Taxonomy & naming
Gymnotus eyra was described by Craig, Correa-Roldán, Ortega, Crampton and Albert in 2018, in a revision of the genus Gymnotus from the Upper Madeira basin of Bolivia and Peru that also described a second new species. It belongs to the family Gymnotidae within the order Gymnotiformes, the New-World electric knifefishes — bony fish, but unusual ones, defined by an electric organ and a single long ribbon-like anal fin rather than a conventional fin arrangement. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name.
The species epithet eyra references a local Peruvian name for the red colour form of the jaguarundi (Herpailurus yagouaroundi), continuing a naming convention within Gymnotus of borrowing names from wild cats — other species carry names like onca, pantherinus and tigre — in each case gesturing at shared nocturnal, predatory habits and a patterned or banded body. Gymnotus eyra was found to be morphologically most similar to G. mamiraua, a species from lowland Amazonia, but distinct enough in its upper-Madeira population to warrant description as new. Despite the shared common name "knifefish", Gymnotiformes are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance in body shape is convergent, not a sign of kinship.
Morphology
As in other Gymnotus, the body is elongate and laterally compressed, tapering to a point, scaleless, and marked with bold dark bars or bands across a paler ground colour — the feature that gives the genus its common name of banded knifefish. There is no dorsal fin and no proper caudal fin; locomotion comes from a long anal fin running most of the length of the underside, undulated in travelling waves that push the fish forward or backward with equal facility.
Running the length of the body is an electric organ, paired with rows of sensory pores in the skin that read the weak field it produces; Gymnotus generates PULSE-type electric organ discharges, delivered as discrete, well-spaced pulses rather than the continuous wave-type signal of the ghost knifefishes (Apteronotidae). Detailed size data specific to G. eyra were not available at the time of writing — data sparse for this recently described species — but Gymnotus as a genus spans a wide range, with most species under 12 in total length, some aquarium-traded species reaching 10–15.5 in, and a few exceeding a metre; G. eyra, a moderate-bodied species from the same clade as G. mamiraua, is presumed to fall within the smaller-to-moderate part of that range pending direct measurement.
Habitat
Gymnotus eyra is known from the Upper Madeira basin of Bolivia and Peru, part of the wider Amazon drainage in the Andean foothill region. Like other banded knifefishes it is expected to occupy small streams, floodplain channels and vegetated margins rather than open river channel — the typical Gymnotus habitat is shallow, structurally complex water among roots, leaf litter and aquatic plants, often turbid or shaded.
Gymnotus is the only genus of gymnotiform to range outside South America, reaching north through Central America to southern Mexico, but G. eyra itself appears to be a narrow-range endemic of the Upper Madeira system, one of many such range-restricted Gymnotus species now being resolved as the genus is revised region by region. Its electric sense suits exactly this kind of dim, structurally cluttered water, where sight is of limited use and a self-generated field is a more reliable way to find a path and locate prey.
Feeding
Gymnotus are nocturnal predators, and G. eyra can be expected to follow the pattern typical of the genus: hunting after dark for aquatic insect larvae, small crustaceans, worms and small fish, located largely by electrolocation rather than sight. Banded knifefishes are active, probing hunters that work over submerged cover and sediment, not ambush specialists.
No feeding data specific to G. eyra were located in the available sources. In the aquarium, Gymnotus species generally take meaty foods readily — bloodworm, chopped earthworm, small crustaceans and frozen or live fare of a similar scale — offered after the lights go down, when the fish naturally becomes active.
Mating
Courtship in Gymnotus, as in other gymnotiforms, is thought to be mediated in part through the electric organ discharge, which carries information about a fish's identity, sex and condition and can be modulated during social encounters. No courtship behaviour specific to G. eyra has been documented; this is inferred from what is known of Gymnotus and its close relatives, where electrocommunication accompanies the physical approach between prospective mates in dark water.
Beyond that inference, data are sparse. As a species described only in 2018 from museum and field survey material, G. eyra has not been the subject of behavioural study, and nothing specific to its courtship has been published.
Breeding
Gymnotidae are unusual among fishes in showing paternal care: in Gymnotus generally, the male builds and guards a nest — often a mass of floating foam or a sheltered hollow among vegetation — into which the female spawns, and he then guards the eggs and larvae until they disperse. Breeding in the wild is seasonal and typically linked to flooding, when new habitat and food become available.
Nothing has been published on breeding in G. eyra specifically, and captive breeding of Gymnotus in general is rare and poorly documented — most aquarium livestock, where it exists, is wild-collected rather than captive-bred. The honest summary for this species is that its reproductive biology is presumed to follow the guarded-nest pattern typical of the genus, not that it has been observed doing so.
In the aquarium
Gymnotus eyra has not established any presence in the aquarium trade, and care information here is necessarily drawn from Gymnotus as a genus rather than from this species directly. Banded knifefishes generally do best in warm water around 72–82 °F, pH roughly 6.0–7.5, in a dimly lit tank with dense cover — driftwood, caves and plants — that lets a naturally nocturnal, secretive fish feel secure by day.
Being scaleless, Gymnotus species are sensitive to ammonia, nitrate and to copper-based or otherwise harsh medications, which typically need reduced dosing. They are nocturnal predators and are not safe with bite-size tankmates, which they will hunt after dark; they are also often territorial toward their own kind and are usually best kept singly or with ample space to establish separate territories. Should G. eyra ever appear in the hobby, prospective keepers should treat any of this as a starting point drawn from the wider genus, not a documented account of the species itself, and should favour captive-bred stock if and when it becomes available.
Conservation
Gymnotus eyra has not been formally assessed by the IUCN Red List; as a species described only in 2018, it is Not Evaluated. No conservation status or population trend data are currently available.
As a narrow-range endemic of the Upper Madeira basin — the pattern typical of many recently split Gymnotus species — it is reasonable to flag the general pressures facing that region's freshwater habitats, including deforestation, water infrastructure development and pollution, as the kind of threat that could affect a single-basin species disproportionately. But without a formal assessment or population data for G. eyra itself, any specific conservation claim would be speculation; the honest position is that its status is simply unknown.