Taxonomy & naming
Sternarchella orthos was described by Francisco Mago-Leccia in 1994 and belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Gymnotiformes are bony fish, but unusual ones: the order is defined by an electric organ carried along the body and by a single long anal fin used for propulsion in place of a normal tail. Apteronotidae is distinguished within the order by an electric organ built from modified nerve tissue rather than muscle, which lets its members produce unusually fast, high-frequency electric organ discharges.
A taxonomic revision of the genus published in the Brazilian journal Neotropical Ichthyology (Scielo) places the type locality of S. orthos in Venezuela, in the Río Apure near the town of Apure, and treats Sternarchella terminalis as a junior synonym of this species. The same revision documents considerable phenotypic diversity across the species' range, with populations from the western Amazon, eastern Amazon and Orinoco basins differing enough in body form that they were once treated as separate taxa. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the standing authority for the valid name. Despite the shared common name "knifefish", Sternarchella and its relatives are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance is convergent, not a sign of kinship.
Morphology
Sternarchella orthos reaches a recorded maximum of about 12 in total length, a mid-sized member of the ghost knifefish family. FishBase records a slightly superior mouth — the lower jaw projects a touch beyond the upper — and a long series of anal-fin rays, 165 to 178 of them, running for most of the length of the body's underside; this ribbon fin, thrown into travelling waves, is the fish's only real means of propulsion, letting it move forwards or backwards with equal ease. As in other apteronotids there is no dorsal fin and no functional tail fin in the conventional sense.
The taxonomic revision covering the genus notes a body cavity extended by 14 to 16 pre-caudal vertebrae, a fan- or rod-shaped second basibranchial bone, and a thorny crown-like projection on the skull shared with the closely related S. patriciae — internal and cranial features used to separate Sternarchella species rather than anything visible to an aquarist. The species is scaleless, like all Gymnotiformes, and carries the electric organ and its associated sensory pores along the body, the anatomical basis of the electrolocation on which it depends. External sex differences are not documented for this species.
Habitat
This species is a fish of the main river channel rather than backwaters or floodplain lakes. FishBase describes it as benthopelagic and tropical, living mostly in the channels of large rivers across the Orinoco basin and, with considerable regional variation, the Amazon basin as well — a distribution spanning a large stretch of northern and central South America. The type locality in the Río Apure, a major Orinoco tributary in Venezuela, sits within that same deep-channel habitat.
Life in the open channel of a big, turbid Neotropical river means darkness and turbulence are the norm rather than the exception: visibility is poor, the water often silt-laden, and an active electric sense is far more useful there than eyesight. Like its relatives, S. orthos is presumed to favour warm, soft to moderately mineralised, near-neutral to acidic water typical of the Amazon and Orinoco systems, though detailed physicochemical data specific to this species has not been published — the record is built mainly from museum specimens and river survey collections rather than field studies of its ecology.
Feeding
The taxonomic revision of the genus reports gut-content analysis showing that Sternarchella orthos feeds on benthic organisms and small fishes, with the details of diet varying geographically across its Amazon and Orinoco populations — consistent with a species that has diversified across a wide range and encounters different prey communities in different rivers. As in other ghost knifefishes, prey is almost certainly located chiefly through electrolocation: the fish's continuous, very high-frequency wave-type discharge, in the range of roughly 772 to 2,180 Hz reported for the genus, lets it sense the presence and position of small animals in the sediment and among cover without needing to see them.
Beyond the confirmation of a benthic-feeding, small-fish-taking diet with regional variation, no further detail on feeding behaviour, activity pattern, or seasonal diet shifts has been published for this species. Data sparse.
Mating
No courtship behaviour has been documented specifically for Sternarchella orthos. What can be said follows from its family: in Apteronotidae the wave-type electric organ discharge functions as more than a sensing tool, carrying information about a fish's identity, sex and physiological state, and in the best-studied genus in the family, Apteronotus, males and females are known to shift discharge frequency during courtship in an electrical exchange. It is reasonable to expect that electric signalling plays a comparable role in S. orthos, given the shared apteronotid electric organ and the high-frequency wave discharge documented for the genus, but this has not been directly studied in Sternarchella.
Given how little of this species' natural history has been recorded even for diet and habitat, its reproductive and courtship behaviour should be considered essentially undocumented rather than inferred with confidence.
Breeding
No information on breeding has been published for Sternarchella orthos; the taxonomic literature covering the genus explicitly notes the absence of documented breeding data. As an apteronotid it is presumed to be an egg-layer, consistent with the rest of the family, but there is no record of spawning behaviour, egg deposition sites, or parental care in the wild or in captivity.
This species is not established in the aquarium hobby, so there is no hobbyist breeding record to draw on either. Data sparse — poorly documented rather than unknown-by-omission.
In the aquarium
Sternarchella orthos is not a trade species; the available sources are museum collections, taxonomic revisions and conservation assessments rather than aquarium literature, and it should be treated as effectively unavailable to hobbyists. Anyone who did encounter it — most likely via a specialist importer working from Amazon or Orinoco river collections rather than through the ordinary trade — should apply the same care principles that govern its better-known relatives such as the black ghost knifefish: a large, dimly lit tank with strong but gentle flow, secure cover such as wide pipes or root tangles for daytime retreat, and soft, warm, near-neutral to acidic water.
As with all Gymnotiformes, it is scaleless and should be presumed sensitive to ammonia, nitrate and to copper-based or other harsh medications, which argues for reduced dosing and immaculate water quality if the species is ever kept. Given it is a deep river-channel fish reaching around 12 in, likely nocturnal and predatory by electrolocation like its relatives, it would not be considered safe with small tankmates. In practice, this is a species for public aquaria, research collections, or specialist keepers rather than a general hobby fish.
Conservation
Sternarchella orthos is assessed as Least Concern on the IUCN Red List, an assessment last updated in 2020, and FishBase records a low vulnerability-to-fishing score. This reflects a wide distribution across the main channels of both the Amazon and Orinoco basins and the apparent lack of a documented major threat specific to the species.
Because it lives in the open channels of two of South America's largest river systems rather than in a single narrow-range tributary, it is not among the single-basin endemics considered most exposed to habitat loss, damming or pollution. That said, the species' basic ecology remains poorly known, and broader pressures on the Amazon and Orinoco — deforestation, mercury contamination from mining, and hydroelectric development — bear on all river-channel fish in these systems, this one included, even without species-specific data to quantify the risk.