Taxonomy & naming
Eigenmannia dutrai was formally described in 2021 by Peixoto, Pastana and Ballen, one of a wave of recent Eigenmannia descriptions that has split what hobbyists and even ichthyologists long lumped together as "the" glass knifefish, Eigenmannia virescens, into a larger set of distinct species. 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 unusual ones: nearly every species, E. dutrai included, generates a weak electric organ discharge (EOD) around its body, used for electrolocation in murky or dark water and for communication with other knifefish. In Sternopygidae, including Eigenmannia, that discharge is wave-type — a continuous, near-sinusoidal hum rather than the intermittent pulses produced by some other gymnotiform families.
The species epithet honours a researcher surnamed Dutra, per convention in the original description, though the exact dedicatee is not detailed in the sources at hand. Molecular and karyotypic evidence (a chromosome complement of 2n=38, with an XY sex system) was used alongside morphology to distinguish E. dutrai from its relatives. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name. Despite the shared common name "knifefish", Eigenmannia and its Gymnotiform relatives are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus) — the resemblance is convergent body shape, not kinship.
Morphology
Like other Eigenmannia, E. dutrai is a slender, laterally compressed, semi-transparent fish with a long, tapering body and no dorsal fin or distinct tail fin. Propulsion comes almost entirely from a single elongated anal fin running along most of the underside, thrown into travelling waves that let the fish glide forward or backward with equal control — a hallmark of the whole order. The body carries the electric organ that generates its wave-type discharge, along with the sensory pores that read the resulting field, and, as in all gymnotiforms, the skin is scaleless.
Detailed external measurements for E. dutrai specifically are limited to the type series (holotype and paratypes catalogued under MZUSP), and comprehensive adult size data are not established in the sources reviewed here; the genus overall spans a wide size range depending on species, with the closely related E. virescens reaching roughly 8–19.5 in total length depending on sex. Sexual dimorphism in Eigenmannia is not primarily visual — males and females are best distinguished by discharge frequency and by size and body depth in mature fish, rather than by colour or fin shape.
Habitat
Eigenmannia dutrai was described from the upper rio Paraná basin of Brazil — the rio Grande, rio Tietê and rio Paranapanema drainages — with its known range since extended to the upper rio Paranaíba (including the rio Corumbá) and further into the rio Paraná system. It is, like the rest of its genus, a fish of South American freshwater river systems, occupying flowing and still-water margins, among roots and vegetation, in warm, often turbid or tannin-stained water where vision is of limited use.
That dim, low-visibility environment is exactly where the electric sense earns its keep: Eigenmannia navigates, hunts and recognises other fish primarily through its electric field rather than through eyesight. As a single-basin or few-basin endemic within the wider genus, E. dutrai's habitat needs are inferred from its close relatives and from the basin it was described in; specific water-chemistry data for the species itself are sparse in the sources reviewed here.
Feeding
Specific dietary data for Eigenmannia dutrai were not found in the sources reviewed here, but the genus as a whole are nocturnal micro-predators. Eigenmannia species hunt small invertebrates — insect larvae, small crustaceans and worms — locating them chiefly by electrolocation as they probe over the substrate and among roots after dark, rather than by sight.
In aquarium care of the genus, this translates to meaty frozen and live foods — bloodworm, brine shrimp, krill and similar small prey items — offered in the evening when the fish become active; there is no reason to expect E. dutrai to differ meaningfully from its close relatives in this respect, though the species itself has not been kept or observed in the aquarium to any documented extent.
Mating
Courtship in Eigenmannia is inseparable from its electricity. The genus's continuous wave-type discharge carries a sex-specific frequency, and it is the classic study system for the jamming-avoidance response in neuroscience — the way two nearby fish with similar discharge frequencies shift them apart to avoid interference. That same electrical channel appears to carry social and courtship information: in the best-studied relative, gravid females respond to playback of a male's courtship discharge, and males modulate their EOD ("chirping") as part of approach and courtship.
No species-specific courtship account exists for E. dutrai in the sources reviewed here; the pattern described is drawn from the wider genus, particularly the long-studied E. virescens, and should be read as generic Eigenmannia biology rather than a confirmed observation of this species.
Breeding
Eigenmannia as a genus is among the more readily bred gymnotiforms in captivity relative to most electric knifefishes, though E. dutrai itself, described only in 2021, has no documented aquarium breeding record in the sources at hand. In the wider genus, spawning is egg-laying: adhesive eggs are scattered among aquatic plant leaves and roots, with clutches reported from around 100 up to several hundred eggs in larger females, often across multiple batches (fractional spawning), typically triggered by rainy-season-like conditioning — warmer, softer, higher water.
Parental care is minimal to absent: eggs are left adhering to vegetation with no evidence of guarding once laid. For E. dutrai specifically, data are sparse, and this account follows the documented pattern for the genus rather than the species.
In the aquarium
Eigenmannia dutrai is not an established aquarium fish — it is a recently described, single-basin Brazilian species with no meaningful trade history, and any specimens in circulation would likely be misidentified as the far more common Eigenmannia virescens complex. General Eigenmannia care principles apply if it is ever kept: like all gymnotiforms it is scaleless and correspondingly sensitive to ammonia, nitrate and to copper-based or harsh medications, which call for reduced doses. As a nocturnal electroreceptive predator it wants dim lighting, plenty of cover — roots, plants, pipes — and tankmates too large to be eaten, since it hunts small fish and shrimp by electrolocation after dark.
Glass knifefish in general are considered one of the easier gymnotiforms to keep, doing well in a small shoal of their own kind rather than singly, which is unusual for the order. Until E. dutrai itself has a documented aquarium history, though, any specific care claims beyond these genus-level generalities would be guesswork.
Conservation
Eigenmannia dutrai has not yet been assessed by the IUCN and is treated here as Data Deficient, reflecting how recently it was described (2021) and the limited data available on its population size, trends and threats. Its known range — the upper rio Paraná basin, including the rio Grande, rio Tietê, rio Paranapanema, rio Paranaíba and rio Corumbá — places it in one of Brazil's most heavily dammed and developed river systems, where habitat alteration from hydroelectric development, agriculture and pollution are the plausible pressures on narrow-range fish, though no species-specific threat assessment exists in the sources reviewed here.
As with many recently split, narrow-range Eigenmannia, the conservative position is to flag data deficiency rather than assert either safety or risk. Recent taxonomic work continues to document diversity and distribution within Eigenmannia across the upper Paraná and Paraguay systems, which should eventually support a proper assessment.