Taxonomy & naming
Sternarchorhynchus severii was described by de Santana & Nogueira in 2006 in Ichthyological Exploration of Freshwaters, based on a holotype male of 6.5 in total length (INPA 22893) collected from the Rio Mucajaí system in Roraima, Brazil. It belongs to the family Apteronotidae — the ghost knifefishes — within the order Gymnotiformes, the New-World electric knifefishes, and more specifically to the subfamily Apteronotinae. The Catalog of Fishes (Eschmeyer, CAS) lists it as a valid name with no synonyms recorded.
The genus Sternarchorhynchus is one of the most species-rich in the family, and its members are notoriously difficult to tell apart, distinguished largely by the length and curvature of the tubular snout and by fine details of dentition and squamation rather than by colour pattern. The species epithet honours William Severi, a Brazilian biologist recognised for his contribution to the study of the fish fauna of northeastern Brazil. Despite the shared common name "knifefish", Sternarchorhynchus and the rest of the Gymnotiformes are not related to the Asian featherback or clown knifefishes (Chitala, Notopterus); the resemblance is convergent body shape, not kinship.
Morphology
Like other members of its genus, Sternarchorhynchus severii has the classic "tube-snout" ghost knifefish body plan: an elongated, laterally compressed, scaleless body that tapers to a fine point at the tail, with no dorsal fin and no true caudal fin. A long tubular snout, ending in a small mouth, is the genus's signature feature and is thought to aid in probing crevices and substrate for hidden prey. Propulsion comes almost entirely from a long ribbon-like anal fin that runs beneath most of the body, its travelling waves driving the fish forward or backward with equal ease. FishBase records a maximum length of 6.5 in total length for the species, modest even by the standards of the tube-snouted knifefishes.
As in all Apteronotidae, the electric organ is derived from modified nerve tissue rather than muscle, which allows it to fire at very high, steady frequencies — among Apteronotidae generally, wave-type discharges recorded in the family run from roughly 420 to 2,179 Hz, continuous and sinusoidal rather than pulsed. That signal, read by sensory pores across the skin, is both a sonar for the dark and a badge of identity broadcast to every other fish nearby. Sexual dimorphism has not been documented for this species.
Habitat
Sternarchorhynchus severii is known from the Rio Mucajaí system in Roraima, in the Brazilian Amazon, with its type locality below a set of rapids (Cachoeira Paredão 2). FishBase describes the water there as acidic (around pH 5.8) and well-oxygenated, flowing past steep banks and a forest-bordered shoreline where beds of aquatic plants of the family Podostemaceae — plants specialised for growth on rocks in fast currents — cling to the streambed.
More broadly, Sternarchorhynchus as a genus is associated with high-energy stream habitats: rapids, waterfalls and the swift water immediately below them, a niche distinct from the sluggish backwaters and flooded forest favoured by some other gymnotiforms. This species in particular is reported from smaller tributaries and streams within the Amazon basin rather than the main river channel. As in other Apteronotidae, the electric sense is put to use in exactly this kind of turbulent, often turbid water, where vision alone would be a poor guide to what lies ahead.
Feeding
Detailed dietary data for Sternarchorhynchus severii has not been published, but the genus's tubular snout is widely interpreted as an adaptation for probing narrow spaces — crevices between rocks, leaf litter and plant beds — for small invertebrate prey. Like other Apteronotidae, it is presumed to hunt primarily by electrolocation, sensing the electrical signature of buried or hidden prey rather than relying on sight, and to be most active after dark.
Given the fast, well-oxygenated water it inhabits, its likely prey base is aquatic insect larvae and other small benthic invertebrates typical of rapids and riffle habitats, in keeping with the diet recorded for related tube-snouted knifefishes. Specific feeding data for this species is data sparse.
Mating
As in other Apteronotidae, the wave-type electric organ discharge in Sternarchorhynchus is understood to carry information about a fish's identity, sex and size, and courtship in the family is thought to unfold partly as an exchange of electric signals in the dark rather than through visual display. This has been studied in detail in a handful of laboratory species such as the brown ghost knifefish, where males and females shift discharge frequency toward one another during courtship.
No courtship or mating behaviour has been documented specifically for Sternarchorhynchus severii. It is presumed to follow the same broad Apteronotidae pattern of electric-signal-mediated approach, but this remains inferred from the family rather than observed in the species itself — data sparse.
Breeding
Sternarchorhynchus severii is an egg-layer, as are all Gymnotiformes, but its reproduction in the wild and in captivity is essentially undocumented. Encyclopedia of Life notes that sexual reproduction is confirmed for the genus but that breeding details remain unpublished, and the species is not established in the aquarium trade, so there is no hobbyist breeding record to draw on either.
No parental care has been reported for the species or, more broadly, for the tube-snouted Apteronotidae. Given how poorly studied this genus is outside of taxonomic description, any specific claim about spawning site, egg number or incubation would be guesswork; the honest summary is that breeding is rarely observed and poorly documented.
In the aquarium
Sternarchorhynchus severii is not a fish of the aquarium trade. It is known only from scientific collection in a remote stretch of the Rio Mucajaí system, and — unlike the popular black ghost knifefish — has no established husbandry record, wild-caught or otherwise. Sources describing the wider genus note that Sternarchorhynchus species are rarely seen in the trade and, when they do appear, are typically wild-caught rather than farm-bred.
Were a specimen ever to be kept, the general precautions that apply to all ghost knifefish would apply doubly here: it is scaleless and therefore sensitive to ammonia, nitrate and copper-based medications, which should be avoided or heavily reduced; and as a fish adapted to fast, well-oxygenated, acidic rapids water, it would need strong flow, high dissolved oxygen and stable, soft, acidic conditions that are difficult to replicate in a typical home aquarium. This is a species to know about, not a species to shop for.
Conservation
Sternarchorhynchus severii is assessed by the IUCN as Vulnerable, reflecting its apparent restriction to the Rio Mucajaí system in Roraima, Brazil — the kind of narrow-range, single-basin distribution that makes many tube-snouted Apteronotidae inherently more exposed than wide-ranging relatives such as the black ghost knifefish. A species confined to one river system can be disproportionately affected by localised pressures — changes in water quality, damming or flow alteration, deforestation of the surrounding catchment — that a continent-spanning species could simply outlast elsewhere in its range.
The species is not part of the aquarium trade, so collection for the hobby is not a relevant threat; conservation attention here belongs with habitat protection in the Mucajaí basin itself. As with many gymnotiforms outside the handful of trade staples, basic ecological and population data remain sparse, which is itself a conservation-relevant gap.