Taxonomy & naming
Petrocephalus grandoculis was described by the Belgian-born British ichthyologist George Albert Boulenger in 1920, in "Poissons recueillis au Congo Belge par l'expédition du Dr C. Christy. Matériaux pour la faune du Congo," published in the Annales du Musée du Congo, Zoologie. The material had been collected during a survey expedition led by Dr. C. Christy for the Congo Museum. It belongs to the family Mormyridae — the elephantnoses and mormyrids — within the order Osteoglossiformes, the bonytongue fishes, one of the oldest living lineages of teleosts. A specific type locality was not located in the sources consulted for this article, and no synonyms are documented for the name.
The species has remained in use in the taxonomic literature since its description. It is one of eleven Petrocephalus species recognized from the Odzala region of the Republic of the Congo in a 2010 revision by Sébastien Lavoué, John Sullivan and Matthew Arnegard, which used a combination of morphology and electric organ discharge waveform to separate the region's superficially similar species.
Morphology
Petrocephalus grandoculis reaches a maximum reported size of about 3.5 in standard length, a small fish even by the standards of a genus made up mostly of compact species. It shares the general Petrocephalus body plan: a laterally compressed, moderately deep body, a short and rounded head, and a small, underslung mouth, set apart from the trunk-nosed Gnathonemus and other mormyrids by the absence of a chin barbel.
The feature that gave the species its name is the eye. In Boulenger's original diagnosis, the eye diameter went into the head length about three times and was roughly equal to the width of the interorbital space — large by the standards of the genus, and the character the epithet grandoculis refers to. As in all mormyrids, an electric organ near the base of the tail generates a weak, pulse-type discharge that the fish reads through electroreceptors across its skin; the 2010 Odzala revision used this discharge waveform, alongside fin-ray counts, mouth width, eye size, tooth counts and melanin pattern, to separate P. grandoculis from the region's other Petrocephalus species, though the specific measured values for this species were not available in the sources reviewed here.
Habitat
Petrocephalus grandoculis is native to the Congo River basin, recorded from its lower and middle sections across the Democratic Republic of the Congo, the Republic of the Congo, and the Central African Republic. One documented locality is the Lékoli River within Odzala-Kokoua National Park, in the Republic of the Congo, sampled during the fieldwork behind the 2010 revision of the region's Petrocephalus species.
Within this range the species occupies forest streams and river channels typical of the lower-middle Congo drainage. Specific temperature and water-chemistry figures have not been published for this species; Congo basin waters of this kind are generally warm and often soft or tannin-stained, but that should be read as a general expectation for the drainage rather than a measured value for this population.
Feeding
No feeding study specific to Petrocephalus grandoculis was located for this article. As in other members of the genus and the wider family Mormyridae, the species is presumed to be a benthophagous invertebrate feeder, using its electric sense to locate aquatic insect larvae and other small invertebrates hidden in mud, sand and leaf litter before taking them with a quick strike of its small mouth.
Mating
No published account of courtship or mate choice in Petrocephalus grandoculis was found for this article. In mormyrids generally, the species-specific electric organ discharge is thought to play a role in courtship and in recognizing the sex and identity of other fish, alongside its better-established roles in navigation and prey detection — but that is a family-level generalization, not an observation reported for this species.
Breeding
Breeding in Petrocephalus grandoculis is unrecorded. No wild spawning observation or captive breeding account for this species was located in the sources consulted for this article, and nothing about egg-laying, parental care, or fry-rearing can honestly be stated beyond that absence.
In the aquarium
Petrocephalus grandoculis has no documented history in the aquarium trade — no hobby literature or care account for this species was found. Should it ever appear, general mormyrid husbandry would apply: a soft substrate for probing, dim lighting, ample cover, and stable, clean water, since the family as a whole is known to be sensitive to poor water quality and to some medications. Live and frozen invertebrate foods offered after dark, matching the group's nocturnal foraging habit, would be the expected approach rather than dried or flake food.
At roughly 3.5 in this is a genuinely small mormyrid, but that says nothing about temperament or compatibility for a species with no aquarium record — any care advice beyond general family practice would be speculation rather than documented experience.
Conservation
The IUCN Red List assesses Petrocephalus grandoculis as Least Concern, based on a 2020 assessment. Specific population trends and threats for this species were not detailed in the sources consulted for this article; as with other freshwater fish of the Congo basin, broad pressures such as habitat degradation and water-quality decline are the kind of long-term concern that could affect it, but the current status is not an urgent conservation flag.