Taxonomy & naming
Sternopygus aequilabiatus was described by Alexander von Humboldt in 1805, one of the earliest named gymnotiforms, and it belongs to the family Sternopygidae — the glass and rattail knifefishes — within the order Gymnotiformes, the New-World electric knifefishes. The genus Sternopygus contains some of the largest-bodied members of the order, in contrast to smaller relatives such as the glass knifefish, Eigenmannia.
Gymnotiformes are bony fish, but unusual ones: they are defined by an electric organ and a long ribbon-like anal fin used for locomotion rather than a normal caudal fin, and despite the common name "knifefish" they are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to a different order entirely and resemble gymnotiforms only by convergence. Within the Sternopygidae, Sternopygus is placed alongside genera such as Eigenmannia, Rhabdolichops and Distocyclus, all of which produce a continuous wave-type electric organ discharge rather than the pulsed discharge seen in families like Gymnotidae. The valid name is that recognised by the Catalog of Fishes.
Morphology
The body is elongated and strongly compressed side to side, deepest near the head and narrowing gradually along its length into a long, thin, whip-like tail — the "rattail" shape typical of the genus. There is no dorsal fin and no functional tail fin; the long anal fin running beneath the body provides essentially all propulsion, thrown into waves that let the fish glide forward, hover, or reverse with equal control. The skin is scaleless, a trait shared across the order and one that matters directly to how the fish must be kept and medicated.
Aquarium and trade sources give a maximum size in the range of roughly 15.5–19.5 in, with reports of exceptional individuals reaching toward 35.5 in, making S. aequilabiatus one of the larger sternopygids kept by hobbyists. Running the length of the body is the electric organ and the sensory pores that read the field it produces; a dedicated study of the species' electroreceptor distribution confirms the well-developed electrosensory system typical of the family. External sexual dimorphism is not well documented for this species; as in related gymnotiforms, body depth and overall size are the more commonly cited (though unreliable) cues to sex.
Habitat
Sternopygus aequilabiatus is a trans-Andean South American species, recorded from river basins in Colombia including the Atrato, Cauca, Magdalena, Ranchería, San Jorge and Tuira systems — drainages that run to the Caribbean and Pacific rather than into the Amazon or Orinoco, distinguishing this species' range from many better-known gymnotiforms. It inhabits flowing river channels and associated habitats within these basins.
Like other gymnotiforms it is most active at night, and its electric sense is built for exactly the conditions of a dim, structurally complex river environment — slow to moderate flow, submerged roots, leaf litter and turbid or tannin-stained water where vision is of limited use. Sources describing its aquarium care recommend a large volume of water (on the order of 78.5–157.5 in of tank length) with hiding places and subdued lighting to approximate this environment, and note the fish spends much of the day sheltering out of sight.
Feeding
As in other gymnotiforms, feeding in Sternopygus aequilabiatus is a nocturnal, predatory activity guided substantially by electrolocation: the fish reads the electric-field disturbances made by prey and by structure in its path, allowing it to hunt effectively after dark. Aquarium sources describe it taking live and frozen foods, in keeping with a carnivorous, meaty diet typical of the genus — worms, insect larvae, crustaceans and similar prey items in the wild, and bloodworm, chopped earthworm and comparable frozen foods in captivity.
It is generally described as tending toward a solitary and somewhat aggressive disposition, feeding best when undisturbed and offered food after the lights go down. Data on its precise natural diet are sparse; the account here reflects the broader feeding pattern documented for large sternopygid knifefishes rather than species-specific dietary studies.
Mating
Electric-organ discharge plays a central role in gymnotiform courtship, and the wave-type signal produced continuously by Sternopygus and its sternopygid relatives is understood to carry information on species identity, sex and reproductive state, in addition to its role in electrolocation. The glass knifefish, Eigenmannia, in the same family, is the classic laboratory subject for this kind of electric signalling and for the jamming avoidance response — the reflex by which two individuals with similar discharge frequencies shift them apart to avoid interference — and related signalling dynamics are presumed to extend across the family, including Sternopygus, though it has not been documented for S. aequilabiatus specifically.
Beyond the electrical component, courtship and pairing behaviour in this species is not well documented in the sources available. Sternopygids as a group are egg-layers without the elaborate physical courtship rituals seen in some other fish groups, and approach and signalling in the dark, rather than visual display, appear to be the primary means of communication between prospective mates.
Breeding
Sternopygidae as a family are egg-layers, typically scattering adhesive eggs among plant roots or floating vegetation, often triggered by the warmer, softer, higher water conditions of the wet season in their native rivers. Eigenmannia is bred in captivity somewhat more readily than most gymnotiforms, but breeding reports specific to Sternopygus aequilabiatus are not documented in the sources available for this account, and captive reproduction of this species should be considered rare to undocumented.
Parental care in the family is minimal to absent: eggs are left among vegetation with no meaningful guarding or brooding behaviour. Given the sparse data, aquarists should not expect this species to breed reliably in a home tank; large, well-established, undisturbed fish in warm, soft water are the closest approximation of favourable conditions suggested by the broader family pattern.
In the aquarium
Sternopygus aequilabiatus is a specialist's fish, not a community staple, and its size drives most of the husbandry requirements: aquarium sources recommend a large tank, on the order of 78.5–157.5 in in length, to accommodate an adult that can reach 15.5–19.5 in and occasionally larger. It needs secure hiding places — pipes, caves, dense root structures — to retreat into during the day, and subdued lighting that lets it become active and confident after dark, in keeping with its nocturnal, electrolocation-driven behaviour.
Being scaleless, it shares the order-wide sensitivity to water quality and to medication: ammonia, nitrite and accumulated nitrate should be kept low through mature filtration and regular water changes, and copper-based treatments should be avoided, with other medications used at reduced doses. Water in the range of roughly 72–77 °F and a pH of 6.0–7.0 is indicated by trade sources. Reports describe the species as tending solitary and somewhat aggressive, so tankmates should be chosen carefully — large, robust, non-aggressive fish that will not be seen as prey and will not nip at the fish's long, vulnerable anal fin. Given the sparse husbandry literature for this species specifically, prospective keepers should treat much of its care as inferred from the wider Sternopygus and sternopygid pattern rather than from dedicated studies.
Conservation
Sternopygus aequilabiatus is assessed by the IUCN as Least Concern, and no significant conservation threats are noted in the sources reviewed for this account. It is recorded across multiple river basins in Colombia, which affords it a broader range than some of the narrow-range, single-river gymnotiform endemics found elsewhere in South America.
As with other Neotropical freshwater fish, ongoing pressures on its native rivers — habitat modification, pollution and land-use change within the Atrato, Cauca, Magdalena and neighbouring basins — bear watching even where the species itself is not currently considered at risk. No trade-driven collection pressure specific to this species is documented in the available sources; it appears only occasionally in the aquarium trade rather than being heavily collected.