Taxonomy & naming
Gymnotus arapaima was described by Albert & Crampton in 2001 and belongs to the family Gymnotidae, within the order Gymnotiformes — the New-World electric knifefishes. These are bony fish, but unusual ones: nearly every species produces a weak electric organ discharge (EOD) around its body, used for electrolocation in dark or turbid water where eyesight is of little use, and for electrocommunication with other members of its species. Gymnotidae, the family that contains both the banded knifefishes (Gymnotus) and the electric eel (Electrophorus), are pulse-type dischargers — their EOD is a series of discrete electric pulses rather than the continuous wave produced by some other gymnotiform families.
The species epithet honours Arapaima gigas, the giant pirarucu, reflecting the fact that G. arapaima was found alongside it in the Mamirauá lake system and Lago Tefé of central Amazonia. Despite the shared common name "knifefish", Gymnotus and its relatives are not related to the Asian featherback or clown knifefishes (Chitala, Notopterus), which belong to an entirely different order and resemble gymnotiforms only by convergent body shape. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name; Gymnotus is a large genus, with the widespread Gymnotus carapo now recognised as a species complex that G. arapaima was split out of related lineages within.
Morphology
Gymnotus arapaima has the classic banded-knifefish build: a stout, laterally compressed body marked with bold dark bars over a paler ground colour, reverse counter-shading (darker below than above, unusual among fish and thought to relate to how the species is viewed by predators and prey from below in open floodplain water), and comparatively large anterior nostrils. As in all gymnotiforms there is no dorsal fin and no distinct tail fin; locomotion comes from a long ribbon-like anal fin run in travelling waves along the underside of the body, letting the fish move forward or backward with equal control. The species is scaleless.
Running the length of the body is the electric organ that generates the fish's pulse-type EOD, paired with electroreceptor pores across the skin that read the field it creates. FishBase records a maximum total length of about 21.5 in, making G. arapaima a solidly built, medium-to-large member of its genus rather than one of the smaller Gymnotus species.
Habitat
The species is known from the Mamirauá lake system, Lago Tefé and adjoining terra firme swamps near Tefé in central Amazonia, Brazil — floodplain (várzea) habitat within the Amazon basin. FishBase records juveniles using the floating meadows of the várzea and adults moving into the flooded forest as the water rises, a seasonal habitat shift typical of many Amazonian floodplain fishes tied to the annual flood pulse.
Water here is warm, generally soft, and close to neutral to slightly acidic (recorded pH roughly 6.3–7.2), often dim and tannin- or sediment-stained — exactly the low-visibility conditions in which an electric sense is more useful than eyesight. Like other Gymnotus, it is a facultative air-breather, able to gulp air at the surface via its gas bladder when the warm, densely vegetated floodplain water becomes oxygen-poor, a trait shared broadly across the genus and important to its survival in seasonally stagnant swamp and floodplain pools.
Feeding
FishBase lists Gymnotus arapaima as carnivorous, with a trophic level (3.2) consistent with a predator of small aquatic animals rather than a specialist. Across the genus, Gymnotus are nocturnal hunters of small fish and invertebrates — insect larvae, worms and crustaceans among them — locating prey after dark largely through electrolocation, sensing the way animals and objects distort the weak electric field around the body, an ability especially valuable in the turbid, low-visibility floodplain water it inhabits.
Direct feeding observations specific to G. arapaima are not documented in the sources reviewed here; its diet is inferred from the general pattern for the genus and the trophic level given by FishBase. As a nocturnal predator built to take fish and invertebrates by surprise, it should not be assumed safe with bite-sized tankmates.
Mating
As in other Gymnotidae, the electric organ discharge in G. arapaima is not purely a sensing tool — it doubles as a communication signal, with waveform differences that can encode species identity, sex and reproductive state, and that likely feature in how individuals recognise and approach potential mates or rival males in the dark. Adult male Gymnotus are also reported to hold territories and to guard nest sites, implying at least some contest or signalling between males around breeding condition and site quality.
Detailed courtship behaviour has not been directly documented for G. arapaima specifically in the sources available here, and the account above draws on the broader pattern described for Gymnotidae and the genus Gymnotus rather than species-specific observation — data sparse for this species.
Breeding
Gymnotidae are unusual among fish in showing genuine parental care, and in Gymnotus this takes the form of a male-guarded nest: the male builds and defends a nest — often a sheltered mass among floating vegetation or a hollow among cover — into which a female spawns, and the male then guards the eggs and larvae. Breeding is understood to be seasonal and tied to the flood cycle typical of Amazonian várzea systems, plausibly linked to the same rising water that pushes G. arapaima's own juveniles into the floating meadows described in its habitat account.
There is no dedicated breeding study for G. arapaima among the sources reviewed here; this account is inferred from parental care documented at the genus and family level rather than confirmed for the species itself. It should be treated as poorly documented for this species specifically, not as an established protocol.
In the aquarium
Gymnotus arapaima is not an established aquarium fish and is essentially unavailable in the ornamental trade; what follows applies the general husbandry needs of the genus rather than species-specific keeping experience. Like all gymnotiforms it is scaleless, which makes it sensitive to ammonia and nitrate and to harsh or copper-based medications — treatments would need reduced dosing, as with the more familiar black ghost knifefish. As a nocturnal predator that reads its surroundings electrically, it would need a dim, well-covered tank with secure hiding places and should not be trusted with fish or invertebrates small enough to be treated as prey.
Given a reported maximum length around 21.5 in and a territorial, nest-guarding disposition in males, a specimen would need considerably more space than a typical community setup, along with the warm, soft, near-neutral water and good oxygenation reflected in its wild habitat. Given how rarely the species reaches the hobby, prospective keepers are better served by the handful of Gymnotus and other gymnotiforms that are actually established in the trade.
Conservation
Gymnotus arapaima is assessed as Least Concern on the IUCN Red List (as of 12 August 2020), consistent with a species known from a defined but not obviously threatened part of central Amazonia. It is not currently flagged as a species of conservation concern.
As with many narrow-range Amazonian floodplain endemics, the broader pressures on its habitat are the ones worth naming even absent a specific threat assessment: deforestation, changes to flood regimes, and pollution affecting várzea lake systems such as Mamirauá and Lago Tefé. The Mamirauá system itself falls within a long-established Brazilian sustainable-development reserve, which offers the species some habitat protection, but detailed population trend data for G. arapaima specifically were not found among the sources reviewed here.