Taxonomy & naming
Eigenmannia guairaca was described by Peixoto, Dutra & Wosiacki in 2015 in a paper that revised the Eigenmannia trilineata species-group using morphological, karyotypic and molecular evidence, splitting what had been treated as one or two widespread species into a monophyletic cluster of several narrower-ranging ones. It belongs to the family Sternopygidae — the glass and rattail knifefishes — within the order Gymnotiformes, the New-World electric knifefishes. These are bony fish, but unusual ones: Gymnotiformes are defined by an electric organ run down the body and by a single long anal fin that provides essentially all of their propulsion, with no functional dorsal or tail fin.
The species epithet guairaca references Guairacá, a protector figure from Guaraní legend associated with the lands the fish was collected in, in Paraná state, Brazil. Despite sharing the common name "knifefish" with the Asian featherback and clown knifefishes (Chitala, Notopterus), the two groups are not related — the blade-like shape is convergent, not a sign of kinship. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name.
Morphology
Eigenmannia guairaca has the classic glass-knifefish build shared by its genus: a slim, laterally compressed, semi-transparent body tapering to a fine point at the tail, with no dorsal fin and no discrete caudal 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 ease and let it hold station with great precision. FishBase records a maximum total length of about 7 in, with a high anal-fin ray count (151–170) that is part of what distinguishes it and its close relatives within the trilineata group.
As in all Sternopygidae, the skin is scaleless and the body carries an electric organ and an array of electroreceptor pores that pick up the fish's own field. The species is separated from its close relatives mainly by fine anatomical, karyotypic and molecular characters documented in the original description rather than by anything obvious to the eye; field identification of Eigenmannia to species level is difficult, and much aquarium-trade "Eigenmannia" is sold under the genus name alone.
Habitat
The species is known from the upper Rio Paraná basin in Paraná state, Brazil, with the type locality in the Riacho Água do Ó, a tributary of the Rio Paranapanema. FishBase reports it more broadly across upper Paraná tributaries, including the rio Paranaíba, rio Grande, rio Tietê and rio Paranapanema, as well as the Paraná mainstem — a fish of running water rather than still backwaters.
As a wave-type sternopygid, E. guairaca most likely favours flowing (lotic) stretches of river and tributary, a preference the literature ties to the higher dissolved-oxygen demands of continuous, high duty-cycle electric discharge compared with the pulse-type gymnotiforms, which tolerate stiller, more oxygen-poor water. Like other glass knifefishes it is presumably a fish of dim or turbid water among roots, submerged vegetation and cover, where electrolocation rather than eyesight does the navigating.
Feeding
No diet study specific to Eigenmannia guairaca has been published; what follows is inferred from its genus and family. Sternopygid glass knifefishes are nocturnal micro-predators, foraging after dark for small invertebrates — aquatic insect larvae, worms, small crustaceans and other micro-fauna — using electrolocation to detect prey hidden in sediment or among roots and plants where vision would be of little use.
In aquarium accounts of the closely related E. virescens, fish take small live and frozen foods readily — bloodworm, brine shrimp, small worms — and are considered easy to feed once settled, generally leaving larger tankmates alone. There is no reason from the available literature to expect E. guairaca to differ meaningfully from its genus in this respect, though this remains an inference rather than a documented fact for the species.
Mating
As in other wave-type Sternopygidae, courtship in Eigenmannia is understood to run substantially through the electric organ discharge itself. In Eigenmannia the discharge is a continuous, near-sinusoidal signal whose frequency differs by sex and individual, and the genus is the classic model for the jamming avoidance response — the way two fish with similar discharge frequencies shift them apart to keep from interfering with each other's electrolocation. That same frequency-sensitive signalling is thought to carry information used in mate recognition and approach in the dark.
No species-specific courtship description exists for E. guairaca; what is documented comes from the better-studied congener E. virescens, in which breeding is associated with soft, acidic water and a drop in temperature and pH simulating the onset of the rainy season — conditions that likely apply across the genus given how uniform its reproductive biology appears to be, but this is an inference, not a confirmed fact for this species.
Breeding
Eigenmannia guairaca itself has no documented aquarium spawning record; data on the species is sparse outside its original taxonomic description. Its genus, however, is among the more approachable of the gymnotiforms to breed: in E. virescens, adhesive eggs are laid among floating plant roots, with females of around 6 in producing 100–200 eggs per spawning event across successive mornings, induced by soft, acid water (roughly DH 2–4) alongside falling temperature and pH.
Parental care in that congener is minimal to absent — eggs are simply left among the vegetation. Given the shared wave-type electric biology and general similarity within Eigenmannia, a broadly similar mode is plausible for E. guairaca, but this should be read as an extrapolation from the genus rather than a reported observation of this species.
In the aquarium
Eigenmannia guairaca is not established in the aquarium trade under its own name; hobby-level Eigenmannia are sold and kept mostly as "glass knifefish" without species-level identification, and virtually everything known about keeping the genus in captivity comes from E. virescens. On that basis, a soft, moderately mineralised, near-neutral to slightly acidic setup in the range of roughly 73–82 °F, with dim lighting and cover such as roots or plants, suits the genus's electrolocating, nocturnal habits well; no marine salt should be used.
As in all Sternopygidae, the species is scaleless and can be expected to be sensitive to ammonia and nitrate, and to copper-based or otherwise harsh medications, which typically need to be dosed at reduced strength. Eigenmannia are gentle, nocturnal micro-predators rather than a threat to fish too large to eat, and the genus is generally considered one of the easier gymnotiforms to keep, doing well in a small shoal of its own kind. Given how little is published on E. guairaca specifically and its narrow known range, keepers should treat any trade specimen labelled with this name with caution about its actual identity.
Conservation
Eigenmannia guairaca is assessed by the IUCN Red List as Data Deficient. It is known mainly from the upper Rio Paraná basin in Paraná state, Brazil, with records spanning several tributaries as well as the Paraná mainstem, but there has been no population-level study of abundance or trend to support a fuller assessment.
The upper Paraná basin has been heavily modified by damming, agriculture and urbanisation, pressures that generally threaten narrow-range riverine fish more than the assessment status alone conveys. Until targeted survey work is done, the honest summary is that its conservation status is genuinely unresolved rather than reassuring, and its status should not be overstated in either direction.