Taxonomy & naming
Sternarchorhynchus chaoi was described by de Santana & Vari in 2010, in a major taxonomic revision of the genus published in the Zoological Journal of the Linnean Society ("Electric fishes of the genus Sternarchorhynchus (Teleostei, Ostariophysi, Gymnotiformes): phylogenetic and revisionary studies"). It belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Like its relatives it is a bony fish, but an unusual one: Gymnotiformes are defined by their electric organs and a single long, ribbon-like anal fin used for locomotion in place of a normal tail and dorsal fin. The species epithet honours Ning Labish Chao, an ichthyologist at the Universidade Federal do Amazonas.
Within Apteronotidae the electric organ is derived from modified nerve tissue rather than muscle, a trait unique to the family that allows a fast, continuous wave-type electric organ discharge (EOD). Sternarchorhynchus is one of the more species-rich genera in this family, distinguished by an elongated, tubular, downward-pointed snout quite unlike the shorter face of the familiar black ghost, Apteronotus albifrons. The Catalog of Fishes is the authority for the valid name. Despite the shared common name, gymnotiform "knifefishes" are not related to the Asian featherback or clown knifefishes (Chitala, Notopterus), which belong to an entirely different order; the resemblance is convergent, not a matter of kinship.
Morphology
Sternarchorhynchus chaoi reaches a modest maximum size of about 6.5 in total length, small even by the standards of a genus whose members mostly stay under 12 in (a few Sternarchorhynchus species reach 23.5 in). Like other members of the genus it has the diagnostic elongated, tube-like, downward-angled snout used to probe crevices and sediment for hidden prey, a body that is laterally compressed and blade-shaped, and no dorsal or proper tail fin — propulsion comes from a long anal fin run in travelling waves along the underside of the body, letting the fish move forwards or backwards with equal ease. Coloration in the genus generally runs medium to dark brown or grey, without bold pattern.
As in other Apteronotidae, an electric organ runs the length of the body and the skin is dotted with electroreceptor pores that read the weak field it produces. The species is scaleless, as all gymnotiforms are. Adult male Sternarchorhynchus develop distinct teeth on the lower jaw used in male-to-male confrontations, a form of sexual dimorphism documented at the genus level; whether this has been specifically confirmed in S. chaoi is data sparse.
Habitat
Sternarchorhynchus chaoi is known from the Jamari River, a tributary of the Amazon basin in Rondônia, western Brazil, with its type locality in a diversion canal associated with the Samuel hydroelectric dam near Porto Velho. As far as published records show it is a narrow-range, single-river endemic rather than a widely distributed species. The genus as a whole occupies a broad range of Amazonian, Orinoco and Guiana river habitats, from small streams under half a metre deep to large rivers, and across whitewater, blackwater and clearwater types, but S. chaoi itself is documented only from this one basin.
Regional water in the Jamari basin runs warm year-round, roughly 73–82 °F with occasional peaks to 88 °F, and Apteronotidae as a family are known for remarkable pH tolerance, with related species recorded across a wide 3.5–8.4 range. Habitat within the genus typically includes undercut banks, submerged wood and densely vegetated margins — structure that suits a tube-snouted fish hunting by touch and electric sense in the dark rather than by sight.
Feeding
Diet in Sternarchorhynchus and the wider family runs to small invertebrates: aquatic insect larvae for juveniles, with shrimp and small fish added to the diet of larger adults in related species. The tube-like, downward-pointed snout of Sternarchorhynchus is built for probing narrow gaps between rocks, root tangles and leaf litter, reaching prey that a shorter-snouted knifefish could not access directly.
As with the rest of the family, hunting is thought to rely heavily on electrolocation rather than eyesight: the weak electric field the fish generates around its body is distorted by nearby prey, letting it detect food hidden in sediment or crevices in dark or turbid water where vision is of little use. Specific dietary data for S. chaoi has not been published; this account follows the documented pattern for the genus and family, and should be read as such.
Mating
No species-specific courtship or mating behaviour has been published for Sternarchorhynchus chaoi. What is well established for the wave-type electric knifefishes generally, and studied closely in related Apteronotidae, is that the electric organ discharge itself carries social information — identity, sex and readiness to spawn — and that courting fish are thought to modulate their discharge as part of approach and recognition in the dark. Whether this has been specifically documented in S. chaoi is unknown; the point is inferred from family-level biology rather than direct observation of this species.
Given how little is published on this species beyond its original description and basic distribution, mating behaviour here should be treated as data sparse rather than assumed to mirror any one relative in detail.
Breeding
Breeding in Sternarchorhynchus chaoi is undocumented. Across Apteronotidae generally, reproduction is by egg-laying, with eggs deposited singly in holes and crevices rather than guarded in a nest, and captive breeding is rare — most trade specimens of the more commonly kept relatives are wild-caught or farm-produced under hormone induction rather than bred by hobbyists. No parental care beyond egg deposition is documented for the family.
S. chaoi itself is not established in the aquarium trade, so there is no hobbyist breeding record to draw on. The honest statement is that its reproductive biology is poorly known and should not be assumed to follow any particular relative's pattern in detail.
In the aquarium
Sternarchorhynchus chaoi is not a trade species; it is known from scientific collection in a single Brazilian river system rather than from aquarium imports, and this account should not be read as a stocking recommendation. Its data sparsity is itself informative: many Sternarchorhynchus species inhabit deep river channels and are only rarely encountered outside targeted ichthyological surveys.
Were a Sternarchorhynchus of this size ever kept, the general rules for scaleless, nocturnal gymnotiforms would apply: no copper-based medications and reduced dosing generally, sensitivity to ammonia and nitrate, warm soft acidic-to-neutral water, dim lighting, and secure cover such as tubes or root tangles to retreat into by day, since these are electrolocating nocturnal hunters uncomfortable in bright, open, shelterless tanks.
Conservation
Sternarchorhynchus chaoi is assessed by the IUCN Red List as Data Deficient, reflecting how little is known of its population size, trend or full range beyond the Jamari River basin where it was described. As a narrow-range, single-river endemic collected near a hydroelectric diversion, it sits in the category of gymnotiform most exposed to localised pressures — dam construction, altered river hydrology, and habitat disturbance in the Jamari basin — even though no formal threat assessment beyond Data Deficient has been published.
The fair reading of the evidence is caution rather than alarm: Data Deficient is not the same as threatened, but a species known from one river reach warrants more monitoring than the label alone conveys. There is no evidence of it being collected for the aquarium trade, which removes one pressure that affects some of its higher-profile relatives.