Taxonomy & naming
Petrocephalus degeni was described by the Belgian-British ichthyologist George Albert Boulenger in 1906. It belongs to Mormyridae, the family of African elephantnoses and mormyrids, within the order Osteoglossiformes — the bonytongues, among the oldest living lineages of bony fish. The order takes its name from a toothed bone on the floor of the mouth that bites against a corresponding pad on the roof, a feature found across its scattered descendants on four continents: the arowanas and arapaima of South America, the featherback knifefishes and elephantnoses of Africa, and the arowanas and saratoga of Asia and Australasia. Petrocephalus degeni sits in the African branch, within the large genus Petrocephalus, whose many species are spread across river and lake basins from the Congo to the East African lakes.
The genus name comes from the Greek petros, "stone", and kephale, "head", referring to the compact, bony head shared by Petrocephalus species. The species epithet degeni honors E. Degen, recorded in taxonomic references as a naturalist and ornithologist who collected specimens in Uganda in the early twentieth century — the presumed source of the type material Boulenger described.
Morphology
Petrocephalus degeni is a small mormyrid, reaching only about 3.1 in in length — modest even by the standards of its own family, and a fraction of the size reached by some of its Mormyridae relatives. Like other Petrocephalus, it has a compact, laterally compressed body and a short, blunt head, quite unlike the elongated snout of Mormyrus or the trunk-like chin appendage that gives Gnathonemus petersii its "elephantnose". The mouth is small and set low, and the genus lacks a schnauzenorgan altogether — identification within Petrocephalus generally relies on fin-ray counts, scale counts and coloration rather than any single obvious external feature.
As in all mormyrids, an electric organ near the base of the tail generates a weak, pulse-type electric field, and an array of skin receptors reads the way that field is distorted by nearby objects, other fish and the surrounding water — electrolocation, a sense that works whether the water is clear or clouded with sediment. Mormyrids also carry an unusually large cerebellum relative to body size, among the highest brain-to-body ratios of any fish, dedicated largely to processing this constant electrical picture of the world. Because many Petrocephalus species look alike, the discharge waveform itself is often a more reliable way to tell them apart than external appearance, and field identification of P. degeni specifically should be treated with some caution.
Habitat
Petrocephalus degeni is known from the Lake Victoria basin of East Africa, including records from the Katonga River and the lake itself — the vast, shallow lake shared by Uganda, Kenya and Tanzania. It lives over sandy and gravelly bottoms in the shallower, near-shore zone rather than open water, and unlike many solitary, territorial mormyrids it forms shoals.
Specific temperature and pH figures have not been published for this species, so any aquarium target has to be inferred from the general conditions of Lake Victoria and its tributaries — warm water, roughly 24-27 degC. Lake Victoria's shallows are often turbid with suspended sediment and phytoplankton, the kind of low-visibility water in which a mormyrid's electric sense is more useful than eyesight for finding food and avoiding obstacles after dark.
Feeding
In the wild, Petrocephalus degeni feeds on algae, plankton and small aquatic insects picked from the lake bed and gravel margins it inhabits. Like other demersal mormyrids it forages along the bottom rather than in open water.
There is no published account of this species in aquaria, so its feeding response in captivity is not documented. Based on the diet and habits of Petrocephalus and other small mormyrids generally, a fine sinking food — crushed flake, small pellets, and frozen foods such as daphnia or finely chopped bloodworm — offered after the lights dim would be the expected approach, but this is inference from the genus rather than a recorded fact about the species.
Mating
Mating in mormyrids is closely tied to their electric sense: the pulse discharge is species-specific and individually variable, and it is thought to function in courtship and mate recognition as well as in navigation, letting a fish identify a potential mate of the right species in murky water where vision is of limited use. Across the family, spawning is generally seasonal, often triggered by rains and the rise in water level and food availability that follows.
No courtship or spawning behavior has been specifically documented for Petrocephalus degeni. The details recorded for better-studied mormyrids are the best available guide, but should not be assumed to apply exactly to this species — data on its reproductive biology are sparse.
Breeding
Mormyrids as a group are essentially unbred in the aquarium hobby, and Petrocephalus degeni is no exception — there is no record of it having been spawned in captivity. Wild reproduction is assumed to follow the family's general pattern of seasonal, flood-associated spawning, with the electric discharge playing some role in pairing, but the finer details remain poorly known even for far more commonly kept mormyrids.
Some mormyrids are thought to scatter eggs among plants or fine root mats and to guard them for a period afterward, but this has not been confirmed for Petrocephalus degeni specifically. Anyone hoping to breed the species in an aquarium should expect to be working from almost no precedent.
In the aquarium
At around 3.1 in, Petrocephalus degeni avoids the size trap that catches out keepers of the larger elephantnoses and the true giants of the bonytongue order — this is not a fish that will outgrow a modest tank. It is, however, still a specialist. Mormyrids in general are sensitive to poor water quality and to many common medications, particularly anything copper-based, and they do best in soft, dim, well-established water with a fine sand substrate they can probe without injury.
Because it forms shoals in the wild, it is best kept as a group rather than singly, in a tank with subdued lighting, plenty of cover, and calm tankmates that will not out-compete it for food. It is rarely available in the aquarium trade, and because so little has been documented about this particular species, anyone keeping it should treat published Petrocephalus and general mormyrid care guidance as a starting point rather than a guarantee.
Conservation
The IUCN Red List assesses Petrocephalus degeni as Data Deficient, an assessment made in December 2015. That designation means there is not enough information on its population size, trend or the extent of threats to it to assign a status of concern or of safety — it is a statement about the state of knowledge, not evidence that the species is currently declining.
Its known range is confined to the Lake Victoria basin, a lake that has undergone major ecological upheaval over the past century, including the introduction of Nile perch, invasive water hyacinth and long-term eutrophication that have reshaped its fish communities. Whether these pressures affect Petrocephalus degeni specifically is not established, but its narrow range within a heavily altered lake system is a reasonable basis for closer study.