Taxonomy & naming
Gymnotus tigre was described by James Albert and William Crampton in 2003, one of seven new Gymnotus species the pair split out of the polytypic Gymnotus carapo species complex in a single revision. Before that work, animals now recognised as several distinct species were long lumped together under the older, more broadly applied name G. carapo; G. tigre is one of the segregates that resulted. The holotype, a fish of 16 in, was collected on the northern shore of the Río Pastaza in Amazonas, Colombia, and is held under the catalogue number UF 25552.
The species belongs to the family Gymnotidae within the order Gymnotiformes, the New-World electric knifefishes — bony fish, but unusual ones, defined by an electric organ run along the body and a long ribbon-like anal fin used for propulsion instead of a tail. Despite the shared common name, gymnotiform knifefishes are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance is convergent body shape, not kinship. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name Gymnotus tigre Albert & Crampton, 2003.
Morphology
Gymnotus tigre is a stout-bodied banded knifefish, reaching a maximum recorded total length of around 16 in — sturdier and more heavily built than the slender ghost knifefishes of the family Apteronotidae. Like all Gymnotus it has no dorsal fin and no distinct tail fin; locomotion comes from a long anal fin running most of the length of the underside, thrown into travelling waves that let the fish move forwards or backwards with equal ease. The body is patterned with the bold dark bars typical of the genus — the source of the tiger knifefish name — set against a lighter ground colour, though exact banding varies between the many species split from the G. carapo complex and precise colour description is not detailed in the available sources.
As in other Gymnotiformes, an electric organ runs the length of the body, generating a weak, pulse-type electric organ discharge (EOD) used for electrolocation and electrocommunication. The skin is scaleless, a trait shared across the order that leaves these fish more exposed to water-quality stress and to certain medications than scaled fish. Gymnotus tigre, like its relatives, is a facultative air-breather, able to gulp air at the surface to supplement gill respiration in oxygen-poor floodplain water.
Habitat
Gymnotus tigre is known from the western Amazon lowlands, with records spanning the Río Pastaza basin in Ecuador to the Tapajós basin in Brazil — an Amazon-basin endemic, described from below 1640 ft elevation. It occupies the swamps, floodplain channels and slow, vegetated margins typical of Gymnotus, water that is often warm, soft to moderately mineralised, tannin-stained and dim, conditions in which electrolocation matters more than eyesight.
Gymnotus is the only genus of gymnotiform knifefish to range outside South America, reaching as far as southern Mexico, but G. tigre itself is a narrower-range Amazonian species rather than one of those far-ranging forms. Its floodplain habitat can become seasonally hypoxic, water low enough in dissolved oxygen to challenge ordinary gill-breathing fish; the facultative air-breathing typical of the genus is an adaptation to exactly that kind of water.
Feeding
Gymnotus tigre is a nocturnal predator, hunting after dark using electrolocation to detect prey by the way it distorts the fish's own weak electric field — a critical sense in turbid, tannin-stained floodplain water where vision is of limited use. As in other banded knifefishes, the diet is expected to consist of bite-sized invertebrates and small fish taken from among submerged cover, though specific dietary data for this species is sparse in the available sources.
In the aquarium, Gymnotus species generally accept meaty foods such as bloodworm, chopped earthworm and other frozen or live invertebrate foods, fed after dark when the fish becomes active. Because it hunts by electrolocation at night, any tankmate small enough to be swallowed should be considered at risk regardless of how the fish behaves during the day.
Mating
Courtship in Gymnotus, as in other gymnotiform knifefishes, is bound up with the electric organ discharge: the EOD is not only a sensing tool but a signal carrying information about identity, sex and reproductive state, and electrocommunication is expected to feature heavily in how tiger knifefish locate and assess one another in the dark. Specific courtship behaviour has not been documented for G. tigre itself, but the genus is known to be territorial, and encounters between conspecifics — including courting pairs — are likely mediated by EOD signalling before any physical approach.
Breeding in Gymnotidae is seasonal and flood-triggered, tied to the rise of floodwaters that open up the shallow, vegetated margins the fish spawn in. Detailed courtship behaviour specific to G. tigre is not documented in the available sources; what follows is drawn from the broader pattern known for the genus.
Breeding
Gymnotidae show a genuinely unusual trait among fishes: parental care. In Gymnotus generally, the male builds and guards a nest — often a floating mass of foam or bubbles, or a sheltered hollow among vegetation and debris — into which the female spawns; the male then guards the eggs and the resulting larvae, a behaviour tied to the seasonal rise of floodwater. Breeding has not been specifically documented for Gymnotus tigre, and no aquarium spawning reports appear in the available sources.
For the aquarist the honest position is that this is a species with no established captive-breeding record: the pattern described here reflects what is known of the genus as a whole rather than anything observed in G. tigre specifically, and anyone attempting to breed it should expect to be working from first principles.
In the aquarium
Gymnotus tigre is a predatory, territorial fish, not a community animal. It is nocturnal, hunts by electrolocation after dark, and is not safe with bite-sized tankmates; it is also often intolerant of its own kind, and is generally best kept solitary or, with care, in a bonded pair rather than a group. A large aquarium with a soft, sandy or bare substrate, dim lighting and secure hiding places — pipes, roots, dense planting at the margins — suits it far better than an open, brightly lit tank, and the tiger knifefish's active nocturnal hunting means cover during the day directly reduces stress.
Being scaleless, G. tigre needs the same caution given to other gymnotiform knifefish: no copper-based medications, reduced doses of anything harsh, and sensitivity to ammonia and nitrate that calls for mature filtration and regular water changes. It tolerates warm water and, thanks to facultative air-breathing, copes with lower dissolved oxygen than many fish — but that tolerance is not licence for poor husbandry. Warm (roughly 75–82 °F), soft, tannin-stained, near-neutral to slightly acidic water suits it best, echoing the floodplain habitat it comes from.
Conservation
Gymnotus tigre is assessed as Least Concern on the IUCN Red List, assessed 23 November 2020, reflecting a range across the western Amazon basin from the Río Pastaza in Ecuador to the Tapajós in Brazil and no evidence of significant population decline. It is not currently a species of conservation concern.
As with other Amazon basin fish, the broader pressures on its habitat — deforestation, water pollution and hydrological change from damming — are the kind of threats that could affect narrow-range or floodplain-dependent species over time, even where a species is not yet flagged as threatened. Least Concern describes the species across its known range, not a guarantee against future habitat loss in the basin it depends on.