Taxonomy & naming
Gymnotus carapo was one of the fishes named by Linnaeus in the founding work of zoological nomenclature in 1758, and it is the type species of the genus Gymnotus, family Gymnotidae, in the order Gymnotiformes. The Gymnotidae hold both the banded knifefishes and the electric eel, which is simply a giant, strongly-electric member of the same family — so the banded knifefish is, in a real sense, the electric eel's small cousin. The Gymnotidae are "pulse-type" electric fish: their discharge is a train of brief pulses rather than the continuous wave of the glass knifefishes.
"Banded knifefish" is applied loosely, because G. carapo as long understood is a widespread complex: more than twenty species have been carved out of it since the 1990s, several of them formerly passing under the name carapo, and work continues. The animal in any given river labelled "Gymnotus carapo" may belong to one of these newer species. The Catalog of Fishes is the authority for the valid name, and — as with all Gymnotiformes — this New-World knifefish is unrelated to the Asian featherback knifefishes.
Morphology
The banded knifefish has the sturdy, cylindrical-to-compressed body typical of Gymnotus, quite different from the slender glass knifefish or the blade-like ghost. Its ground colour is brown to olive, crossed by a series of bold, oblique dark bands — the feature that names it — though the exact pattern varies widely across its range and between the cryptic species lumped under the name. Maximum size is often quoted at around 30 in in total length, but most individuals are far smaller, commonly in the range of a couple of hand-spans.
Like all knifefish it has no dorsal or tail fin and swims on the ripple of a long anal fin, moving readily backwards as well as forwards. The electric organ runs the length of the body and the skin bears the electroreceptors that read its pulses. The body is scaleless. Behind the head, the well-vascularised mouth and gut allow the fish to take in air at the surface — an adaptation to the low-oxygen water it often lives in.
Habitat
No other gymnotiform matches the banded knifefish's range. It occurs from the Río de la Plata basin in northern Argentina and Paraguay, north through the Amazon and Orinoco and the rivers of the Guianas and Trinidad, and up through Central America as far as southern Mexico — the banded knifefishes are the only members of the whole order to have colonised Central America, having crossed after the isthmus of Panama closed. It is a fish of still and slow waters: swamps, floodplain pools, marshes, ditches and the vegetated margins of rivers.
Many of these habitats are warm and poorly oxygenated, and the banded knifefish's ability to breathe air lets it thrive in them and even survive in near-drying pools where other fish suffocate. It is a hardy, adaptable animal, at home in warm, soft to moderately hard, near-neutral water among dense cover. That toughness, and its huge range, are two sides of the same coin — this is a fish built to live where conditions are hard.
Feeding
The banded knifefish is a nocturnal predator with a broad appetite. It eats aquatic insects and their larvae, crustaceans, worms and other invertebrates, and small fish, hunting them at night over the bottom and among vegetation and locating prey partly through its pulsed electric sense. Larger individuals take proportionally more fish. It is an opportunist, and its diet shifts with what its particular swamp or floodplain offers through the seasons.
In the aquarium it is easy to feed on meaty foods — earthworms, prawn, mussel, bloodworm and frozen foods — taken readily in the evening. It is emphatically not a community fish where small tankmates are concerned: anything it can swallow is a potential meal, and its predatory, territorial nature means it is best kept alone or with robust, similarly-sized fish that it cannot eat and that will not harass it.
Mating
As in the rest of the order, electricity is woven into courtship. The banded knifefish's pulsed discharge carries identity and status information, and interacting fish modulate the rate and pattern of their pulses during aggressive and reproductive encounters — a signalling system studied in detail in this genus. Pairs form in the warmer, higher-water season, when floodplains flood and breeding conditions arrive.
Gymnotus is notable, like its relative the electric eel, for parental care. The male takes on the work of reproduction beyond mating: after the female spawns, it is he who tends the developing brood, a level of investment unusual among fishes and shared, within this family, with the electric eel.
Breeding
The banded knifefish builds and guards a nest. In the breeding season the male establishes a nest among vegetation or in a sheltered hollow, the female deposits her eggs there, and the male then guards the eggs and the larvae that hatch, driving off intruders until the young are free-swimming. This paternal care, and the seasonal, flood-triggered timing, echo the pattern seen in the electric eel and set the Gymnotidae apart from the more hands-off glass and ghost knifefishes.
Captive breeding has been achieved and studied, particularly given the genus's importance as a research animal, but it is not routine in the home aquarium and requires mature fish, space and the right seasonal cues. For most keepers the banded knifefish is a hardy, characterful predator to keep rather than a fish to breed, and the guarded nest is best appreciated as one of the more surprising facts about this widespread animal.
In the aquarium
The banded knifefish is one of the hardier knifefishes and, in the right setup, an undemanding one — but it is a predator and needs to be kept as such. It wants a spacious tank with a soft substrate, plenty of cover and shade, secure retreats, and clean, warm, near-neutral water; a tight-fitting lid matters, both because the fish can be a jumper and because it needs undisturbed access to the surface to take air. Subdued lighting suits its nocturnal habits.
It should not be housed with fish small enough to be eaten, and it can be territorial and intolerant of others of its kind, so it is usually best kept singly or with large, robust tankmates. Being scaleless, it shares the family's sensitivity to poor water quality and to harsh medications, so mature filtration and regular water changes are important, and copper-based treatments are best avoided. Given room, cover and appropriate company, it is a tough, long-lived and interesting fish — a chance to keep a close relative of the electric eel at a manageable scale.
Conservation
Gymnotus carapo is assessed as Least Concern on the IUCN Red List. Its enormous range, from Argentina to Mexico, and its tolerance of degraded, warm and poorly-oxygenated habitats that many fish cannot occupy, make it one of the most secure of all the knifefishes, and it is common across much of that range.
The important qualification is taxonomic. Because "Gymnotus carapo" has served as a catch-all under which many distinct species were long hidden, and because new species are still being separated from it, a blanket Least Concern for the complex may not hold for every narrow-range form within it — some of the recently-recognised species have small distributions that warrant their own assessment. For the widespread banded knifefish of the hobby, though, there is no conservation concern, and it is a fish that can be kept and even bred without pressure on wild populations.