Taxonomy & naming
Eigenmannia macrops was described by George Albert Boulenger in 1897 and belongs to the family Sternopygidae, the glass and rattail knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Like all gymnotiforms it is a bony fish but an unusual one, defined by a body-length electric organ and a single long ribbon-like anal fin in place of a normal tail and dorsal fin. The genus Eigenmannia is named for the ichthyologist Carl H. Eigenmann, a major early cataloguer of South American freshwater fishes; the species epithet macrops derives from the Greek for "large eye," describing eyes noticeably bigger than those of its congeners.
Sternopygidae sits apart from the ghost knifefishes (Apteronotidae) and the pulse-type Gymnotidae by its wave-type electric organ discharge, a continuous, near-sinusoidal signal rather than a series of discrete pulses. Despite the shared common name, gymnotiforms like E. macrops are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to an entirely different order of bonytongues; the resemblance is convergent body shape only. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name.
Morphology
Eigenmannia macrops has the classic glass-knifefish silhouette: a laterally compressed, tapering, ribbon-shaped body that is semi-transparent olive to amber in life, with internal organs and the spine sometimes visible through the flanks. There is no dorsal fin and no separate tail fin; instead an extremely long anal fin runs along almost the entire underside of the body from just behind the head to the tip of the tail, and its rippling wave is what propels the fish forwards or backwards with equal facility. The body ends in a fine, whip-like point rather than a rounded tail.
Running the length of the body is the electric organ, paired with electroreceptor pores scattered over the head and flanks that read the field it generates. Reaching up to about 10 in in total length, E. macrops is a mid-sized member of its genus. As its name suggests, it has comparatively large eyes for a glass knifefish, though like other gymnotiforms it relies on electrolocation far more than vision in the dim, often turbid water it inhabits. The species is entirely scaleless, a detail that matters a great deal for its care. External sexual differences are not well documented; discharge frequency, rather than appearance, is the more reliable indicator of sex in wave-type Sternopygidae.
Habitat
Eigenmannia macrops is known from the Guianas and the Amazon and Orinoco basins, with confirmed records from the Cuyuni River in Guyana, the Potaro and Essequibo basins in Guyana, and the Orinoco River in Venezuela. It is a benthopelagic fish of tropical freshwater, typically found around submerged vegetation, roots and other cover in slow to moderate flow.
A study of the species across its range found real microgeographic variation: fish from small tributary caños tend to run smaller than those collected from lagoons and more open, offshore stretches of river, a pattern attributed to differences between these habitat types rather than to separate populations. Across all of these settings the water is warm, tropical and often tannin-stained or turbid, conditions where the electric sense does the navigational work that eyes cannot — even a species with relatively large eyes, like this one, still depends on its electric organ discharge to find food and read its surroundings after dark.
Feeding
As with other Sternopygidae, Eigenmannia macrops is a nocturnal micro-predator, foraging mainly after dark for small invertebrates — aquatic insect larvae, worms and tiny crustaceans — picked out of sediment, leaf litter and vegetation using its electric sense to detect prey that eyes alone would miss in dim or turbid water. It is a continual, probing forager rather than an ambush hunter, consistent with its always-on wave-type discharge.
In the aquarium it accepts small meaty foods readily: bloodworm, blackworm, brine shrimp, daphnia and similar frozen or live fare, along with sinking pellets once settled in. Feeding is best done toward evening, when the fish become more active; a shoal of glass knifefish will often forage together along the bottom and among plant roots rather than compete aggressively, and, being small-mouthed relative to the larger gymnotiforms, they pose little threat to anything but the smallest live foods and fry.
Mating
Courtship in Eigenmannia is inseparable from its electricity. The wave-type electric organ discharge carries a signature frequency that differs by sex and by individual, and Eigenmannia is the classic laboratory subject for the jamming avoidance response — the reflex by which two fish with similar discharge frequencies each shift their own frequency apart to avoid interference when they meet. That same sensitivity to a neighbour's discharge is thought to underpin social and courtship signalling in the genus, letting fish recognise sex, individual identity and reproductive state well before any physical contact.
Beyond the electrical exchange, detailed accounts of pairing and courtship behaviour specific to E. macrops are not well documented; much of what is understood about Sternopygidae courtship comes from electrophysiological study of the genus generally, particularly the well-studied E. virescens, rather than direct field or aquarium observation of this species.
Breeding
Sternopygidae are egg-layers, and Eigenmannia as a genus is bred in captivity more readily than most gymnotiforms — the group's warm, soft, wet-season spawning trigger is comparatively well understood compared with the ghost and pulse-type knifefishes. Adhesive eggs are scattered among fine plant roots or dense floating vegetation, typically prompted by warm temperatures and soft, slightly acidic water resembling the rise in water level during the wet season in its native range.
Parental care afterwards is minimal to none: eggs are left among the vegetation to hatch unguarded. Specific, repeatable breeding reports for E. macrops itself are sparse, and aquarists attempting it are largely extrapolating from the better-documented E. virescens; conditions worth trying are a densely planted, mature, soft-water tank with a gentle rise in temperature to simulate seasonal flooding.
In the aquarium
Eigenmannia macrops is one of the more approachable gymnotiforms for a dedicated keeper, doing well in a modest shoal of its own kind in a tank of a few hundred litres with plenty of fine-leaved planting, driftwood roots and other cover to shelter in by day. Subdued lighting suits it, as does a dark substrate; like other glass knifefish it becomes noticeably more active and confident once the lights dim, foraging along the bottom and around plant roots after dark.
Being scaleless, it needs the same caution around medications and water quality as other knifefishes: copper-based treatments should be avoided altogether and other medications dosed cautiously, and it is sensitive to ammonia, nitrite and elevated nitrate, so mature filtration and regular water changes matter. It is not a fish for a boisterous community — fin-nipping tankmates will damage the long anal fin, and very small fish or shrimp risk being taken as food after dark — but toward tankmates of a similar or larger size it is peaceable and can be housed in a species group. Captive-bred stock is uncommon; most fish in the trade are wild-collected, so care that lets a shoal live out a long life is the best mitigation.
Conservation
Eigenmannia macrops is assessed as Least Concern on the IUCN Red List, reflecting a wide distribution across the Guianas and the Amazon–Orinoco basin and no known major range-wide threats. It is not currently a species of conservation concern.
As with other wide-ranging Neotropical fishes, that overall security does not remove local pressures: habitat degradation, deforestation and pollution affect river systems across its range in Guyana, Venezuela and the wider Amazon basin, and documentation of population trends specific to this species is thin. For the aquarist, favouring sustainably sourced stock and avoiding unnecessary wild collection remains the sound default even for a species not currently listed as threatened.