Taxonomy & naming
Eigenmannia trilineata was described by López and Castello in 1966 and belongs to the family Sternopygidae, the glass and rattail knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Like every gymnotiform it produces a weak electric organ discharge (EOD) used for electrolocation and communication; the Sternopygidae are the wave-type "hummers" of the order, generating a continuous, near-sinusoidal signal rather than the isolated pulses of families such as Gymnotidae. This is a defining trait of the group, not an incidental one, and it underlies almost everything distinctive about how these fish sense their world and interact with each other.
The genus is named for the ichthyologist Carl H. Eigenmann (1863–1927), a major figure in the study of South American freshwater fishes; the species epithet trilineata refers to the three lines visible on the transparent body, most notably the dark stripe that runs along the upper flank. Eigenmannia trilineata turns out to be the type species of a much larger species complex than once thought: a 2015 revision recognised 15 valid Eigenmannia species where earlier literature had lumped a number of cryptic, look-alike forms under this one name. As with the rest of the order, these New-World knifefishes are not related to the superficially similar Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance is convergent, not a sign of kinship. The Catalog of Fishes is the authority for the valid name.
Morphology
Eigenmannia trilineata has the classic glass-knifefish build: a long, laterally compressed, ribbon-like body that tapers smoothly to a fine point at the tail, with no dorsal fin and no distinct caudal fin. The body is semi-transparent, giving the fish its common name, and marked with the three lines that gave rise to its species name — most conspicuously a dark stripe running along the upper midline of the flank. FishBase lists a maximum length of about 10 in total length, a mid-sized species within its family, smaller than the larger rattail knifefishes of the related genus Sternopygus.
Propulsion comes almost entirely from a single elongated anal fin running most of the length of the underside, undulated in travelling waves that allow the fish to move forward, hover, or reverse with equal control — useful for backing out of the roots and crevices it hides among. Running beneath the skin is the electric organ that generates its wave-type discharge, monophasic and continuous, together with the electroreceptor pores that read the resulting field. The species is scaleless, a trait shared across the order that leaves the skin unusually exposed and has real consequences for how it must be kept.
Habitat
This species is native to the lower Paraguay–Paraná River basin in South America, a freshwater, benthopelagic fish of the flowing channels and margins of that system rather than of still water. Like other Sternopygidae it favours turbid or dimly-lit stretches of river, often around submerged roots, aquatic vegetation and leaf litter, where an electric sense is far more useful than eyesight for finding food and avoiding obstacles after dark.
Warm, soft, near-neutral to slightly acidic water is typical of its range, consistent with the wider pattern seen across Neotropical gymnotiforms. The species has not been reported as an especially narrow-range endemic within the lower Paraná–Paraguay system, and it is described as tolerant of a variety of habitats within that basin, which — together with its wide occurrence there — underlies its favourable conservation status.
Feeding
As with the rest of the order, Eigenmannia trilineata is a nocturnal micro-predator, most active after dark when it hunts small invertebrates in and around cover using its electric sense to locate prey that eyesight alone would miss in dim, tannin-stained water. The cache consulted for this profile does not detail its diet item-by-item, but the general pattern across Sternopygidae — small aquatic insect larvae, worms, crustaceans and other benthic invertebrates picked from the substrate and among roots — is a reasonable expectation for this species too.
In the aquarium, glass knifefish are generally reported to take standard small meaty foods — bloodworm, brine shrimp, chopped worms and similar frozen or live fare — though feeding behaviour specific to E. trilineata is not covered in the sources used here. Given its nocturnal habits, feeding in low light or after the tank lights dim is likely to see the most activity.
Mating
Courtship and signalling in this species are built around its electric organ discharge. As a wave-type gymnotiform, Eigenmannia produces a continuous, near-sinusoidal discharge at a steady individual frequency, and this genus is one of the classic subjects for the study of the jamming avoidance response — the reflex by which two nearby Eigenmannia shift their discharge frequencies apart to stop their signals from interfering with each other. Discharge frequency in Eigenmannia differs between the sexes, and that difference is thought to play a role in how individuals recognise and approach one another in the dark.
A 2010s reproductive-biology study of E. trilineata found it to be a batch spawner with an extended, parcelled spawning strategy rather than a single annual event, laying down eggs progressively as the breeding season allows; relative fecundity was measured at roughly 0.27 oocytes per milligram of body weight, and first sexual maturity was reached at about 3 in total length in females and 2.5 in in males. Spawning itself takes place among the roots of aquatic plants, and is associated with conditions mimicking the rainy season — warmer, softer, higher water — a trigger shared broadly across the family.
Breeding
Eigenmannia trilineata is an egg-layer, and unusually for a gymnotiform it is reported to breed in captivity more readily than most of its relatives — most knifefish species reaching the aquarium trade are wild-caught or farm-produced rather than home-bred, but glass knifefish are a partial exception. Spawning takes the form of adhesive eggs scattered among the roots of live or floating plants, with relative fecundity around 0.27 oocytes per milligram of body weight and extended, batch-style spawning rather than one large clutch, per the SciELO reproductive-biology study of this species.
Parental care afterwards is minimal to absent: eggs are left among the vegetation with no evidence of guarding once spawning ends. For the aquarist wanting to try, the practical cues to replicate are the ones the wild population responds to — a rise in temperature, softening and slight acidification of the water, and dense fine-leaved or root cover to receive the eggs, echoing a wet-season pulse. Detailed rearing protocols for this species specifically are not documented in the sources used here, so results should be expected to vary.
In the aquarium
Being scaleless, Eigenmannia trilineata needs the same caution around water quality and medication as other gymnotiforms: it is sensitive to ammonia, nitrite and accumulated nitrate, and to copper-based treatments, which should be avoided or used at reduced dose. Aquarium sources recommend a temperature around 72–82 °F, soft to moderately hard water (roughly 5–12 dGH) and a pH of about 6.0–7.0 — soft, acidic to near-neutral — with pristine water quality (nitrate kept under about 20 ppm) and no marine salt.
It is one of the more manageable knifefish for a home aquarium: peaceful toward fish too large to be seen as prey, and reportedly does well kept in a small shoal of its own kind rather than as a lone specimen, an option not available with many larger, more solitary gymnotiforms. As a nocturnal predator it should still be given dim lighting, secure hiding places such as roots, pipes or dense planting, and tankmates that are not small enough to be hunted after dark. Given its captive-breeding track record relative to other electric knifefish, captive-bred stock is worth seeking out where available.
Conservation
Eigenmannia trilineata is assessed as Least Concern on the IUCN Red List, an assessment consistent with its fairly wide occurrence across the lower Paraguay–Paraná basin and general habitat tolerance within that system. It is not currently considered a species of conservation concern.
As with other Neotropical freshwater fishes, the longer-term pressures on the basin it inhabits — damming, pollution and habitat modification along the Paraná–Paraguay system — are worth noting even where the species itself is not flagged as threatened; Least Concern describes the species' status across its range today, not a guarantee against future habitat change.