Taxonomy & naming
Eugnathichthys macroterolepis was described by the Belgian-based ichthyologist George Albert Boulenger in 1899. It sits in the genus Eugnathichthys within the family Distichodontidae, order Characiformes, and the Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats the genus as the taxonomic authority for the valid combination — no synonymy or generic reassignment is noted in the cached sources, so the original combination stands.
The genus name distinguishes several closely related species that are separated largely by scale size and squamation pattern rather than by dramatic differences in body shape. The specific epithet macroterolepis references comparatively large scales, distinguishing this species from its congener E. eetveldii; a molecular phylogenetic study of the genus (Zootaxa) has examined relationships among Eugnathichthys species, situating this fin- and scale-eating lineage within the broader, largely plant-grazing Distichodontidae.
Morphology
Eugnathichthys macroterolepis is a small, elongate characin reaching a maximum recorded length of about 5.5 in total length — a modest size compared to its plant-grazing relatives in Distichodus, some of which exceed 31.5 in. As its name suggests, it carries comparatively large scales, including scaling that extends onto the caudal fin and onto the base of the adipose fin, a feature used to separate it from the similar E. eetveldii.
The jaw structure is the genus's defining feature: rather than the blunt, herbivore-adapted mouth of Distichodus, Eugnathichthys species carry specialised dentition suited to shearing scales and nipping fin tissue from other fish in a single rapid pass — an ectoparasitic feeding strategy rather than a predatory one in the sense of consuming whole prey.
Habitat
The species is endemic to the Congo basin, with records spanning the lower Congo River up through the upper Lualaba and into tributaries including the Kasai and Nsele rivers. This places it squarely within the vast, warm, tropical river system of central Africa rather than any of the Congo's isolated lake systems.
Water conditions across its range are tropical and comparatively warm, in the range of roughly 73–81 °F. The cached sources do not report specific pH or hardness figures for this species, and rather than guess at typical Congo-basin water chemistry, that is best left as data sparse pending a source that states it directly.
Feeding
Eugnathichthys macroterolepis is a fin- and scale-eater — an ectoparasitic feeding strategy in which the fish rasps scales and shears fin tissue from other, usually larger, fish rather than hunting and consuming whole prey. This sets it and its Eugnathichthys relatives apart sharply from the plant- and algae-grazing habits of Distichodus, the far better-known genus in the same family.
In practical terms this means the species' natural diet is built around brief, repeated attacks on the fins and flanks of tankmates in the wild community of Congo basin fishes, rather than around detritus, algae or plant matter. Any aquarium diet should account for this specialised feeding behaviour rather than assume a generic omnivore pattern.
Mating
The cached sources do not document courtship behaviour, pairing, or sexual dimorphism for Eugnathichthys macroterolepis. Rather than infer detail from related genera, this is best reported honestly as data sparse: nothing in the available record describes how this species pairs or spawns.
Breeding
No breeding account for Eugnathichthys macroterolepis appears in the cached sources. Its family, Distichodontidae, is broadly characterised by egg-scattering with no parental care, and Distichodus species in particular are known to be very difficult to breed in captivity — but no source here confirms this specifically for Eugnathichthys, so no breeding mode should be asserted for this species. It is not known to be bred in the aquarium hobby based on the available record.
In the aquarium
Eugnathichthys macroterolepis is rarely encountered in the aquarium trade, and its biology argues for caution rather than casual community-tank placement. At roughly 5.5 in maximum, it does not need an enormous tank on size alone, but its fin- and scale-eating habit means it poses a real risk to slow-moving, long-finned or docile tankmates — the kind of fish that would make an easy, repeated target for an ectoparasitic fin-nipper. A keeper considering this species should plan for robust, fast-moving, short-finned tankmates or a species-only setup, and should not assume it will behave like a peaceful community characin.
Water should be kept warm, in line with its tropical Congo basin origin (roughly 73–81 °F); specific pH and hardness targets are not documented in the available sources, so a keeper should default to soft, slightly acidic to neutral conditions typical of Congo basin water pending firmer data.
Conservation
The IUCN Red List assesses Eugnathichthys macroterolepis as Least Concern (2020), consistent with a species that, while endemic to the Congo basin, occupies a broad range across the lower Congo, upper Lualaba, Kasai and Nsele rivers rather than a single threatened locality. The cached sources do not detail a specific threat assessment beyond the status itself.
As with many Congo basin endemics, the species' long-term outlook is tied to the health of the wider Congo River system, though nothing in the available record flags species-specific pressure such as targeted collection or habitat loss.