Taxonomy & naming
Sternarchorhynchus goeldii was described by de Santana & Vari in 2010, part of a single revisionary paper (Zoological Journal of the Linnean Society) that overhauled the genus and added 22 new species at once — a measure of how poorly resolved this group of fish had been until recently. It belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes, the New-World electric knifefishes. Apteronotidae are set apart from the rest of the order by an electric organ built from modified nerve tissue rather than muscle, which gives them the fastest, most refined wave-type electric discharges of any knifefish.
The genus name Sternarchorhynchus refers to the long snout that defines the group; species-level identification within it is difficult and rests on fine details of snout shape and proportion, which is why misidentification is common even among specialist importers. These fish share only a superficial resemblance with the Asian featherback and clown knifefishes (Chitala, Notopterus) — unrelated bonytongues from a different order entirely — and the Catalog of Fishes is the authority for the valid name.
Morphology
Like other members of its genus, S. goeldii has the long, downcurved, tubular snout that gives Sternarchorhynchus its name, ending in a small terminal mouth adapted for probing rather than for grasping large prey. The body is elongate, laterally compressed and knife-like in the manner of all Apteronotidae, tapering to a slender tail with no dorsal fin and no true 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. A thin dorsal filament runs along the back in place of a fin.
The skin is scaleless and smooth, a feature shared across the order and one with real consequences for how the fish must be kept. Reported total length reaches around 12 in. Running the length of the body is the electric organ and the pores of the associated sensory system, which the fish relies on far more than vision to navigate and hunt in the dark, sediment-laden waters it inhabits.
Habitat
Sternarchorhynchus goeldii is a Neotropical species from the Amazon basin, with specimens documented from the Rio Purus drainage spanning Brazil and Peru and further material collected from the Peruvian Amazon. As with its relatives, its precise range is imperfectly mapped — narrow-range, single-basin distributions are typical of this genus, and the difficulty of separating similar-looking Sternarchorhynchus species in the field means published records likely understate or blur the true extent of the species.
It occupies the same kind of habitat as most Apteronotidae: warm, soft, generally acidic to near-neutral Amazonian river water, reported around 75–82 °F and pH 6–7. These are typically deeper channels and slower stretches with cover from roots, submerged wood and leaf litter, where light penetration is poor and an electric sense does the work that eyes cannot. Activity is nocturnal to crepuscular, consistent with the rest of the family.
Feeding
In the wild, S. goeldii almost certainly hunts the way other tube-snouted Apteronotidae do: probing sediment, root tangles and leaf litter after dark for small invertebrates, using its long snout and electrolocation to find prey that is out of sight. The elongated, curved snout is a specialisation for reaching into narrow gaps and interstitial spaces that a straight-mouthed fish cannot access.
In captivity, aquarists who have kept the species report it takes frozen bloodworm and tubifex readily once settled, alongside other small live or frozen meaty foods typical of electric knifefish diets. As a nocturnal predator it feeds most confidently after lights-out, and — as with its relatives — is not a safe companion for anything small enough to be mistaken for food during the night shift.
Mating
No courtship behaviour has been documented specifically for Sternarchorhynchus goeldii. What can be said with confidence follows from its family: Apteronotidae communicate and navigate with a fast, wave-type electric organ discharge, and in the best-studied ghost knifefishes that discharge carries information about identity, sex and reproductive state, with males and females adjusting discharge frequency toward one another as part of courtship. It is reasonable to expect the same broad pattern in S. goeldii, but this has not been confirmed for the species itself.
Aquarium reports for this species note pronounced intraspecific aggression, including snout-to-snout "sword-fighting" between individuals — behaviour more clearly tied to territoriality than to documented courtship, though the two may be connected in a fish that signals identity and status electrically.
Breeding
Breeding of Sternarchorhynchus goeldii in the aquarium is undocumented. Like the great majority of Apteronotidae, it is presumed to be an egg-layer, but no spawning has been reported in captivity, and the species is not commercially bred — all specimens reaching the trade are wild-collected. Data on its reproductive biology, even in the wild, is sparse to non-existent beyond its formal description.
Given this, the honest position for the aquarist is that S. goeldii should be kept and enjoyed as a wild-collected specialist fish, not approached with any expectation of breeding it. As in its better-known relatives, electric-organ signalling almost certainly plays some courtship role, but no protocol or observed spawning exists to describe.
In the aquarium
Sternarchorhynchus goeldii is a specialist's fish, both because of its size and temperament and because correctly identifying it in the trade is genuinely difficult — several similar-looking Sternarchorhynchus species are commonly sold under this name or vice versa. Hobbyist reports describe keeping individuals successfully in large tanks (around 200 x 60 x 25.5 in has been reported), reflecting a fish that, while not huge by knifefish standards at roughly 12 in, is long-bodied and needs real swimming room plus dim, sheltered surroundings by day.
Being scaleless, it demands the same caution with medications and water quality as other Apteronotidae: no copper-based treatments, reduced dosing generally, and stable, mature water low in ammonia and nitrate. Reports note marked aggression toward conspecifics and other elongate knifefish, so it is often best kept singly or with great care in stocking; it is also a nocturnal predator not suited to housing with small or slow tankmates. Captive husbandry knowledge is thin compared with the popular black ghost, so this is a fish for the experienced keeper willing to accept some trial and error, and to source stock carefully given how often it is mislabelled.
Conservation
Sternarchorhynchus goeldii is assessed on the IUCN Red List as Least Concern. As a Rio Purus / Peruvian Amazon species tied to intact river-channel and root-tangle habitat, it faces the same general pressures as other Amazonian fish — deforestation, water-quality decline and hydrological change from damming — but no species-specific data exists to quantify these threats.
Many species in this genus were described only in the 2010 revision and remain poorly surveyed across the Amazon basin, so this Least Concern listing should be read against that general backdrop rather than as evidence of dense, dedicated population study.