Taxonomy & naming
Eigenmannia limbata was described by Schreiner & Miranda Ribeiro in 1903 and belongs to the family Sternopygidae, the glass and rattail knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. It sits in the subfamily Eigenmanniinae alongside its many congeners, a genus that has undergone repeated taxonomic revision as new species have been split out from what were once broader species concepts. The genus name honours the ichthyologist Carl H. Eigenmann, a major early student of South American freshwater fishes; the specific epithet limbata is Latin for "edged" or "bordered", referring to a pale border on the anal fin.
As with all Gymnotiformes, the common name "knifefish" invites a false comparison: the Asian featherback and clown knifefishes (Chitala, Notopterus) are bonytongues in an unrelated order, and the resemblance to Eigenmannia is convergent, not a sign of kinship. Mitochondrial genome sequencing of specimens from the Suriname River (10,921 bp, 13 protein-coding genes) confirms the species' placement within Sternopygidae and the broader Gymnotiformes. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name.
Morphology
Eigenmannia limbata has the classic glass-knifefish build: a laterally compressed, elongate body tapering to a fine, whip-like tail, with skin translucent enough in life to show a hint of the internal anatomy — hence "glass knifefish" for the genus as a whole. There is no dorsal fin and no true tail fin; locomotion comes from a very long anal fin running most of the length of the underside, thrown into travelling waves that drive the fish forwards or backwards with equal facility, letting it back out of tight cover as easily as it swims into it. The species name refers to a pale border on this anal fin, a useful field mark against the otherwise plain, silvery-olive body.
Running along the body is the electric organ, paired with an array of skin sensory pores (electroreceptors) that read the field it produces. Eigenmannia produces a wave-type electric organ discharge — a continuous, near-sinusoidal hum rather than the discrete pulses of some other gymnotiform lineages — and it is this constant signal that both Eigenmannia and its relative Sternopygus have made classic subjects for research into electrolocation and jamming avoidance. Like all knifefishes in the order it is scaleless. Reported maximum size is around 19 in total length, though most individuals seen in the wild and the trade are considerably smaller.
Habitat
Eigenmannia limbata is a South American species recorded from the Guianas and the Amazon River basin, part of a genus whose various species collectively range from the Río Magdalena in Colombia south to the La Plata basin in Argentina. It favours flowing freshwater habitats — undercut banks, tangles of old submerged wood, small creeks, river channels, floodplains and lagoons — rather than open, unstructured water, and it is typically found sheltering among cover during the day.
As with other gymnotiforms, darkness and structure suit it: these are dim, often turbid or shaded waters where an electric sense has a real advantage over vision, and Eigenmannia relies on its wave-type discharge to navigate and to detect other fish and prey in exactly these conditions. It is a benthopelagic, tropical freshwater fish, active over and among the same submerged wood and root tangles that offer it daytime refuge.
Feeding
Glass knifefish are nocturnal micro-predators, foraging chiefly after dark over sediment, wood and root tangles for small invertebrates — aquatic insect larvae, worms and other tiny benthic prey — much of it located by electrolocation rather than sight in the dim water they inhabit. Like other Gymnotiformes they are gentle towards fish too large to be considered prey, hunting only what fits their mouth.
In the aquarium Eigenmannia limbata takes small meaty foods readily — bloodworm, brine shrimp, chopped worms and suitable sinking prepared foods — and, being one of the more manageable gymnotiforms in a group otherwise dominated by shy or demanding species, tends to settle and feed well in a shoal given adequate cover and subdued lighting.
Mating
Courtship and social signalling in Eigenmannia are built almost entirely around its electric organ discharge. The wave-type EOD carries a stable, individually and sexually distinctive frequency, and Eigenmannia is the textbook genus for the jamming avoidance response: when two fish with similar discharge frequencies come into range and their signals begin to interfere, each shifts its own frequency apart from the other's rather than let the two collide — a mechanism first characterised in this genus and still used as a model system in sensory neuroscience. Sex-specific differences in discharge frequency are documented in the genus, and electric signalling is thought to carry information about identity, sex and reproductive state during approach and courtship in the dark.
Beyond that electrical courtship, direct field observations of pair formation and spawning behaviour in Eigenmannia limbata specifically are sparse; much of what is confidently known comes from electrophysiology and laboratory study rather than from watching wild fish spawn.
Breeding
Sternopygidae are egg-layers, and within the family Eigenmannia is reported to be bred in captivity somewhat more readily than most other gymnotiforms, typically triggered by warm, soft, higher-water conditions that mimic a wet-season rise. Eggs are adhesive and are scattered or deposited among fine cover — plant roots, floating vegetation and similar structure — rather than guarded in a nest.
Parental care is minimal to absent: once eggs are laid among the vegetation, the adults provide little or no further protection. Species-specific breeding accounts for Eigenmannia limbata are limited in the available literature, so aquarists attempting to spawn it should expect to work from the genus's general pattern — soft, warm, well-planted or root-tangled water and a stable, mature setup — rather than a fixed protocol, and should treat any success as noteworthy given how poorly documented breeding remains across most of the order.
In the aquarium
Eigenmannia limbata is regarded as one of the easier gymnotiforms to keep, and unlike many of its relatives it does well as a small shoal rather than as a solitary specimen, with individuals appearing to tolerate — and possibly benefit from — the company of their own kind. As with every fish in this order it is scaleless, so extra care is needed with medications: avoid copper-based treatments and reduce dosing generally, and keep a close eye on ammonia and nitrate, to which scaleless fish are more sensitive than scaled ones.
Being a nocturnal electroreceptive hunter, it wants dim lighting and plenty of cover — driftwood, root tangles, dense planting or floating plants that cut overhead light — to feel secure during the day and to recreate the shaded, structured water it comes from. It is not a fish for a brightly lit, bare community tank, and while it is peaceful towards similarly sized tankmates, very small fish or shrimp sharing the tank at night should be assumed to be at risk. Warm, soft, gently moving water in a well-established, mature aquarium suits it best; captive-bred stock, where available, is preferable to wild-caught.
Conservation
Eigenmannia limbata is assessed by the IUCN as Least Concern, consistent with its relatively broad South American distribution across the Guianas and Amazon basin and the general pattern among glass and rattail knifefishes, many of which are assessed as Least Concern or remain Data Deficient owing to limited survey effort across the group.
As with other wide-ranging Amazonian and Guianan fishes, that status reflects range rather than an absence of local pressure — habitat degradation, deforestation and water-quality changes affect the smaller streams and floodplain habitats this species and its relatives depend on, even where the species as a whole is not currently considered threatened.