior in the wild is otherwise undocumented.
Breeding
Breeding has not been recorded for Petrocephalus steindachneri, in the wild or in captivity. No account of spawning, egg development, or parental care exists for this species in the literature consulted for this article, and nothing should be inferred beyond that honest gap.
In the aquarium
Petrocephalus steindachneri has essentially no documented history in the aquarium hobby; it does not appear in the trade literature or care guides available for this account, and no established husbandry record exists for the species. At roughly 2.5 in standard length it would be a small fish by mormyrid standards, but that is an inference from its measured size rather than a report of anyone having kept it.
Mormyrids as a family are known to be sensitive to poor water quality and to certain medications, to prefer soft substrate for probing and dim lighting with ample cover, and to take live or frozen invertebrate foods far more readily than dried diets. Those are reasonable starting points if this species were ever encountered in the trade, but they describe the genus in general rather than anything established for Petrocephalus steindachneri specifically.
Conservation
No public IUCN Red List assessment has been located for Petrocephalus steindachneri. The species is known only from Tanzania, with its type locality restricted to a single tributary stream, Msola Brook, within the Ulanga River system of the Rufiji basin — a narrow known range that would warrant a formal assessment, but no such evaluation exists in the sources available for this article. No species-specific threats have been documented.