Taxonomy & naming
The species was described by the Belgian-British ichthyologist George Albert Boulenger in 1911 as Mormyrus castelnaui, from specimens collected in the Lake Ngami basin of what is now northwestern Botswana. It was later moved into Pollimyrus, the genus of small mormyrids commonly called dwarf stonebashers, and sits within the family Mormyridae in the order Osteoglossiformes, the ancient bonytongue lineage.
Work on the wider species group has identified a pattern of parapatric speciation between populations in the Okavango system and the Zambezi River, with the Zambezi population later split off as a distinct sibling species, Pollimyrus marianne. Pollimyrus castelnaui in the current, narrower sense refers to the Okavango-centred population and its close relatives across the upper Zambezi drainage. The dossier available for this account does not state the etymology of the epithet castelnaui; it most likely honors a naturalist surnamed Castelnau, but that dedication is not confirmed by the sources at hand.
Morphology
Pollimyrus castelnaui is a small mormyrid, reaching about 3 in in standard length, which places it toward the lower end of the size range even for a genus built around small species. Like other dwarf stonebashers it has a laterally compressed, deep-bodied shape, a short blunt head, and a small mouth — a build suited to picking through submerged vegetation and leaf litter rather than open-water cruising.
As in all mormyrids, an electric organ in the caudal peduncle generates a weak, pulse-type discharge that the fish reads through electroreceptors across its skin, an ability used to navigate and sense surroundings where vision is unreliable. The electric organ discharge is also thought to be involved in species recognition within this species group — a mechanism plausible enough that it helped identify the Zambezi population as a separate species from the Okavango one — but no discharge waveform or other detailed morphological data specific to Pollimyrus castelnaui is available in the sources for this account.
Habitat
Pollimyrus castelnaui is native to the Okavango basin and the upper Zambezi system, with records extending to the Kafue and Cunene rivers, northern Lake Malawi, the Luongo River (part of the lower Luapula/upper Congo drainage) in Zambia, and the Kasai River in Angola. It was historically recorded from the Lake Ngami basin of northwestern Botswana, the type locality for the original description.
Within this range the species favors dense vegetation along river margins and floodplain lagoons, typically in shallow water sheltered beneath surface plant cover. This is the same kind of structurally complex, low-visibility habitat associated with mormyrids generally, where electrolocation gives a clear advantage over eyesight. Specific temperature and water-chemistry data for the species are not documented in the available sources.
Feeding
No species-specific feeding study is available for Pollimyrus castelnaui, but its diet almost certainly follows the pattern typical of small mormyrids: benthic invertebrates such as insect larvae and small crustaceans, located by electrolocation among vegetation and detritus and taken with a quick strike of the small mouth. General mormyrid care literature recommends live and frozen invertebrate foods such as bloodworm and brine shrimp in the aquarium, with dried or flake foods usually ignored, though none of this has been documented specifically for this species.
Mating
No courtship behavior has been documented for Pollimyrus castelnaui specifically. The species-recognition role attributed to the electric organ discharge in this species group — the basis on which the Zambezi population was distinguished as Pollimyrus marianne — suggests the discharge is likely also involved in mate recognition, following a pattern reported more broadly across mormyrids, but this has not been confirmed for Pollimyrus castelnaui by direct observation.
Breeding
Breeding in Pollimyrus castelnaui is unrecorded. No wild or captive spawning account for this species is available in the sources for this dossier, and mormyrids as a family are rarely bred in the aquarium; fish in the trade are almost always wild-caught. Nothing about egg-laying site, clutch size, or parental care can be stated for this species without guessing.
In the aquarium
Pollimyrus castelnaui has essentially no documented track record in the aquarium hobby; it does not appear in the general mormyrid-care literature by name, and any account of keeping this particular species would be speculative. What can be said comes from general mormyrid husbandry: a tank of at least a meter in length, a fine sand or smooth gravel substrate to protect the sensitive snout, dim or subdued lighting suited to a nocturnal or crepuscular fish, and ample cover from plants, wood, or rockwork.
Mormyrids as a group are notoriously sensitive to poor water quality and to many common medications — copper-based treatments in particular are frequently fatal — so stable, clean water and cautious dosing matter more here than with most community fish. At around 3 in this species would not need a large footprint, but given the total absence of a hobby record, anyone attempting it should treat it as an experimental fish requiring close observation rather than a species with established care parameters.
Conservation
The IUCN Red List assesses Pollimyrus castelnaui as Least Concern, based on an assessment dated 13 July 2018. The species is known across a fairly wide area of the Okavango and upper Zambezi drainages, including the Kafue, Cunene, Luongo, and Kasai rivers and northern Lake Malawi, without evidence of significant population decline.
No major species-specific threats are documented. As with other freshwater fish of southern African floodplain systems, broader pressures such as habitat degradation and changes to seasonal flooding regimes are long-term concerns for the wider ecosystem, but the current Least Concern status reflects a species with no immediate conservation flag.