Taxonomy & naming
Eigenmannia sayona was described by Peixoto & Waltz in 2017 and placed in the family Sternopygidae, the glass and rattail knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. It belongs to the Eigenmannia trilineata species group, a cluster of similar, hard-to-distinguish glass knifefishes from northern South America that has been the subject of ongoing taxonomic revision as researchers use fine anatomical characters and electric-signal differences to separate what were once treated as a single wide-ranging species.
Gymnotiformes are bony fish, but unusual ones: nearly every species in the order generates a weak electric organ discharge (EOD) around its body, used for electrolocation — sensing prey, obstacles and other fish in the dark — and for communication, since each species and sex carries a characteristic discharge. They swim by rippling a single long anal fin rather than a tail, can move backwards as readily as forwards, and are entirely scaleless. Despite the shared common name "knifefish", they are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to a different order entirely; the resemblance is convergent, not a matter of kinship. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name. The species epithet, sayona, refers to La Sayona, a vengeful spectral woman of Venezuelan legend.
Morphology
Eigenmannia sayona shows the classic glass-knifefish build: a slender, laterally compressed body tapering to a fine, whip-like tail, with an elongated anal fin running most of the length of the underside and no separate tail fin. There is no dorsal fin. The body is semi-transparent in life, a trait shared across the genus that gives glass knifefishes their common name, and the pectoral and anal fins are described as hyaline (glassy, near-colourless). FishBase records 198–217 soft rays in the anal fin and a maximum length of 6.5 in total length, modest for the order.
As in all gymnotiforms, an electric organ runs along the body and the skin is studded with electroreceptor pores that read the field it produces; the fish itself is scaleless, a point that matters directly to how it should be kept. Eigenmannia produces a wave-type discharge — a continuous, near-sinusoidal hum rather than the intermittent pulses of some other knifefish families — and closely related species in the trilineata group are often separated as much by subtle differences in discharge frequency and anatomy as by outward appearance.
Habitat
Eigenmannia sayona is known from the río Orinoco basin of Venezuela. Glass knifefishes as a group favour slow-flowing stretches of river — quiet channels, river margins, floodplains and deep water — typically among submerged roots and aquatic plants that offer cover and structure for their electric sense to work against. Data on this particular species' habitat use beyond its type locality is sparse.
The wider genus lives in warm, often softwater, dimly lit Neotropical rivers where vision is of limited use and electrolocation does the work of finding food and avoiding obstacles after dark. As a benthopelagic fish it moves through the water column rather than staying tight to the bottom, and like other Sternopygidae it is a nocturnal micro-predator, generally peaceful toward tankmates too large to be prey.
Feeding
Specific dietary studies on Eigenmannia sayona are not available, but the genus is well characterised: glass knifefishes are nocturnal micro-predators that hunt small invertebrates — aquatic insect larvae, worms and tiny crustaceans — locating them in the dark largely through electrolocation, sensing the way prey distorts the electric field around the body rather than relying on sight.
In keeping with that ecology, captive Eigenmannia take small meaty foods such as bloodworm, brine shrimp and finely chopped invertebrate matter, generally becoming most active and willing to feed as the lights dim. Data on this species' feeding behaviour specifically is sparse, and the account here follows the well-documented pattern of its close relatives.
Mating
Courtship in Eigenmannia is closely tied to its electric sense. The wave-type discharge is not just a navigation tool but a social signal: males and females in the genus carry sex-specific discharge frequencies, and the well-studied jamming-avoidance response — in which two fish with similar discharge frequencies shift them apart to avoid interference — doubles as a way individuals recognise and respond to one another in close encounters, including during courtship. Discharge frequency in the genus is also temperature-sensitive, tying reproductive signalling to the warm, rainy-season conditions that typically trigger spawning.
Detailed courtship behaviour specific to Eigenmannia sayona has not been documented; the account here follows what is known for the genus, in particular the closely related E. virescens, which is better studied in the laboratory.
Breeding
Sternopygidae are egg-layers, and Eigenmannia is one of the more readily bred genera in the order: spawning is typically triggered by warm, soft, higher-water conditions echoing the wet season, with adhesive eggs scattered among fine plant roots or floating vegetation rather than guarded in a nest. Parental care is minimal to absent — eggs are left among the vegetation once laid.
No species-specific breeding account exists for Eigenmannia sayona; its reproductive biology is inferred from the wider genus, and E. virescens in particular has been bred under warm (around 77 °F), rainy-season-style conditions in research and hobby settings. Aquarists working with this or related glass knifefish species should expect the genus pattern rather than a documented protocol specific to E. sayona.
In the aquarium
Eigenmannia sayona is not established in the aquarium trade, and care information specific to this species is not available; the following follows the well-documented husbandry of its genus. Glass knifefishes are considered among the easier gymnotiforms to keep, doing well in a modest shoal of several individuals in a planted tank with dim lighting, gentle flow and plenty of cover — roots, plants and shaded retreats that suit both their nocturnal habits and their electric sense.
Being scaleless, Eigenmannia is sensitive to poor water quality and to many standard fish medications; copper-based treatments should be avoided and other doses reduced. It needs mature filtration, stable warm water and tankmates too large to be preyed on, since it remains a nocturnal micro-predator even at its modest maximum size. Given how little is documented about this specific, narrow-range species, any aquarist encountering it should treat husbandry as provisional and lean on the wider Eigenmannia literature rather than assume specifics.
Conservation
Eigenmannia sayona is assessed as Least Concern on the IUCN Red List, an assessment dated 28 August 2020. It is treated as endemic to Venezuela, known from the río Orinoco basin.
As with many recently described, narrow-range Sternopygidae, the practical conservation picture depends heavily on the health of its home basin — habitat change, pollution and hydrological alteration are the general pressures facing Orinoco fishes — but no species-specific threat data is available beyond the Least Concern assessment itself. The designation reflects current knowledge rather than certainty about long-term trends for such a recently recognised species.