Taxonomy & naming
Eigenmannia magoi was described in 2020 by Herrera-Collazos, Galindo-Cuervo, Maldonado-Ocampo and Rincón-Sandoval as one of three new species split from the Eigenmannia trilineata species group in northwestern South America. The description used an integrative approach — osteology, the mitochondrial COI barcode gene and geometric morphometrics of body shape — to separate it from its close relatives, a level of scrutiny typical of recent gymnotiform taxonomy, where many glass knifefishes look nearly identical externally and are distinguished only by combining several independent lines of evidence.
The species 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 generates a weak electric organ discharge (EOD) around its body, used for electrolocation in place of vision and for communication with other fish of its kind. Despite the shared common name, Gymnotiformes are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance in body shape is convergent, not a sign of kinship. The genus name honours German-American ichthyologist Carl H. Eigenmann, a pioneer of South American fish taxonomy; the species epithet magoi honours Francisco Mago Leccia (1931–2004), the Venezuelan biologist whose work underpins much of what is known about gymnotiform diversity and biology. The Catalog of Fishes is the authority for the valid name.
Morphology
Eigenmannia magoi 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. As in the rest of the genus the body is semi-transparent in life, giving Eigenmannia its common trade name of "glass knifefish" where it is sold. Known specimens (paratypes) range from about 3–5.5 in in total length, with one diaphanized (cleared-and-stained) reference specimen reaching 6.5 in — modest sizes typical of the genus rather than the larger build seen in some Sternopygidae, such as Sternopygus.
Propulsion comes entirely from a very long anal fin running most of the length of the underside, undulated in travelling waves that let the fish glide forward or backward without bending its body — the same swimming mode shared across the order. Down the flanks runs the electric organ, paired with rows of sensory pores (the electroreceptors) that read the resulting field. The species is scaleless, like all gymnotiforms, which leaves the skin exposed and is a major consideration for water quality and medication in captivity. E. magoi produces a wave-type discharge — a continuous, quasi-sinusoidal signal typical of Eigenmannia, generally in the range of roughly 200–700 Hz reported for the genus — rather than the intermittent pulses of some other knifefish lineages.
Habitat
The species is known from the Catatumbo River basin on the Colombia–Venezuela border, specifically the type locality at the confluence of the quebradas (streams) Agualazal and Agualasal with the Río Zulia, in Norte de Santander department, Colombia. The Catatumbo system drains north into Lake Maracaibo, and the genus Eigenmannia as a whole is distributed far more broadly, from the Río Tuíra in Panama south to the Río de la Plata in Argentina — but E. magoi itself, as currently documented, appears to be restricted to this single trans-Andean basin, making it a narrow-range endemic rather than a widespread species.
Glass knifefishes in general favour slow-flowing streams, river margins, deep channels and floodplain habitat, sheltering among submerged roots and vegetation by day and foraging after dark. As with other gymnotiforms, the electric sense is suited to exactly this kind of dim, structurally complex water, where sight is of limited use but a self-generated electric field lets the fish map its surroundings and locate prey and rivals in the dark or in turbid conditions.
Feeding
No feeding study specific to Eigenmannia magoi has been documented; its diet is inferred from the genus and from Sternopygidae generally. Glass knifefishes are nocturnal micro-predators, foraging mainly after dark for small aquatic invertebrates — insect larvae, worms, and tiny crustaceans — picked out of sediment, leaf litter and root tangles largely by electrolocation rather than sight.
Related Eigenmannia species, notably the widely kept E. virescens, take frozen and live meaty foods readily in the aquarium — bloodworm, brine shrimp, small worms — and E. magoi would be expected to behave similarly if it were ever kept, though this remains an inference rather than an observed fact for this particular species. Data on E. magoi's specific prey and foraging behaviour in the wild is sparse.
Mating
Courtship in Eigenmannia is closely tied to its electric signalling. Wave-type discharges carry information about a fish's species, sex and individual identity, and Eigenmannia is the textbook genus for the jamming avoidance response — when the discharge frequencies of two nearby fish are close enough to interfere, each shifts its own frequency to reduce the overlap, a behaviour extensively studied in neuroscience using the genus as a model system. Sex-specific differences in discharge frequency are documented in Eigenmannia generally, and are presumed to play a role in mate recognition and approach, alongside physical contact and chemical cues, though this has not been studied directly in E. magoi.
No published account describes courtship behaviour in E. magoi specifically; what is known is inferred from the wider genus. As with most gymnotiforms, actual spawning behaviour in the wild is difficult to observe and poorly documented even for far better-known relatives.
Breeding
Eigenmannia magoi has no documented breeding record, in the wild or in captivity — unsurprising for a species described only in 2020 from a single river system and essentially unknown to the aquarium trade. Sternopygidae as a family are egg-layers, typically scattering adhesive eggs among plant roots and floating vegetation during periods of warm, soft, higher water associated with the wet season, with little or no parental care afterwards.
Within the wider genus, Eigenmannia species such as E. virescens are among the more readily bred gymnotiforms in captivity relative to most knifefishes, which are notoriously difficult to spawn at home. Whether E. magoi would follow that pattern is untested; for this species the honest position is that breeding biology is entirely unrecorded, not that a protocol exists and is simply undocumented here.
In the aquarium
Eigenmannia magoi is not established in the aquarium hobby — it is a recently described, narrow-range species known from scientific collections rather than the trade, and this profile should be read as a description of a wild species rather than a stocking guide. Aquarists are far more likely to encounter its close relative, the glass knifefish E. virescens, which is genuinely popular and is often used as a stand-in for the genus's general husbandry needs.
What can be said with confidence, extrapolated from the genus and the order, is this: as a scaleless fish it would need scrupulous water quality and great care with medications, since copper-based and other harsh treatments that ordinary scaled fish tolerate can be dangerous, usually requiring reduced doses. As a nocturnal electroreceptive predator it would want dim lighting, cover such as roots or pipes to retreat into by day, and tankmates too large to be eaten, since bite-sized fish and shrimp are at risk after dark despite the fish's outwardly peaceful demeanour. Any specimen that did reach the hobby should ideally be captive-bred rather than wild-collected, given how little is known about the health of its single known population.
Conservation
Eigenmannia magoi has not been formally assessed by the IUCN Red List; its status is Not Evaluated. This is common for gymnotiforms described in the last few decades, many of which remain data deficient or unassessed simply because no dedicated survey has yet been carried out.
The species is known only from its type locality in the upper Catatumbo basin, a river system that drains into Lake Maracaibo across the Colombia–Venezuela border — a single-basin endemic of exactly the kind that tends to be most vulnerable to habitat change, since it has nowhere else to go if its home stream is degraded. The wider Catatumbo region has a history of deforestation, agricultural expansion and oil-related development pressure, though no study specifically quantifies the impact of these pressures on E. magoi. In the absence of a population assessment, the appropriate conservation stance is caution rather than alarm: the species' true status is simply unknown, and it warrants a proper Red List evaluation given its apparently restricted range.