Taxonomy & naming
Petrocephalus ansorgii was described by the Belgian-British ichthyologist George Albert Boulenger in 1903. It belongs to the family Mormyridae — the elephantnoses and mormyrids — within Osteoglossiformes, the order of bonytongue fishes named for a toothed bone on the tongue that bites against toothed plates on the roof of the mouth. Osteoglossiformes is one of the oldest surviving branches of the teleost fishes, and its living members are scattered across South America, Africa, Southeast Asia and Australasia by deep time rather than by any single shared origin. The Catalog of Fishes lists Petrocephalus ansorgii as the currently valid name.
The genus name Petrocephalus comes from Greek petra, "rock", and kephale, "head", a reference to the hard, bony skull shared by the roughly thirty species placed in the genus. The species epithet ansorgii honors William John Ansorge, a British physician and naturalist who collected extensively in West and Central Africa in the early 1900s and whose name is attached to several fish and bird species from the region.
Morphology
Petrocephalus ansorgii has the compact, deep-bodied build typical of its genus, which sets it apart at a glance from the more elongated, trunk-snouted mormyrids such as Gnathonemus: the mouth is small and terminal, and Petrocephalus lacks the fleshy chin appendage, the schnauzenorgan, that gives true elephantnoses their name. FishBase records a maximum size of about 5.6 in, modest even by mormyrid standards, and aquarium specimens are usually smaller still, rarely topping 3.9 in.
The body carries the family's electric organ near the base of the tail, which produces a weak, species-specific pulse-type electric organ discharge (EOD). Mormyrids read the distortions this field creates as objects and other fish move through it, an active sense called electrolocation that lets them navigate and hunt in dark, silty water where vision is close to useless. The family is also known for an unusually large cerebellum relative to body size, among the highest brain-to-body ratios recorded in fishes, dedicated to processing this constant stream of electrosensory information. Many small Petrocephalus species look enough alike that reliable field identification often comes down to the shape of the EOD pulse rather than external appearance.
Habitat
Petrocephalus ansorgii is a West African river fish, recorded from the Upper and Lower Niger, the Bénoué (Benue), and coastal rivers of Nigeria. It is a demersal species of tropical freshwater, living near the bottom of flowing river channels and associated floodplain habitat rather than in open water.
FishBase gives a temperature range of roughly 68–77 °F for the species. Detailed water-chemistry data for wild populations are sparse, but like most river-dwelling mormyrids it comes from turbid, sediment-laden or tannin-stained water with limited visibility, the kind of environment in which electrolocation is thought to have evolved as a substitute for eyesight. Precise pH figures are not well documented for this species; in the aquarium it is generally kept in soft, slightly acidic to neutral water rather than hard or alkaline conditions.
Feeding
In the wild, Petrocephalus ansorgii forages along the riverbed for small invertebrates — insect larvae, worms and other benthic prey — locating food partly by electrolocation as it probes soft substrate in low-visibility water, a general foraging strategy documented across the mormyrid family.
In the aquarium it takes frozen bloodworm and brine shrimp readily and does best on a varied diet of small live or frozen invertebrate foods rather than flake or pellet alone. Because it feeds mainly at dusk and after dark, offering food once the lights dim helps it compete for meals in a mixed tank.
Mating
Mating in Petrocephalus ansorgii, as in mormyrids generally, is bound up with its electric sense: the species-specific EOD pulse is thought to help fish recognize and select mates of their own kind among the several mormyrid species that can share a single river system. Beyond that general pattern, courtship behavior specific to P. ansorgii has not been documented.
Wild spawning in mormyrids is typically seasonal and triggered by rising water during the flood season, when floodplain habitat opens up and food becomes more abundant for developing fry. The finer details for this species — courtship displays, spawning sites, egg-laying behavior — are sparse in the available literature.
Breeding
Petrocephalus ansorgii is essentially not bred in captivity, and its reproductive biology is poorly known even by the standards of a family that is generally difficult to spawn. Mormyrids as a group have a poor aquarium breeding record; the electric organ discharge appears to play some role in courtship and species recognition, but the finer points of nest building, egg-laying and parental behavior remain sparse for most species, this one included.
What little can be inferred from the family points to flood-triggered, seasonal spawning in the wild, with some mormyrids thought to scatter or deposit eggs among vegetation. No reliable, repeatable method exists for breeding P. ansorgii in home aquaria, and hobbyists should not expect to reproduce it; essentially all specimens in the trade are wild-collected.
In the aquarium
Petrocephalus ansorgii is a small, peaceful mormyrid suited to a specialist setup rather than a generic community tank. It rarely exceeds 3.9 in in captivity, so there is no size trap with this species, but it is more demanding than its modest dimensions suggest: it wants dim lighting, a soft substrate it can probe with its snout, and plenty of cover — driftwood, roots and leaf litter — to retreat into during the day, since it is most active at dusk and after dark.
Water quality matters more here than for many aquarium fish. Mormyrids are notably sensitive to poor or fluctuating water conditions and to some medications, so treatments should be dosed conservatively, and copper-based medications in particular are best avoided. Group dynamics need attention too — like many mormyrids, this species can be territorial and occasionally aggressive toward its own kind, so tank size and numbers should give individuals room to establish their own space rather than crowding several fish together. A varied diet of bloodworm, brine shrimp and other small invertebrate foods, offered in the evening, keeps it in good condition.
Conservation
The IUCN Red List assesses Petrocephalus ansorgii as Least Concern, based on an assessment dated August 29, 2019. It is not listed under CITES.
The species is known from a reasonably wide span of West African river systems — the Niger, the Bénoué and coastal rivers of Nigeria — rather than a single restricted locality, which supports the Least Concern rating. As with much of West Africa's river fish fauna, longer-term pressures such as damming, pollution and habitat degradation in the Niger basin are general concerns for the region, though no specific population decline has been documented for this species.