Taxonomy & naming
Sternarchorhynchus caboclo was described by de Santana and Nogueira in 2006, one of four new species erected in their revision of the genus Sternarchorhynchus, which placed the group's phylogenetics and species boundaries on a firmer footing within the family Apteronotidae. The genus name combines Sternarchus — an older genus name for related ghost knifefishes, built from Greek roots referring to the chest and to rule or primacy — with rhynchos, Greek for snout, describing the long tubular nose that defines the genus. The species epithet caboclo honours the caboclos, the mixed-heritage river people of northern Brazil whose traditional knowledge and presence in the region the describing authors chose to recognise.
The species sits in the order Gymnotiformes, the New-World electric knifefishes — bony fish, but very unusual ones, defined by an electric organ and a long ribbon-like anal fin used for propulsion in place of a normal tail. Within that order, Apteronotidae — the ghost knifefishes — are set apart by an electric organ derived from modified nerve cells rather than muscle, giving them the fastest, most refined wave-type discharges of any knifefish. The distinguishing anatomical characters cited in the original description include a hyoid arch with four branchiostegal rays and a posttemporal bone not fused to the supracleithrum, details that separate S. caboclo from its close congeners. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name. These New-World knifefish are not related to the Asian featherback or clown knifefishes (Chitala, Notopterus), despite the shared common name — the resemblance is convergent, not a sign of kinship.
Morphology
Sternarchorhynchus caboclo reaches about 9 in in total length. Like its relatives it has the classic ghost-knifefish silhouette: a laterally compressed, blade-like body tapering to a whip-thin tail, no dorsal fin, and a long anal fin running most of the length of the underside that provides essentially all of its propulsion, letting the fish move forwards or backwards with equal ease. The genus is immediately recognisable by its elongated, tubular snout, an adaptation that concentrates electroreceptors near the tip for close-range probing of crevices and sediment.
Running beneath the skin along the body is the electric organ, and scattered over the skin surface are the tuberous and ampullary receptor pores that read the field it produces — in Apteronotidae this system is unusually fast and precise, built from modified nerve cells rather than muscle tissue. As in other gymnotiforms the fish is scaleless, a detail that matters for husbandry wherever the species is kept. Published discharge-rate studies across the genus Sternarchorhynchus report wave-type electric organ discharges in the roughly 944–1992 Hz range, among the highest frequencies recorded in the order.
Habitat
Sternarchorhynchus caboclo is known from the Rio Mucajaí in the Brazilian Amazon, with its type locality both above and below a waterfall on that river. The habitat described at the type locality is a river of steep banks running through dense forest, with beds of aquatic Podostemonaceae — the river-weed family that roots to rock in fast, well-aerated flow — and acidic, well-oxygenated water (pH around 5.8, oxygen saturation around 91%).
That combination of swift current, acidic blackwater chemistry and structurally complex, plant-and-rock substrate is typical of the deep-channel and rapids habitats favoured by Sternarchorhynchus generally, a genus of narrow-range Amazonian endemics rather than wide-ranging generalists. As in other apteronotids, the electric sense is put to full use here: in the low light and turbulence of these forested river reaches, electrolocation does the work that vision cannot.
Feeding
Specific dietary data for Sternarchorhynchus caboclo is data sparse, but FishBase places it at a trophic level of about 3.3, consistent with a carnivorous diet of small invertebrates rather than fish. Across the genus and the wider ghost-knifefish group, the pattern is a nocturnal, probing predator that works sediment, root tangles and crevices after dark for aquatic insect larvae, worms and small crustaceans, locating them largely by electrolocation rather than sight — a long, tubular snout of the kind this species carries is well suited to that close-range probing work.
Hobby accounts of the closely related Sternarchorhynchus goeldii, kept occasionally by specialist aquarists, describe a diet of frozen and live invertebrate foods — tubifex and bloodworm among them — taken at night, and it is reasonable to expect S. caboclo to behave similarly if kept, though no husbandry account for this particular species has been documented.
Mating
As in other Apteronotidae, reproduction in Sternarchorhynchus is bound up with the wave-type electric organ discharge, which carries information on identity, sex and readiness well beyond its role in electrolocation. Studies across the family show males and females shifting discharge frequency during encounters, an electrical signalling system that most likely plays a role in courtship and mate assessment in the dark river channels these fish inhabit, though no courtship account specific to S. caboclo has been published.
No direct observations of courtship behaviour in this species are available in the literature reviewed here; what is understood about gymnotiform courtship generally comes from electrophysiological study of better-known relatives such as the brown ghost knifefish, rather than from field or aquarium observation of Sternarchorhynchus itself.
Breeding
Breeding biology in Sternarchorhynchus caboclo is poorly documented. Apteronotidae as a family are egg-layers, and where spawning has been observed in the group it typically involves eggs scattered among root tangles, plant cover or crevices in flowing water, with no substantial parental care afterwards. There is no record of this species being bred in captivity, and given its rarity in the trade and its specialised deep-channel habitat, a defined breeding protocol should not be assumed.
The honest summary is that S. caboclo's reproduction is essentially unstudied outside its role as a taxonomic subject: what exists in the literature concerns its description and its place in the genus, not its spawning behaviour.
In the aquarium
Sternarchorhynchus caboclo is not an established aquarium fish; it is essentially unknown in the hobby under its own name; and this account draws on husbandry experience with its close relative Sternarchorhynchus goeldii, which is occasionally kept by specialist aquarists. That experience points to a demanding species: a large aquarium (200×60×25.5 in tanks have been discussed for the genus), a soft, sandy or muddy substrate rather than gravel — the long, sensitive snout is easily damaged probing a coarse substrate — dense planting or driftwood and caves for daytime cover, and meticulous water quality with strong filtration and frequent water changes, since these tube-snouted knifefish are reported as extremely sensitive to declining water conditions.
Because it is scaleless like all gymnotiforms, any Sternarchorhynchus needs careful medication practice: no copper-based treatments, and other medications at reduced dose. It is a nocturnal predator that electrolocates its food after dark, so tankmates should be too large to be eaten and the tank should offer subdued lighting. Given its rarity, price and specialised needs, this is a fish for experienced keepers only, and captive-bred stock does not appear to exist — any specimen in the trade is very likely wild-caught.
Conservation
Sternarchorhynchus caboclo is assessed as Vulnerable on the IUCN Red List, an assessment made in November 2018. This reflects the narrow, localised distribution typical of the genus — the species is known only from the Rio Mucajaí drainage in Brazil rather than across the wider Amazon basin, which leaves it far more exposed to localised habitat change than a widespread river fish would be.
FishBase records the species as low fishing vulnerability and no threat to humans, and it is not a target of any fishery; the conservation concern instead traces to habitat pressures common to narrow-range Amazonian tributary endemics, such as deforestation, water-quality change and hydrological disruption of the specific river reaches it depends on. With so little published on population trends, the Vulnerable listing should be read as a precautionary reflection of its restricted range rather than a documented decline.