Taxonomy & naming
Gymnotus onca was described by Albert and Crampton in 2001, one of a wave of species split from the long-conflated "Gymnotus carapo" species complex once electric organ discharge patterns and detailed morphology began to be used to tell Neotropical banded knifefishes apart. It belongs to the family Gymnotidae within the order Gymnotiformes, the New-World electric knifefishes — a group of bony fishes defined by an electric organ and a ribbon-like anal fin rather than by any resemblance to the unrelated Asian featherback and clown knifefishes (Chitala, Notopterus), which are bonytongues from an entirely different lineage.
The genus name Gymnotus is Greek for "naked back", describing the fish's lack of a dorsal fin. The species epithet onca refers to Panthera onca, the jaguar, a nod to the bold, irregular dark blotching across the body that recalls a big cat's rosette pattern. The type locality is Cano do Lago Rato in the Mamirauá Sustainable Development Reserve, Amazonas, Brazil, and the Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name.
Morphology
Gymnotus onca has the classic banded-knifefish build: a stout, laterally compressed, elongate body tapering to a whip-like tail, with the boldly barred or blotched dark-on-pale pattern that gives the genus its common name. As in all gymnotiforms there is no dorsal fin and no separate tail fin; locomotion comes from a single long anal fin running most of the body's underside, thrown into travelling waves that drive the fish forward or backward with equal facility. The skin is entirely scaleless.
An electric organ runs along the body and drives a pulse-type electric organ discharge (EOD) — brief, discrete pulses rather than the continuous wave signal used by ghost knifefishes (Apteronotus) — with sensory pores across the skin reading the field it creates. That pulse EOD is used for electrolocation in the dark, turbid water Gymnotus favours, and for signalling to other fish of its own species. Detailed size data for G. onca specifically are sparse in the literature, consistent with a species-complex genus where field identification is often difficult and many species remain little studied.
Habitat
Gymnotus onca is known from the upper Amazon River basin in Brazil, with its type locality in the seasonally flooded forest of the Mamirauá Reserve — a landscape of blackwater channels, flooded forest and floodplain lakes that shift dramatically between high- and low-water seasons. Like other Gymnotus it favours swampy, vegetated, often oxygen-poor margins and floodplain habitat rather than open river channel.
Gymnotus as a genus is unusual among gymnotiforms for facultative air-breathing, an adaptation that lets these fish survive in stagnant, low-oxygen floodplain water where many other fish cannot. Water is warm and typically soft to moderately mineralised, and the dim, tannin-stained or turbid conditions typical of Amazon floodplain habitat put a premium on the electric sense over vision — precise water chemistry data for this species are not well documented beyond the general conditions of its Amazon floodplain range.
Feeding
Gymnotus onca is presumed, on the strength of its genus, to be a nocturnal predator that hunts small invertebrates and other prey after dark using electrolocation to detect movement and buried animals in turbid or dark water. Banded knifefishes as a group are tough, opportunistic hunters of floodplain and swamp habitat, taking aquatic insect larvae, worms, crustaceans and small fish.
Species-specific feeding data for G. onca are not available in the sources consulted here; its diet should be assumed to follow the general pattern of the genus rather than being independently documented.
Mating
Courtship and mate recognition in Gymnotus, as in other gymnotiforms, is thought to run substantially through the electric organ discharge — each species, and often each sex, carries a distinctive pulse signature, and rivals and prospective mates are likely to identify and assess one another electrically before or during physical approach. This electrocommunication role of the EOD is well established for the genus in general, but courtship behaviour has not been specifically documented for G. onca.
Data are sparse for this species; what follows in Breeding reflects the known reproductive mode of Gymnotus as a genus rather than direct observation of G. onca.
Breeding
Gymnotidae are unusual among fishes for genuine parental care, and in Gymnotus that care falls to the male: he builds and guards a nest — often a sheltered hollow in vegetation or a mass of floating material — into which a female spawns, and he then guards the eggs and subsequent larvae until they disperse. Spawning in the genus is typically seasonal and linked to the flood cycle of the rivers and floodplains these fish inhabit.
No species-specific breeding account exists for G. onca in the sources available here, and it is not known to be bred in the aquarium; the general Gymnotus pattern of male nest-guarding is the best-supported expectation for this species, not a documented protocol for it.
In the aquarium
Gymnotus onca is not established in the aquarium trade, and there is no meaningful hobbyist husbandry record for the species; it should be treated as effectively unavailable rather than as a fish to plan a tank around. What can be said follows from its genus: Gymnotus are tough, nocturnal, air-breathing predators that need dim lighting, secure cover to retreat to by day, and tankmates too large to be hunted after dark — they are not community-safe with small or bite-size fish, and adults can be territorial toward their own kind.
Being entirely scaleless, banded knifefishes are sensitive to poor water quality and to copper-based or harsh medications, which should be used at reduced doses if at all. Anyone who does encounter this species should prioritise captive-bred or responsibly sourced stock and expect to keep, not breed, it — reliable spawning triggers are not documented.
Conservation
Gymnotus onca is assessed as Least Concern, though as with much of the genus the assessment reflects limited data as much as confirmed security; the species is known with certainty only from the upper Amazon basin in Brazil, including the well-studied Mamirauá Reserve floodplain.
Broader Amazon floodplain habitat faces ongoing pressure from deforestation, water-quality change and shifting flood regimes, and narrow-range or poorly surveyed gymnotiforms are exactly the kind of species that pressure could affect without being noticed. The Least Concern rating should be read as an absence of demonstrated decline rather than a thorough population assessment.