Taxonomy & naming
Petrocephalus catostoma was described by the German-British ichthyologist Albert Günther in 1866. 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, named for a bony, tooth-bearing structure on the tongue that bites against toothed plates on the roof of the mouth. The Catalog of Fishes lists Petrocephalus catostoma as the valid name in a genus of several dozen small African mormyrids.
The genus name Petrocephalus comes from the Greek petros (rock, stone) and kephale (head), a reference to the hard, blunt cranium typical of these fish. The species epithet catostoma is built from the Greek kata (down, downward) and stoma (mouth), describing the small, underslung mouth set low on the head — a feature shared by most Petrocephalus species and distinct from the trunk-like chin barbel (the schnauzenorgan) that defines their better-known relative Gnathonemus.
Morphology
Petrocephalus catostoma is a modest-sized mormyrid, reaching about 5.9 in in fork length according to FishBase, which puts it among the smaller members of the family rather than the trunk-nosed giants of the trade. The body is laterally compressed and deep for its length, with a short, blunt head, a small, underslung mouth, and a single dorsal fin set well back over the anal fin — a body plan built for slow, deliberate swimming rather than speed.
As in all mormyrids, an electric organ near the base of the tail produces a weak, species-specific pulse discharge. The fish reads the resulting field with electroreceptors spread over its skin, using it to sense obstacles, prey, and other fish in murky or nighttime water where eyesight is of limited use. Mormyrids are also known for carrying one of the highest brain-to-body mass ratios of any vertebrate group, with an enlarged cerebellum devoted to processing this electrosensory stream — visual identification between similar Petrocephalus species is often unreliable, and the discharge itself is frequently the more dependable marker.
Habitat
The species is native to southeastern Africa, documented from the Rovuma River system on the Tanzania–Mozambique border, including waters within the Niassa Reserve, and nearby coastal drainages. Its type locality is the Rovuma River. Records from the Congo basin are unconfirmed, and the species should be treated as an East African endemic rather than a broadly continental one until better data says otherwise.
Within that range it favours shallow, muddy water: sheltered bays, quiet backwaters and lagoons, and slow river reaches with standing or fringing vegetation, at warm temperatures around 73–79 °F. This is the same kind of low-visibility, structure-rich habitat associated with mormyrids generally, where an electric sense outperforms eyesight. Precise pH data for the species is not well documented; East African floodplain and coastal river systems of this kind typically run soft and close to neutral, but that should be read as a general expectation rather than a measured figure for this particular population.
Feeding
Petrocephalus catostoma is a bottom-foraging invertebrate feeder, in keeping with the family. Mormyrids typically use their electric sense to locate small aquatic invertebrates — insect larvae, worms and tiny crustaceans — hidden in mud, silt and leaf litter, then take them with a quick, precise strike of the small downturned mouth.
In captivity, the aquarium-care literature on mormyrids generally recommends live and frozen invertebrate foods such as bloodworm and brine shrimp; dried or flake foods are usually ignored or accepted only reluctantly. Feeding after the lights go down tends to suit this style of fish better, since mormyrids forage more actively in dim conditions and can be out-competed for food by more visually oriented tankmates in daylight.
Mating
Mormyrids are thought to use their electric organ discharge for more than navigation: the pulse pattern is species-specific and is believed to play a role in courtship and in recognising the sex and identity of other fish, alongside general shoal cohesion. Petrocephalus catostoma is described as occurring in shoals in the wild, which fits this pattern, but the courtship behaviour of this particular species has not been studied in detail.
As with most mormyrids, reproduction in the wild is presumed to be seasonal and tied to the rains and rising river levels, when floodplain habitat opens up for spawning. Beyond that general picture, the specifics of pairing, spawning site choice, and any signalling behaviour in Petrocephalus catostoma are not well documented in the literature available for this account.
Breeding
Breeding in mormyrids as a group is poorly understood and rarely, if ever, achieved in captivity, and Petrocephalus catostoma is no exception — there is no reliable account of this species being bred in an aquarium. Almost all mormyrids in the trade, this species included, are wild-caught rather than farmed.
What little is known from the family points to seasonal, flood-triggered spawning tied to rising water and expanding floodplain habitat, with the electric discharge likely involved in courtship signalling. Some mormyrids are thought to scatter eggs among submerged plants and provide little or no further care, but that pattern has not been confirmed specifically for this species, and it would be a mistake to present it as established fact rather than a reasonable extrapolation from relatives.
In the aquarium
Petrocephalus catostoma occasionally turns up in the trade as one of several small, similar-looking mormyrids sold generically as "elephantnose" or under a Petrocephalus label, and misidentification between species in the genus is common — the electric discharge, not appearance, is the reliable way to tell them apart, and that is not something a hobbyist can check. Tank setup should follow general mormyrid practice: soft substrate for probing, dim lighting, plenty of cover, and stable, clean water with minimal turbidity — mormyrids as a group are notably sensitive to poor water quality and to some medications, so gradual acclimation and conservative dosing are worth the extra care.
At around 5.9 in this is a manageable size rather than a size trap, but mormyrids can be territorial toward their own kind and are best kept either singly with peaceful, non-nipping tankmates or in an established group large enough to diffuse aggression — a lone pair often does not work. Feeding live or frozen invertebrate foods after dark, rather than relying on daytime flake feeding, suits its natural foraging rhythm better.
Conservation
The IUCN Red List assesses Petrocephalus catostoma as Least Concern, based on a 2022 assessment. It is documented across a reasonably wide stretch of the Rovuma River system and adjacent East African coastal drainages, including protected habitat within the Niassa Reserve, without evidence of a significant population decline.
No major species-specific threats are documented for this fish. As with other freshwater species tied to African river and floodplain systems, general pressures such as habitat degradation, siltation and water-quality decline in the wider Rovuma basin are the kind of long-term concerns that could affect it, but the current Least Concern status should not be read as an urgent conservation flag.