Taxonomy & naming
Sternopygus xingu was described by Albert and Fink in 1996, placed in the family Sternopygidae — the glass and rattail knifefishes — within the order Gymnotiformes, the New-World electric knifefishes. The type locality lies in the Rio Xingu basin of Mato Grosso, Brazil, which gives the species its epithet. Sternopygidae is one of several gymnotiform families defined by a wave-type electric organ discharge, a continuous, hum-like signal quite different from the pulse-type discharge used by families such as Gymnotidae.
The genus Sternopygus — the "rattail" knifefishes — contains some of the largest-bodied members of the order, in contrast to the more slender, often semi-transparent glass knifefishes of the related genus Eigenmannia. Despite the shared common name "knifefish", Gymnotiformes are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to an entirely different order and resemble gymnotiforms only by convergent body shape. The Catalog of Fishes is the authority for the valid name.
Morphology
Sternopygus xingu has the classic rattail build: a body that is deepest just behind the head and tapers gradually and continuously into a long, thin, whip-like tail, without the abrupt narrowing seen in some other knifefish genera. There is no dorsal fin and no separate tail fin; the fish is propelled by a very long anal fin that runs along most of the ventral length of the body, thrown into travelling waves that let it move forward or backward with equal ease. FishBase records a maximum total length of about 20.5 in, making it one of the larger species in its family.
As in all gymnotiforms, the skin is scaleless and carries the sensory pores of the electrosensory system, alongside the electric organ that runs the length of the body and produces the fish's wave-type discharge. Detailed colour pattern and sexual dimorphism data for this species are data sparse; congeners in the genus show little obvious external difference between the sexes outside of body depth and the breeding condition of the electric organ.
Habitat
The species is endemic to the Xingu and lower Tocantins river basins in Brazil, both major clearwater-to-blackwater tributary systems of the Amazon in the eastern part of the basin. Within its family, Sternopygus species typically inhabit slow-moving to still tropical waters — river margins, backwaters and floodplain habitat as well as deeper channels — rather than fast, well-oxygenated rapids.
As an electric fish it is adapted to life where vision is of limited use: dim, often turbid or tannin-stained water in which prey, obstacles and other knifefish are read through the electric field it generates rather than by sight. Specific water chemistry data for S. xingu itself is not documented in available sources; the genus as a whole occupies warm, tropical South American freshwater in the roughly 22–28 degrees C range.
Feeding
Sternopygus xingu is, like its relatives, a nocturnal micro-predator. FishBase places the genus at a trophic level around 3.2, consistent with a carnivorous diet of small invertebrates — aquatic insect larvae, worms and small crustaceans — taken from the substrate and among cover after dark. Prey is located primarily through electrolocation rather than sight, the electric field revealing the presence of animals hidden in sediment or vegetation that the fish's eyes could not detect in the gloom.
No aquarium feeding data specific to this species is available; congeneric Sternopygus in captivity take a similar range of meaty foods — bloodworm, blackworm, chopped earthworm and other frozen or live invertebrate foods — offered after dark when the fish are active.
Mating
Courtship in Sternopygus is closely tied to the wave-type electric organ discharge. In the well-studied congener S. macrurus, males hold territory and use changes in discharge rate, along with brief interruptions of the signal, to advertise for and approach mates in the dark; the discharge itself carries information about sex, size and reproductive state. There is no reason to expect S. xingu differs greatly from this pattern, though it has not been documented directly for the species.
Spawning in the genus is associated with the onset of the rainy season, when rising, softer water appears to be a cue. Courtship plays out largely as an exchange of electrical signals between fish that may never see each other clearly, rather than through visual display.
Breeding
Reproduction has not been documented for Sternopygus xingu specifically; data is sparse. In the congener S. macrurus, spawning occurs around the rainy season, with adhesive eggs scattered among plant roots and floating vegetation — reported fecundity of several thousand eggs, hatching after about four days, larvae beginning to feed around day eleven, and sexual maturity reached at roughly a year old. There is no meaningful parental care after spawning; eggs are left among vegetation to develop unguarded.
Sternopygus xingu is not known to be bred in the aquarium hobby, and no captive-breeding protocol has been published for the species. Given the pattern in its genus, any spawning would likely require large, mature fish, warm soft water and the seasonal cues of the wet season rather than a simple aquarium trigger.
In the aquarium
Sternopygus xingu is essentially unknown in the hobby and, where offered at all, would come with the same caveats that apply to the whole rattail group: it is a scaleless fish, sensitive to ammonia, nitrite and nitrate, and to copper-based and other harsh medications, which should be dosed at reduced strength or avoided. It is a nocturnal predator that wants dim lighting and secure cover — pipes, roots or dense planting — to retreat into by day, becoming active only after dark.
Growing past half a metre in total length, it needs a large, long aquarium with plenty of open swimming space, warm and gently moving water, and tankmates too large to be eaten; small fish and shrimp are at risk after dark regardless of how placid the knifefish appears by day. As with other little-kept knifefishes, any specimen in the trade should be treated as wild-caught, and given the space and stable water quality that a long-lived, large-bodied electric fish demands.
Conservation
Sternopygus xingu is assessed as Least Concern on the IUCN Red List, an assessment dated December 3, 2020. It is a narrow-range endemic, restricted to the Xingu and lower Tocantins basins in Brazil, which makes its Least Concern status less a sign of security across a huge range and more a reflection of a still relatively intact habitat and no known major population decline at the time of assessment.
The Xingu basin in particular has been the focus of large-scale hydroelectric development, and single-river endemics in the region are the species most exposed to habitat alteration from damming, altered flow regimes and downstream pollution. No specific threats to S. xingu are documented in available sources, but the general pressures on Xingu-basin fishes — dam construction, deforestation and mining-related pollution — are relevant context for a species found nowhere else.