Taxonomy & naming
Eigenmannia antonioi was described in 2015 by Peixoto, Dutra & Wosiacki in a monograph on the "electric glass knifefishes" of the Eigenmannia trilineata species-group, published in the Zoological Journal of the Linnean Society. That paper split what had been treated as one or two widespread, variable species into seven distinct new taxa, of which E. antonioi is one. It belongs to the family Sternopygidae, the glass and rattail knifefishes, within the subfamily Eigenmanniinae, and sits in the order Gymnotiformes — the New-World electric knifefishes, a group of bony fish defined by their electric organs and a single long anal fin used for propulsion rather than the usual complement of fins.
The species epithet honours Antônio da Silva Wanderley, the grandfather of the paper's first author, a naming convention distinct from any anatomical or geographic descriptor. Eschmeyer's Catalog of Fishes lists no synonyms and no history of generic reassignment for E. antonioi — it has held a stable taxonomic position since its description. It is worth stressing, as with all Gymnotiformes, that the shared common name "knifefish" does not imply any relationship to the Asian featherback and clown knifefishes (Chitala, Notopterus); those are bonytongues in an unrelated order, and the resemblance in body shape is convergent, not a mark of kinship.
Morphology
Eigenmannia antonioi has the classic glass-knifefish build: a long, laterally compressed, ribbon-like body that tapers to a fine point at the tail, with no dorsal fin and no true caudal fin. Locomotion comes entirely from a very long anal fin running along most of the underside, which the fish undulates in travelling waves to move forward or backward with equal ease — a hallmark of the whole order. Like its congeners it is semi-transparent to some degree, the body wall thin enough to show internal structure, which is where the group's "glass knifefish" name comes from.
FishBase records a maximum length of 8 in total length for the species, a modest size within the genus. As in all Gymnotiformes, an electric organ runs the length of the body and the skin carries the electroreceptor pores that read the field it generates; the species produces a wave-type electric organ discharge — a continuous, near-sinusoidal hum rather than the discrete pulses of pulse-type gymnotiforms such as Gymnotus. The fish is scaleless, a trait shared across the order that has direct consequences for how sensitive it is to water quality and medication.
Habitat
The type locality of Eigenmannia antonioi is the Rio Anapu, within the Floresta Nacional de Caxiuanã, a tributary system of the lower Amazon in Pará state, Brazil. On current data the species is known only from this river system — a narrow-range Amazon basin endemic rather than a continent-spanning species like some of its relatives. FishBase describes it as freshwater and benthopelagic, living in the water column near the bottom rather than right on the substrate or at the surface.
As with other glass knifefishes, this is warm, blackwater-influenced Amazonian river habitat: soft, acidic to near-neutral, often stained with tannins from decaying leaf litter, and dim even by day. That darkness is central to how the fish lives — electrolocation does the sensory work that vision cannot in turbid or tea-coloured water, letting the fish navigate around roots, submerged wood and other structure and detect prey and rivals by the disturbance they cause in its electric field.
Feeding
Detailed dietary data specific to Eigenmannia antonioi is not available in the sources consulted here, but the genus's feeding ecology is well established and there is no reason to expect this species departs from it. Glass knifefishes are nocturnal micro-predators, active after dark and locating prey largely by electrolocation rather than sight — small aquatic invertebrates such as insect larvae, worms and tiny crustaceans picked from the water column and among cover near the bottom.
By day the fish is expected to retreat into cover — root tangles, submerged wood, dense marginal vegetation — and become active only as light fades, a pattern consistent across Eigenmannia and the wider Sternopygidae. Data on this particular species' feeding behaviour in the wild is sparse; it should be treated as broadly typical of the genus until better documented.
Mating
No species-specific courtship account exists for Eigenmannia antonioi, but the genus is unusually well studied electrophysiologically through its widespread relative E. virescens, and the pattern almost certainly extends to this species. Wave-type electric organ discharge is central to Eigenmannia courtship and social behaviour: each fish's discharge carries a signature frequency that differs between males and females and shifts with reproductive state, and fish use these frequency differences — along with the well-known jamming-avoidance response, in which two individuals with similar discharge frequencies shift them apart to avoid interference — to recognise sex, assess rivals and communicate readiness to spawn.
Courtship in the genus therefore plays out substantially as an exchange of electric signals in the dark, layered over physical approach, rather than through visual display. Congeneric data (E. virescens, Sternopygus macrurus) show males increasing discharge rate and producing signal interruptions to attract females, a plausible model for how E. antonioi behaves, though it has not been documented directly in this species.
Breeding
Eigenmannia antonioi has no documented captive-breeding record; nothing in the available sources describes it being spawned in an aquarium. What can be said comes from the family's general reproductive strategy and from the better-studied congener E. virescens: Sternopygidae are egg-layers, not livebearers or mouthbrooders, typically spawning adhesive eggs scattered among aquatic-plant leaves and roots during the warm, high-water wet season, when soft, plentiful floodwater conditions favour larval survival. E. virescens is reported to lay on the order of 100–200 adhesive eggs per spawn under these conditions, with essentially no parental care afterwards — the eggs are left among vegetation to develop unguarded.
Given that E. antonioi is a narrow-range, recently described species with no aquarium trade presence, its breeding biology should be considered data sparse and inferred rather than directly known. Eigenmannia as a genus is regarded as one of the more readily bred gymnotiforms in captivity relative to other knifefish groups, largely on the strength of E. virescens; whether E. antonioi shares that tractability is untested.
In the aquarium
Eigenmannia antonioi is not an established aquarium species — it was described only in 2015 from a single Amazon tributary, and the fish sold under "glass knifefish" in the trade is almost invariably the widespread E. virescens, not this narrow-range relative. Anything said here about its aquarium requirements is inferred from the genus rather than drawn from hobbyist experience with this exact species.
Were it kept, the general Eigenmannia template would apply: warm water in the roughly 68–86 °F range (many keepers aim nearer 81 °F), soft and slightly acidic (pH around 6.0–7.0), with dim lighting and dense cover such as plants, roots or pipes to shelter in by day, since these are nocturnal fish that read their surroundings electrically rather than visually. Being scaleless, the whole family is unusually sensitive to ammonia and nitrate and to copper-based or harsh medications, which need reduced dosing. Eigenmannia species generally do well in a small shoal of their own kind rather than singly, and are peaceful toward tankmates too large to be eaten — though as with all gymnotiforms, small fish and shrimp are at risk after dark. Given the uncertainty around this species' status and needs, captive-bred stock of a well-established congener is the more responsible choice for most aquarists rather than seeking out E. antonioi itself.
Conservation
Eigenmannia antonioi is assessed by the IUCN as Least Concern. That status sits somewhat uneasily alongside its apparent range: the species is currently known only from the Rio Anapu within the Floresta Nacional de Caxiuanã, a tributary of the lower Amazon in Pará, Brazil, which is the profile of a narrow-range single-river endemic rather than a widely distributed fish — the more typical profile for a Data Deficient or geographically restricted assessment. As with many recently described, poorly surveyed Amazonian fishes, the true range may simply be under-documented rather than genuinely small.
The Floresta Nacional de Caxiuanã is a protected federal forest reserve, which affords the type locality some formal protection from deforestation and direct habitat conversion, though broader Amazonian pressures — logging, land-use change, mining-related pollution and hydrological disruption from damming elsewhere in the basin — remain relevant background risks for any narrow-range tributary species in the region. There is no aquarium trade pressure on this species to speak of, since it is not commercially collected or sold.