Taxonomy & naming
Apteronotus macrolepis was described by the Austrian ichthyologist Franz Steindachner in 1881 and belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Like the rest of the order it is a bony fish, but a strange one: Gymnotiformes are defined by an electric organ and a long ribbon-like anal fin rather than by any resemblance to typical aquarium fish. Within the order, the Apteronotidae are distinguished by an electric organ built from modified nerve cells rather than muscle, giving them the fastest, highest-frequency discharges of any knifefish group — a wave-type signal that runs continuously rather than in pulses.
The species name macrolepis comes from the Greek makrós (large) and lepís (scale), a reference to the comparatively large scales on the upper body — notable because most Apteronotidae are effectively scaleless-looking and smooth-skinned to the eye. It sits in the same genus as the aquarium-trade black ghost (A. albifrons) and brown ghost (A. leptorhynchus), but is one of the many far less-documented Apteronotus species. As with all New-World knifefishes, it is unrelated to the superficially similar Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to an entirely different order; the Catalog of Fishes is the authority for the valid name.
Morphology
Apteronotus macrolepis follows the ghost knifefish body plan: a laterally compressed, blade-shaped body that tapers to a fine tail, with no dorsal fin and no distinct caudal fin doing the work of propulsion. Instead, an elongated anal fin running along most of the underside throws travelling waves that drive the fish smoothly forward or backward, letting it manoeuvre precisely in tight cover without needing to turn around. Maximum recorded size is modest for the genus, reaching around 7 in in total length — considerably smaller than the black ghost's roughly 19.5 in.
Running the length of the body is the electric organ, paired with sensory pores across the skin that read the field it generates. The species carries an electric organ discharge typical of Apteronotidae — a fast, continuous wave-type signal — that underpins its electrolocation sense. Data are sparse on external sexual differences in this species; as in most ghost knifefishes, males and females are difficult to distinguish without close examination.
Habitat
Apteronotus macrolepis is documented from the Amazon River basin in South America, where it occupies the same broad category of habitat as other Apteronotidae: benthopelagic tropical freshwater, meaning it spends its time near the bottom and in midwater rather than at the surface. Amazonian ghost knifefish habitat is typically warm, dim and often tannin-stained or turbid, with slow-to-moderate flow and cover from submerged roots, leaf litter and overhanging vegetation.
That darkness matters: it is the reason the electric sense exists at all. Where eyesight offers little advantage in murky or nocturnal conditions, an electric field lets these fish navigate, avoid obstacles and detect prey with precision. Detailed locality data for A. macrolepis specifically are sparse beyond its Amazon basin range; finer habitat descriptions rely on the broader pattern seen across the genus.
Feeding
As with other ghost knifefishes, Apteronotus macrolepis is presumed to be a nocturnal carnivore that hunts small invertebrates — insect larvae, worms and small crustaceans — largely by electrolocation, detecting prey hidden in sediment or tucked among roots and debris after dark. Specific dietary studies for this species are not available; the feeding ecology described here follows the well-documented pattern for the genus rather than direct observation of A. macrolepis.
In captivity, ghost knifefishes of similar size typically take meaty frozen and live foods — bloodworm, small crustaceans and chopped invertebrate foods — offered in the evening when they become active. Data on A. macrolepis in aquarium settings specifically are sparse, as the species is seldom kept.
Mating
Courtship in Apteronotus is closely tied to the electric organ discharge, which carries information beyond simple electrolocation — signalling a fish's identity, sex and, in the best-studied relative, the brown ghost knifefish, its readiness to spawn. Males and females of that related species are known to shift their discharge frequency during courtship in a kind of electrical duet, while rival fish produce distinctive interruptions in each other's signals on encounter. It is reasonable to expect a broadly similar electric courtship behaviour in A. macrolepis given its close relationship to the better-studied ghost knifefishes, though this has not been directly documented for the species.
Beyond the electric signalling, specific courtship and spawning behaviour for A. macrolepis is data sparse — no dedicated field or aquarium account of its mating behaviour was found in the sources consulted.
Breeding
Apteronotus macrolepis is presumed to be an egg-layer, consistent with the rest of the genus, but its breeding biology is essentially undocumented and it is not established in the aquarium trade in any numbers that would generate hobbyist breeding reports. Ghost knifefishes as a group are only rarely bred in captivity even where they are commonly kept — most trade specimens of the popular species are wild-caught or produced on commercial farms under hormone induction — and A. macrolepis, being a far less commonly traded species, has essentially no reported captive breeding history.
Where spawning has been observed in related Apteronotus species, eggs are scattered among cover such as roots and plants, with no substantial parental care afterwards. Applying that pattern to A. macrolepis is an inference from its relatives rather than a documented fact for this species; the honest summary is rarely bred, poorly documented.
In the aquarium
Apteronotus macrolepis is not an established aquarium species and is rarely, if ever, seen in the trade, so aquarium care information specific to it is sparse. What can be said follows from its family: like all Apteronotidae it is scaleless and therefore unusually sensitive to poor water quality and to copper-based or harsh medications, which should be avoided or used at reduced doses. As a nocturnal electroreceptive hunter it would need a dim, quiet tank with secure cover — pipes, caves or root tangles — to feel settled during the day.
Given its smaller adult size (around 7 in) relative to the black ghost, a specimen of this species would in principle need less swimming space than its larger cousin, but any tankmates would still need to be too large to be eaten and not prone to nipping at its long anal fin. Prospective keepers should treat any offered specimen as a fish about which little is confidently known, and source captive-bred stock wherever possible rather than wild-caught fish from a poorly documented, narrow-range population.
Conservation
Apteronotus macrolepis is assessed as Least Concern, consistent with its occurrence across the wide-ranging Amazon River basin rather than a narrow single-river endemic range. This reflects the general pattern for wide-ranging Amazonian gymnotiforms, which tend to fare better on conservation assessments than narrow-range endemics restricted to a single tributary.
As with many lesser-known Amazon basin fish, that Least Concern status should be read against the backdrop of ongoing pressures across the basin — deforestation, water pollution and habitat change — even where the species itself is not the subject of dedicated conservation study. Because A. macrolepis has little presence in the aquarium trade, collection pressure from the hobby is not considered a meaningful threat to the species.