Taxonomy & naming
Petrocephalus microphthalmus was described by the French ichthyologist Jacques Pellegrin in 1909 (the year is also cited as 1908 in some sources) in the Bulletin du Museum National d'Histoire Naturelle. The type locality is the lower Ogooue River at Ngomo, Gabon, and the holotype, MNHN 1908-0211, is held at the Museum National d'Histoire Naturelle in Paris. The species belongs to Mormyridae, the elephantnoses and mormyrids, within Osteoglossiformes, the bonytongue fishes.
Petrocephalus microphthalmus has held up as a valid species through every major revision of the genus, including work by Gosse (1984), Lavoue and colleagues (2004, 2010, 2012), Hopkins and colleagues (2007), Kramer and colleagues (2012), Skelton (2019), Stiassny and colleagues (2021), Fermon and colleagues (2022), and Walsh and colleagues (2022). A dedicated revision of the Gabonese Petrocephalus fauna recognized four valid species in the country — P. microphthalmus, P. simus, P. balayi, and the newly described P. sullivani — separated largely by fin-ray counts, dentition, and electric organ discharge rather than by external appearance alone.
Morphology
At a maximum recorded length of about 2 in total length, Petrocephalus microphthalmus is one of the smaller members of a genus already built of compact, round-bodied fish. The body is silvery to purplish and iridescent, with a black blotch on the anterior rays of the dorsal fin near its origin, a useful field mark in a genus where species otherwise look very alike. The dorsal fin is short, with 15–18 branched rays, set opposite a much longer anal fin of 23–27 branched rays, the same asymmetric fin arrangement found throughout Mormyridae. The mouth is moderately wide, with 9–11 teeth in the upper jaw and 14–20 in the lower. As the epithet microphthalmus ("small-eyed") suggests, the eye is reduced compared with related species.
Like all mormyrids, Petrocephalus microphthalmus carries a weak electric organ in the caudal peduncle. The organ produces a brief pulse discharge, read by electroreceptors spread over the head and body; in this species those receptors sit as isolated pores rather than clustered into rosettes, one of the characters used to separate it from related Petrocephalus. The discharge itself is very short — recorded at 0.252–0.511 milliseconds in a Republic of Congo population from Odzala National Park and 0.380–0.561 milliseconds in a Gabonese population — and no difference between the sexes has been reported in either.
Habitat
Petrocephalus microphthalmus is known from Gabon, the Republic of Congo, the Democratic Republic of Congo, and Angola. Its range spans coastal river systems from the Sanaga River in the north to the Kouilou-Niari River in the south, the Ntem River basin, the Ogooue River basin — where it is recorded throughout the drainage — and the middle Congo River downstream to the lower Congo near Brazzaville.
The species is demersal, living on or near the bottom of small tributary creeks running through lowland forest, and it is described as locally very abundant in parts of its range, including within Odzala National Park in the Republic of Congo. Specific temperature and pH figures have not been published for this species; forest streams across its range are generally soft and mildly acidic, but that is a regional expectation rather than a measurement taken from this population.
Feeding
Diet has not been directly documented for Petrocephalus microphthalmus. FishBase places it at a trophic level of 3.1, consistent with a carnivorous, invertebrate-based diet typical of the genus, in which prey is located in mud, sand, and leaf litter by electrolocation rather than by sight. No specific prey items and no captive feeding account have been published for this species — the record here is genuinely thin.
Mating
No courtship or pairing behavior has been described for Petrocephalus microphthalmus. In mormyrids generally, the electric organ discharge is thought to carry information used in species and sex recognition and possibly in courtship, but this has not been studied in this species specifically. No sex-based difference in its discharge has been reported, which argues against a strong role for the EOD in sex recognition here, though that is an inference rather than a tested conclusion.
Breeding
Breeding is unrecorded. No wild spawning account and no captive breeding account exists for Petrocephalus microphthalmus in the published literature, and basic maturity data such as size or age at first reproduction have not been documented either. This gap is typical of the genus: most Petrocephalus species, especially the smaller and more narrowly distributed ones, have never been studied closely enough to describe their reproduction.
In the aquarium
Petrocephalus microphthalmus has essentially no aquarium history — it is not established in the trade, and no husbandry record specific to this species has been published. What can be said comes from general mormyrid care rather than from this species: a soft substrate for probing, dim lighting and ample cover to reduce stress, and live or frozen invertebrate foods such as bloodworm, since mormyrids largely ignore dried food. Mormyrids as a group are known to be sensitive to poor water quality and to some medications, so cautious acclimation and conservative dosing would apply if the species were ever kept.
At around 2 in it would be a genuinely small tankmate rather than a size concern, but that has not been tested against the practical realities of sourcing or keeping the species, since it currently plays no part in the hobby.
Conservation
The IUCN Red List assessed Petrocephalus microphthalmus as Least Concern in 2022. The species is documented across a wide range spanning four countries and several major Central African river systems — the Ogooue, Congo, and Ntem basins and coastal drainages from the Sanaga to the Kouilou-Niari — and is locally abundant in at least part of that range, including inside a protected area at Odzala National Park. No species-specific threats are noted in the assessment used for this account.