Taxonomy & naming
Apteronotus macrostomus was described by Henry Weed Fowler in 1943 as Sternarchus macrostomus, and was later transferred into Apteronotus and placed within the Apteronotus leptorhynchus species-group — the same cluster of closely related ghost knifefishes that contains the brown ghost, A. leptorhynchus, its closest relative in the hobby literature. It belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes, the New-World electric knifefishes. These are bony fish, but unusual ones: distantly related to the other freshwater aquarium groups and defined by an electric organ and a long ribbon-like anal fin used for locomotion rather than a normal tail.
Within Gymnotiformes, the Apteronotidae are distinguished by an electric organ built from modified nerve cells rather than muscle tissue, which produces a very fast, continuous wave-type discharge — among the highest-frequency signals of any electric fish. The genus name Apteronotus is Greek for "without a fin on the back," describing the missing dorsal fin; the species epithet macrostomus combines the Greek makros ("long, large") and stoma ("mouth"), a straightforward reference to the fish's large mouth, which extends past the eye to roughly half the length of the head. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name. Despite the shared common name, gymnotiforms are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance is convergent body shape only.
Morphology
Like its Apteronotidae relatives, A. macrostomus has the classic ghost-knifefish silhouette: a laterally compressed, blade-shaped body tapering to a slender tail, matte-dark in colour, with no dorsal fin and no conventional caudal fin. Propulsion comes almost entirely from a long anal fin running along most of the underside of the body, thrown into travelling waves that let the fish glide forward or backward with equal control — a trait shared across the order. The skin is scaleless, a feature that matters directly to how the species must be cared for in captivity.
The defining physical trait that gives the species its name is its mouth: distinctly large for the genus, extending back past the eye to about half the head's length, a useful field character for separating it from the similarly built A. leptorhynchus. Maximum recorded size is around 10 in total length. As in other Apteronotidae, the electric organ runs the length of the body beneath the skin, paired with rows of electroreceptor pores that read the field it creates; external sexual dimorphism is not documented for this species and, as in its relatives, is likely subtle at best.
Habitat
Apteronotus macrostomus is known from the Meta River basin of Colombia, with its type locality near Villavicencio on the Río Meta — a tributary system that drains into the Orinoco. Unlike the widespread black ghost, this appears to be a much more geographically restricted species, and the available records point to a single major river basin rather than a continent-spanning range; the range should be treated as narrow until further survey work says otherwise.
As with other Apteronotidae, the species is presumed to inhabit flowing, tropical freshwater — the kind of turbid, structurally complex river channel and margin habitat, often among submerged roots and woody debris, where an electric sense is more useful than eyesight. No published water-chemistry data (temperature, pH) specific to this species were found in the available sources; the notes below follow the well-documented conditions of its close relatives in the same species-group and river system, and should be read as a reasonable inference rather than a species-specific measurement.
Feeding
No species-specific dietary study of A. macrostomus was found in the available sources. Its close relatives in the Apteronotus leptorhynchus group are nocturnal carnivores that hunt small benthic invertebrates — aquatic insect larvae, worms and small crustaceans — locating prey in the dark largely through electrolocation rather than sight, and there is no reason to expect this species to differ meaningfully in its general feeding ecology.
In the rare event it is kept in the aquarium, the working assumption should follow the rest of the genus: a nocturnal, meaty-food carnivore best offered bloodworm, chopped earthworm, blackworm and similar frozen or live foods after dark, when Apteronotidae become active. Data on this exact species' feeding behaviour in captivity is sparse to nonexistent.
Mating
As in other Apteronotidae, courtship in this genus is understood to run substantially through the electric organ discharge itself. The wave-type EOD carries information beyond simple electrolocation — it signals identity, sex and reproductive state to other fish sharing the same water, and in the best-studied relative, the brown ghost A. leptorhynchus, males and females shift their discharge frequencies toward one another during courtship in a kind of electrical duet, while rival fish produce distinctive interruptions on encounter. There is no reason to think A. macrostomus, a close relative within the same species-group, behaves very differently, though this has not been directly documented for the species itself.
Beyond that electrophysiological picture, specific courtship and pairing behaviour for A. macrostomus is not described in the available literature. Data sparse.
Breeding
No breeding record for Apteronotus macrostomus — in the wild or in captivity — was found in the available sources. As with most Apteronotidae, it should be assumed to be an egg-layer that scatters eggs among submerged cover such as roots and crevices, with no substantial parental care documented for the genus as a whole; but this specific species is essentially unbred and unstudied in aquarium settings, and the trade (what little exists) is presumed to draw on wild-caught fish rather than captive breeding.
Given the near-total absence of published data, the honest statement for this species is that its reproductive biology in captivity is undocumented, and no breeding protocol can be responsibly offered.
In the aquarium
Apteronotus macrostomus is not an established aquarium species; unlike the black ghost or brown ghost, it is rarely if ever seen in the trade, and the fishkeeping literature available treats it mainly as a taxonomic sibling of A. leptorhynchus rather than as a fish with its own care history. Anyone who does encounter it should apply the same baseline that governs the whole family: it is scaleless, so ammonia, nitrite and nitrate must be kept low, and medications containing copper — along with many standard treatments at full dose — should be avoided or reduced.
As a nocturnal electroreceptive predator, it would need a dim, quiet tank with secure cover such as tubes, caves or root tangles to retreat into by day, and tankmates chosen carefully: anything small enough to be hunted after dark is at risk, while boisterous or nippy fish threaten its unprotected skin and long anal fin. Given how little is documented about this particular species' temperament, adult size ceiling in practice, and long-term husbandry, prospective keepers should treat it as an advanced, experimental fish and lean on the well-documented needs of its Apteronotus relatives as a starting point rather than a guarantee.
Conservation
Apteronotus macrostomus is assessed by the IUCN Red List as Data Deficient, reflecting how little is known about its population size, trends and the extent of any threats — a common status for narrow-range, poorly surveyed South American electric fish. Its apparent restriction to the Meta River basin in Colombia means that, even without a confirmed threat level, it fits the general pattern for single-basin gymnotiform endemics: localised habitat change, pollution or damming in that one system could matter disproportionately to the species, since there is nowhere else for the population to fall back on.
There is no evidence in the available sources of a significant aquarium trade in this species, so collection pressure is not flagged as a documented concern. The main conservation need, as the Data Deficient rating implies, is simply more survey and monitoring work in the Meta basin to establish a real population baseline.