Taxonomy & naming
Eigenmannia nigra was described by the Venezuelan ichthyologist Francisco Mago-Leccia in 1994, in his monograph on the electric fishes of the continental waters of America — the standard taxonomic treatment of the Gymnotiformes for decades afterward. It belongs to the family Sternopygidae, the glass and rattail knifefishes, within the order Gymnotiformes, the New-World electric knifefishes. Gymnotiformes are bony fish but an unusual lineage: they lack a dorsal fin and a conventional tail fin, swim by undulating a single long anal fin, and — with one famous exception, the electric eel — generate only a weak electric field around the body rather than a stunning discharge.
Sternopygidae is the wave-type family within the order: its members produce a continuous, near-sinusoidal electric organ discharge (EOD), nicknamed the "hummers," as opposed to the intermittent pulses of families like Gymnotidae. The genus Eigenmannia — the glass knifefishes — is the best studied of these electrical hummers, largely because of the jamming-avoidance response, the classic neuroscience behaviour in which two Eigenmannia individually shift their discharge frequencies apart to avoid interfering with each other's electrolocation. Eigenmannia nigra should not be confused with the Asian featherback or clown knifefishes (Chitala, Notopterus), which despite the shared "knifefish" name belong to an entirely different, distantly related order and are not electric fish at all. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name.
Morphology
Eigenmannia nigra shares the classic glass-knifefish body plan: laterally compressed, ribbon-thin toward the tail, with no dorsal fin and no true caudal fin. Locomotion comes almost entirely from a very long anal fin running most of the length of the underside, which the fish throws into travelling waves to move forward, backward or hover in place with equal facility. Most Eigenmannia species are semi-transparent, an adaptation that presumably reduces visibility to predators in open water, but E. nigra departs from that pattern — as its name states, it carries a distinctive dark or black coloration, without the horizontal stripes seen in some relatives, a dark blotch across the shoulder (humeral) region, and an anal fin that is black or dark-margined. FishBase records a maximum total length of 18.5 in for males and unsexed individuals, making it a mid-sized to large member of the genus.
As in all gymnotiforms, an electric organ runs along much of the body, paired with an array of electroreceptor pores across the skin that read the field it produces. The species is scaleless, like its relatives, which leaves the skin more exposed and has direct consequences for water quality and medication tolerance in captivity. External sexual dimorphism has not been documented for this species in the sources available.
Habitat
Eigenmannia nigra is known from the Rio Negro basin and the connecting Rio Casiquiare in Venezuela, from Colombian Amazonia near Leticia, and from the Rupununi River in Guyana — a range centred on the upper Rio Negro and adjacent blackwater and clearwater systems of the northwestern Amazon and Guiana Shield. It is a benthopelagic freshwater fish, occupying the water column over and near the bottom rather than living buried in sediment or clinging to the surface.
The Rio Negro system is characteristically tannin-stained blackwater: warm, extremely soft, and acidic, with visibility often reduced to a matter of centimetres. In water this dark, electrolocation rather than eyesight is the primary sense for navigating, finding prey and detecting other fish, which is presumably why the wave-type electric sense of the Sternopygidae is so well developed in this basin. Specific microhabitat details for E. nigra are not documented in the sources at hand, but glass knifefishes in general favour slower margins, root tangles and submerged vegetation over open, fast-flowing channels.
Feeding
Detailed diet data specific to Eigenmannia nigra is not available in the sources consulted. Glass knifefishes as a group are nocturnal micro-predators, foraging after dark for small invertebrates — aquatic insect larvae, worms, small crustaceans and similar prey — located largely through electrolocation rather than sight, in the same manner as the better-studied Eigenmannia virescens and other genus members.
In the aquarium, Eigenmannia species readily accept small meaty foods — bloodworm, brine shrimp, chopped worms and similar frozen or live fare — offered in the evening or after lights-out, when the fish naturally becomes active. There is no specific reason to expect E. nigra to differ from this genus-wide pattern, though it has not been documented in the hobby to the same extent as its more commonly kept, transparent relatives.
Mating
As in other wave-type Gymnotiformes, courtship and social signalling in Eigenmannia are thought to run substantially through the electric organ discharge itself. The EOD carries information about species, sex and individual identity, and in the well-studied E. virescens, males and females show sex-typical differences in discharge frequency and modulate their signals during social encounters — the same jamming-avoidance mechanism used to navigate around interference from other fish is also implicated in courtship and rival interactions. It is reasonable to expect a broadly similar pattern of electrical courtship in E. nigra, though this has not been specifically documented for the species.
Beyond the electrical component, no behavioural account of pairing, courtship display or spawning approach specific to Eigenmannia nigra is available in the sources at hand; the species remains poorly documented outside its original taxonomic description and subsequent distribution records.
Breeding
Sternopygidae as a family are egg-layers, typically scattering adhesive eggs among plant roots or floating vegetation rather than building a nest or mouthbrooding, and Eigenmannia in general is bred in captivity somewhat more readily than most other gymnotiforms — usually associated with warm, soft, slightly acidic water and a rise in water level or flow mimicking the wet season. There is little to no parental care reported in the genus once eggs are laid.
Breeding of Eigenmannia nigra specifically is not documented in the sources available, and given how little the species appears in the aquarium trade, captive spawning has almost certainly not been recorded or is exceptionally rare. The honest position for this species is that its reproductive biology in captivity is essentially unknown; the genus-typical pattern of egg-scattering among vegetation with no meaningful parental care is the best available inference, not a confirmed account.
In the aquarium
Eigenmannia nigra is essentially unestablished in the hobby, and specific aquarium care data for this species does not appear in the sources consulted. What can be said comes from the family and genus: Sternopygidae generally want warm water, roughly 73–82 °F, soft to moderately mineralised and pH around 6.0–7.0, with no measurable ammonia or nitrite — parameters that echo the soft, warm, tannin-stained blackwater of its native Rio Negro basin. Dim lighting and secure cover suit its electrosensory, nocturnal habits, as with other glass knifefishes.
Being scaleless, it should be assumed sensitive to the same risks as its relatives: no copper-based medications, and reduced doses of many standard treatments. Given its unknown temperament and its adult size approaching half a metre, it should be treated cautiously around small or slow tankmates until proven otherwise, and — as with all poorly known Gymnotiformes — any specimen in the trade is most likely wild-caught, making thoughtful, well-researched husbandry particularly important.
Conservation
Eigenmannia nigra is assessed by the IUCN Red List as Least Concern, with the assessment dated December 2, 2020. This reflects a range that spans several distinct freshwater systems — the Rio Negro and Rio Casiquiare in Venezuela, Colombian Amazonia near Leticia, and the Rupununi River in Guyana — rather than a single isolated population, which under IUCN criteria typically supports a lower-concern rating even for a poorly studied species.
No specific threats to Eigenmannia nigra are documented in the sources available. As with other fishes of the Rio Negro and Guiana Shield drainages, the general pressures facing the wider region — habitat change, mining-related pollution including mercury, and deforestation — are relevant to the basin as a whole, but there is no evidence in hand that they are acting on this species at a level of concern beyond its current Least Concern status.