Taxonomy & naming
Petrocephalus tenuicauda was described by the Austrian ichthyologist Franz Steindachner in 1894, originally as Mormyrus tenuicauda, before later work moved it into Petrocephalus, where it has stood since. It belongs to the family Mormyridae within the order Osteoglossiformes, the bonytongue fishes — one of the most ancient surviving lineages of teleosts, named for the toothed bony structure on the tongue that bites against a matching plate on the roof of the mouth.
Petrocephalus itself makes up the subfamily Petrocephalinae on its own, a lineage regarded as the sister group to all other mormyrids and distinguished from genera such as Gnathonemus by lacking any trunk-like chin appendage. The genus has undergone repeated taxonomic revision, with electric organ discharge waveform now used alongside traditional morphology to separate species that look nearly identical externally; tenuicauda has remained a recognized valid species through those revisions.
Morphology
Petrocephalus tenuicauda is a small mormyrid, reaching about 4.5 in in standard length. The body is compressed and moderately deep, roughly 1.4 times as deep as it is long in standard length, with a single dorsal fin set well back over the anal fin — dorsal soft rays number 24 to 29, anal soft rays 16 to 21, and there are no dorsal spines. Coloration is silvery with little to no iridescence, marked by a dark bar below the eye, a dark triangular patch at the base of the dorsal fin, and darkened dorsal fin tips.
The species epithet tenuicauda, "thin tail," points to the caudal peduncle, which is notably slender relative to the body — its height runs only 1.7 to 2.9 times into the standard length. As in every mormyrid, an electric organ near the base of the tail generates a weak, pulse-type discharge, read by electroreceptors across the skin and used to build a working picture of the surroundings where eyesight offers little help.
Habitat
Petrocephalus tenuicauda is native to the coastal rivers of the Atlantic Guinea slope in West Africa, recorded from drainages in Guinea, Sierra Leone, and Liberia. It is a tropical freshwater species associated with river habitat rather than lakes or estuaries.
Within these systems it is described as a fish of dark, murky river bottoms — turbid, low-visibility water where an electric sense does more work than vision. Detailed temperature and pH figures specific to this species are not documented; West African coastal rivers of this kind are generally warm, soft, and slightly acidic to neutral, but that is a regional expectation rather than a measured value for tenuicauda itself.
Feeding
Petrocephalus tenuicauda is a specialized micro-predator of small invertebrates. It hunts over dark river bottoms using its weak electric organ discharge to locate prey it cannot see directly, a foraging strategy shared across the mormyrids and especially useful in the murky water this species favors.
No aquarium feeding record specific to this species is documented. Mormyrids generally take live and frozen invertebrate foods such as bloodworm and brine shrimp far more readily than dried foods, and there is no reason to expect tenuicauda to differ, but that remains an inference from the family rather than an observation of this fish.
Mating
No courtship or mating behavior has been published for Petrocephalus tenuicauda specifically. In mormyrids generally, the electric organ discharge is thought to carry social information beyond simple navigation — its waveform is often species- and even sex-specific, and researchers studying the genus have used it as a diagnostic character precisely because it varies in ways body shape does not.
Whether that signal plays a role in pairing or courtship in this species has not been studied. Nothing about shoaling behavior, sex ratio, or seasonal timing of reproduction in the wild is documented for tenuicauda in the available literature.
Breeding
Breeding has not been recorded for Petrocephalus tenuicauda, either in the wild or in captivity. Mormyrid reproduction as a whole is poorly documented and rarely achieved in aquaria, and this species is no exception — no spawning account, egg description, or fry-rearing record exists in the literature reviewed for this profile.
No details of parental care can be stated for this species. Any expectation drawn from better-studied mormyrid relatives would be speculation rather than fact, and none is offered here.
In the aquarium
Petrocephalus tenuicauda has essentially no track record in the aquarium hobby and is not a species most keepers will ever encounter offered for sale. What follows is general mormyrid husbandry, not a documented account of this fish in captivity.
Mormyrids as a group do best over a soft, fine substrate they can probe with electrolocation, in dimly lit tanks furnished with cover such as driftwood and broad-leaved plants, and in stable, well-filtered water — the family is known for sensitivity to poor water quality and to some common aquarium medications, so any acclimation or treatment should be conservative and gradual. Live or frozen invertebrate foods offered after the lights dim suit the natural foraging rhythm of these fish far better than daytime flake feeding. None of this has been verified for tenuicauda itself, but it reflects standard practice for the small Petrocephalus species that do occasionally appear in the trade.
Conservation
The IUCN Red List assesses Petrocephalus tenuicauda as Least Concern, based on an assessment dated 9 January 2020. No species-specific threats are documented in that assessment.
As with other small freshwater fishes restricted to West African coastal river systems, longer-term pressures such as habitat degradation, deforestation of riparian corridors, and water-quality decline in the Upper Guinea region are the kind of general concerns that could eventually affect it, but the current Least Concern rating reflects no immediate cause for alarm.