Taxonomy & naming
Hippopotamyrus macrops was described by the Belgian-British ichthyologist George Albert Boulenger in 1909 from the Upper Congo, with the holotype held at the Natural History Museum in London (BMNH 1899.6.27.12). It belongs to the family Mormyridae, the elephantnoses and mormyrids, within the order Osteoglossiformes — the bonytongues, one of the most ancient living branches of the teleost fishes, named for a toothed bone on the floor of the mouth that bites against a toothed plate on the roof. Mormyridae is an exclusively African radiation that, together with the related Gymnarchidae, makes up the weakly electric Mormyriformes; despite a superficial resemblance in some relatives, it is entirely unrelated to the electric knifefishes (Gymnotiformes) of South America, a case of pure convergent evolution across two separate continents and lineages.
The species has been shuffled between genera more than once. It was originally described in the broad genus Marcusenius, and the Catalog of Fishes currently lists the accepted combination as Cyphomyrus macrops, reflecting a later split of the old, wide-ranging genus Hippopotamyrus into several narrower genera on the basis of skull and electric-organ characters; the combination Hippopotamyrus macrops used here follows the older, still widely cited placement, and the genus-level revision is a reminder of how unsettled mormyrid systematics remains. The epithet macrops is Greek for "large-eyed" (makros, large + ops, eye), describing eyes roughly twice the length of the snout — a genuinely diagnostic feature of the species.
Morphology
Hippopotamyrus macrops is a modestly sized mormyrid, reaching about 7.5 in in standard length according to FishBase; no total-length figure has been published, and there are no aquarium specimens to measure independently. The body is elongate and laterally compressed in the typical mormyrid pattern, with a small, roughly terminal mouth — it lacks the long, probing chin appendage (the schnauzenorgan) that characterises Gnathonemus petersii and some related genera — and the oversized eyes for which the species is named.
Like every mormyrid, it carries an electric organ in the caudal peduncle, derived from modified muscle tissue, that produces a continuous, low-voltage, species- and often individually distinctive pulse-type electric organ discharge (EOD). The EOD is picked up by electroreceptors spread across the skin and used to electrolocate — building a moving picture of obstacles, prey and open water in dark, sediment-stained rivers — and to signal to other fish of the same species. Mormyrids as a family have among the highest brain-to-body mass ratios of any vertebrate, with an enlarged cerebellum devoted largely to processing this electrosensory picture; H. macrops shares this general mormyrid neuroanatomy, though its own brain has not been separately studied.
Habitat
The species is known from the Congo River basin of Central Africa, with its type locality in the Upper Congo and further records reported from the Democratic Republic of the Congo and Cameroon. It is a demersal, freshwater fish of tropical rivers and their floodplains, living close to the bottom rather than in open water.
No detailed water-chemistry measurements have been published for H. macrops specifically. Like most Congo Basin mormyrids it almost certainly inhabits warm, soft, only mildly acidic to neutral water that is frequently turbid with suspended sediment or stained by decaying vegetation — the low-visibility conditions an electric sense is best suited to reading. The wider Congo system is one of the most species-rich and least-studied river basins on Earth, and much of its mormyrid diversity, including this species, remains poorly surveyed.
Feeding
FishBase places Hippopotamyrus macrops at a trophic level of roughly 3.2, consistent with an omnivore leaning carnivorous — a typical mormyrid diet of small aquatic invertebrates such as insect larvae, worms and tiny crustaceans, picked out of sediment and cover using electrolocation rather than eyesight, mostly after dark.
No aquarium feeding record exists for this species, since it has apparently never been kept in captivity. Other small mormyrids in the trade generally take live and frozen bloodworm, brine shrimp and similar small invertebrate foods readily once settled; that is a reasonable expectation for H. macrops rather than a documented fact for this species.
Mating
Mormyrid courtship is built around the electric organ discharge: in better-studied genera such as Pollimyrus and Brienomyrus, males use their EOD waveform and pulse pattern in courtship displays to signal species identity and spawning readiness, and females appear to select mates partly on EOD characteristics. Whether Hippopotamyrus macrops follows this pattern has not been documented — it is inferred from the wider family rather than observed directly in this species.
In the wild, Congo Basin mormyrids are generally understood to be seasonal spawners, with reproduction cued to seasonal rains and the resulting rise in river level and flooding of marginal habitat, which is thought to provide both spawning cover and abundant food for fry. No details of pairing, courtship duration or spawning site have been recorded for H. macrops.
Breeding
Breeding biology in Hippopotamyrus macrops is essentially undocumented. FishBase notes only that it is a seasonal spawner in the wild with poorly understood reproductive ecology, and unlike a handful of other mormyrids that have bred incidentally under aquarium conditions, this species has never been imported into the hobby at all — so there is no captive record to draw on.
Some mormyrids are thought to scatter eggs among submerged plants or root mats with little further parental care, while others may guard a clutch briefly; without direct observation it would be guesswork to assign either pattern to H. macrops specifically. The honest summary is that captive reproduction of this species has never been attempted, and its wild breeding behaviour remains a research gap rather than a documented fact.
In the aquarium
Hippopotamyrus macrops is not, in any practical sense, an aquarium fish. FishBase and hobbyist surveys of the elephantnose trade agree that species in this Cyphomyrus/Hippopotamyrus group have never been imported for the hobby — unlike the popular Gnathonemus petersii, this is a fish known almost entirely from museum specimens and field surveys, not from tanks.
General mormyrid husbandry principles would apply if it were ever available: soft, slightly acidic to neutral, dimly lit water over a sandy bottom with plenty of cover, since mormyrids are shy, largely nocturnal fish that grow nervous and stressed under bright light or in bare tanks; a species tank or small group is usually favoured over boisterous community tankmates. Mormyrids as a family are notably sensitive to poor water quality and to many medications, so any hypothetical care would call for conservative dosing and pristine water. None of this has actually been tested on H. macrops itself, and prospective keepers should treat it as a species with essentially no aquarium track record rather than assume general mormyrid advice transfers perfectly.
Conservation
The IUCN Red List assesses Hippopotamyrus macrops as Least Concern, an assessment dated to 2009. FishBase records it as a fish of high resilience and low vulnerability to fishing pressure, and it is not the target of any dedicated fishery or aquarium trade that would draw down wild numbers.
That Least Concern status should be read against the wider context of the Congo Basin, one of the world's least-studied major freshwater systems, where habitat pressures such as deforestation, mining-related sedimentation and future hydropower development are recognised long-term threats to the basin's mormyrid diversity even where individual species assessments remain reassuring.