Taxonomy & naming
Gymnotus cylindricus was described by Francesca La Monte in 1935 and belongs to the family Gymnotidae, within the order Gymnotiformes — the New-World electric knifefishes. The Gymnotidae are one of several families in the order and are set apart by their pulse-type electric organ discharge (EOD): rather than the continuous, wave-like signal produced by ghost knifefishes (Apteronotidae), Gymnotus fires discrete, well-separated pulses, each one a brief, information-rich waveform rather than a steady hum.
The genus Gymnotus is large and still being actively split — G. carapo, long treated as one widespread species, has in recent decades been resolved into a complex of 20 or more distinct species, and G. cylindricus belongs to a related cluster of forms restricted to Middle America. Despite the shared common name, gymnotiform "knifefishes" are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus), which are bonytongues in an entirely different order; the resemblance is convergent, not a matter of kinship. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name.
Morphology
True to its genus, G. cylindricus is a stout-bodied, almost cylindrical knifefish rather than the flattened blade shape of Apteronotus — the name cylindricus itself points at that rounded cross-section. The body carries the bold dark barring typical of banded knifefishes, breaking up its outline against the leaf litter and root tangles it lives among. FishBase records a maximum length of 9.5 in standard length, making it a modestly sized member of the genus. As in all gymnotiforms there is no dorsal fin and no functional tail fin; locomotion comes from a long anal fin run as a travelling wave along the underside of the body, letting the fish move forwards or backwards with equal control. The skin is entirely without scales.
Running the length of the body is the electric organ, and the skin is studded with electroreceptor pores that read the field it creates. Published work on the genus's electric organ discharge shows G. cylindricus produces a monophasic pulse — a single-phase waveform, simpler than the multi-phase pulses of many relatives — which researchers have suggested may reflect relaxed predation pressure on this species relative to others in the genus. That waveform is distinctive enough to serve as a form of species identification in electrocommunication between individuals.
Habitat
Gymnotus cylindricus is a Central American endemic, known from the Motagua River basin of Guatemala, and its wider group of related Middle American forms is treated as restricted to the isthmus rather than ranging into South America. This matters taxonomically: Gymnotus is the only genus in the entire order Gymnotiformes to have colonized Central America, with most gymnotiforms confined to South America proper.
As is typical for the genus, G. cylindricus is a benthopelagic freshwater fish of slow, vegetated water — swamps, floodplain pools and river margins rather than fast, open channels — and like other Gymnotus it is a facultative air-breather, able to gulp air at the surface to survive in warm, oxygen-poor water where many other fish cannot. That combination of toughness and a taste for still, structure-rich water underpins its ecology and, by extension, how it should be kept.
Feeding
Data on this species' diet specifically are sparse, but the genus Gymnotus is well documented as a nocturnal predator, and there is no reason to expect G. cylindricus differs. Banded knifefish hunt after dark, using electrolocation to detect aquatic insect larvae, worms, small crustaceans and other invertebrates hidden in sediment or tangled root mats, and larger individuals across the genus are known to take small fish as well.
In captivity, Gymnotus species readily accept meaty foods — bloodworm, chopped earthworm, prawn and other frozen or live fare — offered after the lights go down, when these fish become active and confident. As active nocturnal hunters they are not suited to housing with anything small enough to be treated as prey.
Mating
As in other Gymnotiformes, electric organ discharge is not purely a sensory tool in this genus — it also functions in communication, and the literature on Gymnotus EOD diversity notes that species- and sex-specific waveforms are used both for electrolocation and for signalling between individuals. It is reasonable to expect courtship in G. cylindricus, as in its relatives, involves an exchange of electric signals alongside physical approach in the dark, though this has not been specifically documented for the species and the cache does not support more than that inference.
Detailed courtship behaviour for G. cylindricus itself is not well described in the available literature; data sparse.
Breeding
The Gymnotidae are unusual among fishes for showing genuine parental care, and the general pattern for the genus Gymnotus is that the male builds and guards a nest — often a floating mass of foam or bubbles, or a sheltered hollow among vegetation — into which the female spawns, with the male then guarding the eggs and larvae until they disperse; spawning is typically seasonal and tied to floodplain conditions. Whether this exact pattern has been directly observed in G. cylindricus is not confirmed in the sources reviewed here, but it is the documented mode for the genus and there is no indication this species departs from it.
G. cylindricus is essentially unbred and undocumented in the aquarium hobby. Prefer honesty over invention: this is a poorly documented species in captivity, and any aquarist attempting to breed it should expect to be working from the general Gymnotus pattern rather than a proven protocol.
In the aquarium
Gymnotus cylindricus is seen only rarely in the trade, usually via specialist importers rather than general fish shops, and should be approached as an advanced fishkeeper's animal. Being scaleless, it needs the same caution around medications as other gymnotiforms — avoid copper-based treatments and reduce doses of anything harsh — and it is sensitive to ammonia and nitrate, so mature filtration and clean, warm water are essential. Its facultative air-breathing gives it more tolerance of low-oxygen conditions than most aquarium fish, but that should be treated as a survival trait, not licence for poor water quality.
Give it a dimly lit tank with plenty of cover — root tangles, wide pipe, dense planting or leaf litter — to shelter in by day, since like its relatives it is a nocturnal predator that becomes active after dark. It is not community-safe with small or bite-size tankmates, which it will hunt at night by electrolocation regardless of how placid it appears during the day, and it can be territorial with other Gymnotus. Captive-bred stock is not generally available for this species; specimens in the trade should be assumed wild-collected.
Conservation
Gymnotus cylindricus is assessed by the IUCN as Least Concern, last evaluated in 2019. As a species restricted to Central America — known from the Motagua River basin of Guatemala and the broader Middle American range of its species group — it is narrower-ranging than many of its South American relatives, but it is not currently flagged as threatened.
As with other narrow-range Middle American freshwater fish, the main long-term risks to this kind of species are habitat degradation, water pollution and changes to floodplain hydrology in its home basin, rather than any pressure from the aquarium trade, which barely touches this species. No specific conservation programme targets G. cylindricus, and the available assessment does not describe an active decline.