Taxonomy & naming
Sternarchorhynchus axelrodi was described by Casey B. Dillman de Santana and Richard P. Vari in 2010, as part of a broad phylogenetic and revisionary study of the genus Sternarchorhynchus published in the Zoological Journal of the Linnean Society. It belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Gymnotiformes are bony fish, but unusual ones: distantly related to the more familiar aquarium groups, and defined by an electric organ and a long ribbon-like anal fin used for propulsion in place of a normal tail.
The genus Sternarchorhynchus is diagnosed by a distinctively long, decurved, tube-like snout, and now numbers around 32 valid species distributed through Neotropical South America, with more than two-thirds of them in the Amazon basin and a further seven in the Orinoco and Gulf of Paria drainages. Sternarchorhynchus axelrodi was established as a valid species in the 2010 revision and is endemic to the Rio Tocantins basin, a large river system that drains central Brazil between the Amazon and Orinoco drainages. As with all Gymnotiformes, it is unrelated to the Asian featherback and clown knifefishes (Chitala, Notopterus) despite sharing the common name "knifefish" — the resemblance is convergent body shape, not kinship. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name.
Morphology
Sternarchorhynchus axelrodi shows the tube-snouted body plan that defines its genus: an elongated, laterally compressed body tapering to a fine tail, with the head drawn out into a long, downward-curving snout. FishBase records a holotype male of 17.5 in total length and a paratype female of 19.5 in, with a maximum recorded length of about 23.5 in TL — a considerably larger fish than the familiar black ghost knifefish of the aquarium trade. As in other Apteronotidae it has no dorsal fin and no conventional caudal fin; locomotion comes from a long anal fin run as a travelling wave along the underside of the body, letting the fish move forward and backward with equal ease.
The electric organ that runs the length of the body is built from modified nerve tissue rather than muscle, the trait that sets the Apteronotidae apart from other gymnotiform families and gives them the fastest, most refined wave-type electric organ discharge in the order. Sensory pores across the skin read the resulting field, letting the fish sense objects, prey and other knifefish around it. The species is scaleless, like all Gymnotiformes, leaving the skin exposed and sensitive to water quality and to certain medications.
Habitat
The species is known only from the Rio Tocantins basin in Brazil, where it is reported broadly across both the upper and lower portions of the river system. FishBase describes its habitat as clear water over a stony substrate, consistent with the flowing, well-oxygenated river channels the genus Sternarchorhynchus generally favours over still or heavily vegetated water.
Like other Apteronotidae, it is a fish of warm, soft to moderately mineralised, near-neutral to acidic Neotropical river water, in the roughly 23–28 degrees Celsius range typical for the family. Life in clear but often dimly lit river channels, where vision is of limited use for a nocturnal fish working around rocks and current, is exactly the setting in which an electric sense earns its keep — sensing structure and prey by the way they distort the fish's own electric field rather than by sight.
Feeding
Detailed dietary studies specific to Sternarchorhynchus axelrodi are not available, but FishBase records it feeding on insects and aquatic invertebrates, and describes the species as nocturnal — consistent with the feeding pattern documented across the genus and the wider family. Sternarchorhynchus species are generally understood to feed by a grasp-and-suction action of the elongated snout, probing crevices and substrate for small invertebrates located by electrolocation rather than sight.
As with its relatives, the tube snout is thought to be an adaptation for reaching into narrow spaces among rocks and debris that a shorter-headed knifefish could not access, extending the reach of its electrosensory hunting into tight cover. Beyond this general pattern, the precise composition of its diet in the wild is data sparse.
Mating
No courtship or spawning behaviour has been documented specifically for Sternarchorhynchus axelrodi. What is understood about courtship in the Apteronotidae more broadly centres on the wave-type electric organ discharge itself: in well-studied relatives such as the brown ghost knifefish, the discharge frequency carries information about a fish's identity, sex and readiness to spawn, and males and females are known to shift their discharge frequencies toward one another during courtship interactions. It is reasonable to expect a similar electric signalling component in S. axelrodi's courtship, given its shared electric-organ physiology, but this has not been studied in the species itself and should be treated as inference from the family rather than an observed fact.
The species is essentially unrecorded in captivity, so there are no aquarium observations of pairing or approach behaviour to draw on. This remains one of the many Sternarchorhynchus species whose reproductive biology is simply data sparse.
Breeding
There is no documented account of Sternarchorhynchus axelrodi breeding in captivity, and the species does not appear to be established in the aquarium trade at all. Like other Apteronotidae it is presumed to be an egg-layer, but nothing specific is known about its spawning site, egg number, or any parental care, and no captive spawning has been reported.
Given how rarely this genus is seen outside scientific collections, breeding biology for S. axelrodi is best treated as unknown rather than extrapolated. The honest position is that its reproduction is poorly documented, consistent with its Data Deficient conservation status and its status as a taxonomically described but otherwise little-studied river fish.
In the aquarium
Sternarchorhynchus axelrodi is not a fish of the aquarium hobby. It is not established in the trade, and there are no husbandry records to draw on for tank size, diet or compatibility specific to this species. Anyone encountering it would be dealing with a large, deep-channel river fish reported to reach up to roughly 23.5 in total length, considerably bigger than the ghost knifefishes more familiar to aquarists.
What can be said follows from its family and its biology rather than direct experience: as a scaleless Gymnotiform it would need great care with medications, since Apteronotidae are known to be sensitive to copper-based treatments and to ammonia and nitrate; as a nocturnal, electroreceptive predator it would need dim lighting, secure cover, and would not be a safe companion for small tankmates. Given its size, its rarity, and the complete absence of husbandry data, this is a species best appreciated through the scientific literature rather than kept.
Conservation
Sternarchorhynchus axelrodi is classified as Near Threatened by the IUCN Red List. As a species apparently confined to the Rio Tocantins basin, it is exposed to whatever pressures affect that system — the Tocantins has seen substantial hydroelectric damming and associated habitat alteration in recent decades, a pattern consistent with a Near Threatened rather than a more secure listing, though no study specifically quantifies population trends in this species against those changes.
Single-river endemics within Sternarchorhynchus generally lack the dense population, threat and trend data that exists for more visible fish, so this listing should be read as reflecting genuine cause for concern rather than a comprehensively studied decline.