Taxonomy & naming
Sternarchorhynchus villasboasi was described by de Santana & Vari in 2010, one of a wave of new Sternarchorhynchus species recognised as the genus was revised and its many superficially similar, tube-snouted members were teased apart. It belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes, the New-World electric knifefishes — a group of bony fish defined not by scales or fin count but by an electric organ and a long ribbon-like anal fin used for locomotion. The Apteronotidae are distinguished within the order by an electric organ built from modified nerve-cell axons rather than muscle tissue, which lets them generate an unusually fast, continuous, wave-type discharge.
The genus Sternarchorhynchus is recognisable by its long, tubular, decurved snout, an adaptation for probing crevices and sediment that sets it apart from the blunter-snouted ghost knifefishes such as Apteronotus. Despite the shared common name, Sternarchorhynchus and the rest of the 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 Catalog of Fishes is the authority for the valid name.
Morphology
Sternarchorhynchus villasboasi is a small, laterally compressed ghost knifefish reaching around 5.5 in in total length, with females running marginally smaller than males. Its most obvious feature is the long, narrow, downward-curving snout typical of the genus, an elongation of the head used to probe into rock crevices and coarse sediment for hidden prey. As in all Apteronotidae, there is no dorsal fin and no distinct tail fin; the body tapers to a slender caudal region, and swimming is carried out almost entirely by undulations of a long anal fin running along the underside, which lets the fish move forward and backward with equal control.
The skin is scaleless, and an electric organ runs along the body beneath it, paired with sensory pores that read the resulting field. This is a wave-type electric organ discharge generated by modified nerve cells, among the fastest and most energetically demanding of any fish's, at a reported frequency in the roughly 800–1200 Hz range typical of the family — a trait that also explains why these fish need well-oxygenated water. Data are sparse on external sexual differences beyond the modest size gap between the sexes.
Habitat
This species is a single-river endemic, known only from the Xingu River basin in Brazil, and specifically associated with the Volta Grande — the river's great rapids and waterfall stretch — alongside its close relative Sternarchorhynchus kokraimoro. It is a rheophilic specialist, meaning it is adapted to fast, well-oxygenated, turbulent flow rather than the still backwaters and floodplain lakes many other Amazonian fish favour, and it typically holds position among rocks and in the swift channels of the rapids themselves.
Water here is warm and, like most Apteronotidae habitat, likely soft and circumneutral, broadly in the range of 73–82 °F and pH 6.0–8.0 reported for the family. The rapids are turbid and structurally complex, exactly the kind of low-visibility environment where an electric sense outperforms eyesight — foraging by electrolocation lets the fish find prey wedged into crevices that sight alone could not resolve. The species' entire known range is compact enough that this one stretch of river effectively defines its habitat.
Feeding
Detailed dietary data for Sternarchorhynchus villasboasi are sparse, but the genus's feeding ecology is well characterised: these are benthic foragers that use the long, curved snout to probe crevices, coarse substrate and root tangles for aquatic insect larvae and other small invertebrates, detecting prey largely by electrolocation rather than sight in the dark, fast-moving water they inhabit. FishBase places the species at a trophic level of roughly 3.2, consistent with a diet of small invertebrate prey rather than fish.
As a rheophilic rapids specialist, it likely feeds while holding station in or near strong current, picking prey from rock surfaces and crevices rather than actively cruising open water — a foraging style shaped by the same fast-flow habitat that makes it a narrow-range endemic. No specific published account of stomach contents or feeding behaviour for this species was found in the sources consulted here.
Mating
As in other Apteronotidae, the wave-type electric organ discharge in Sternarchorhynchus villasboasi almost certainly carries information about identity, sex and reproductive state, and courtship in the family is understood to involve an exchange of electric signals — shifts in discharge frequency between prospective mates, and distinctive interruptions between rivals — alongside physical approach in the dark. This electric courtship is the best-documented reproductive behaviour across the family, though it has been studied in far more detail in commonly kept relatives than in this narrow-range wild species.
Specific courtship behaviour for S. villasboasi itself has not been documented in the sources reviewed here; data are sparse, and what can be said follows from the broader biology of wave-type ghost knifefishes rather than direct observation of this species.
Breeding
No documented captive breeding exists for Sternarchorhynchus villasboasi. It is an egg-layer, as are all Gymnotiformes, but reproduction in the wild — spawning sites, timing, and any parental involvement — is essentially unrecorded for this species in the available literature. Where Apteronotidae have reached the aquarium trade at all, most stock has been wild-caught or, on commercial farms, produced under hormonal induction rather than natural spawning in captivity; there is no indication this species has ever been bred outside the wild.
Given its restricted range in a rapids habitat now altered by dam development, breeding biology is best treated as data sparse and poorly documented rather than guessed at. Set against the well-studied electric courtship signalling of the family in general, the actual spawning behaviour of this species remains an open question.
In the aquarium
Sternarchorhynchus villasboasi is not an aquarium fish in any practical sense — it is a deep-channel, rheophilic Amazonian species with no established presence in the trade, and the sources reviewed here record no aquarium husbandry information for it at all. Its needs can only be inferred from its family and habitat: like other ghost knifefishes it is scaleless and would need careful, reduced-dose medication (no copper-based treatments) and very clean water, and as a fast-water rapids specialist it would additionally require strong flow and high dissolved oxygen that most home aquaria are not built to sustain.
As with other Apteronotidae, any hypothetical care would call for a large, dimly lit tank with abundant cover, since these are nocturnal, electrolocating predators unsuited to bite-size or nippy tankmates. In practice, this is a species to know about rather than to keep: its value is as a marker of a unique and threatened rapids ecosystem, not as a candidate for the hobby.
Conservation
The IUCN Red List assesses Sternarchorhynchus villasboasi as Vulnerable (VU) under criterion D2, reflecting a very small, restricted range confined to the Volta Grande section of the Xingu River. The principal threat is hydroelectric development — the Belo Monte dam complex has fragmented and altered flow through the Volta Grande, converting stretches of the rapids the species depends on into reservoir-like conditions and reducing the fast, oxygen-rich flow that this and other rheophilic Xingu endemics require.
Because the species is a rapids specialist restricted to a single, heavily engineered river reach, it is considered highly vulnerable to further habitat degradation even though it faces no direct fishing or trade pressure. It shares this exposure with other Xingu endemics, including its congener Sternarchorhynchus kokraimoro, making the Volta Grande a conservation priority for freshwater biodiversity in the Amazon basin.