Taxonomy & naming
Sternarchorhynchus gnomus was described by de Santana & Taphorn in 2006, from material collected in the lower Río Caroní, a major right-bank tributary of the Orinoco in Venezuela. 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: the Gymnotiformes are defined by an electric organ and a long ribbon-like anal fin used for propulsion, and they bear no relation to the superficially similar Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to an entirely different order.
Within Apteronotidae, the genus Sternarchorhynchus is set apart by an elongated, tube-like snout, and Sternarchorhynchus gnomus was described as the smallest species in the genus at the time of its naming. The species epithet reflects that diminutive size relative to its congeners. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name Sternarchorhynchus gnomus de Santana & Taphorn, 2006.
Morphology
As in all Apteronotidae, the body of Sternarchorhynchus gnomus is elongated and laterally compressed, without a dorsal fin and without a true tail fin; forward and backward movement comes from undulations of a single long anal fin running along most of the underside. The defining feature of the genus is the drawn-out, tubular snout, an adaptation thought to aid probing into narrow spaces and sediment while the fish's electric sense does the actual work of locating prey. The species is described as the smallest member of Sternarchorhynchus, though a precise maximum length is not established in the available literature — data on this point remain sparse.
Like other apteronotids, S. gnomus carries an electric organ derived from modified nerve cells (a neurogenic organ) rather than from muscle tissue, which is what allows Apteronotidae to produce the fastest, highest-frequency electric organ discharges in the order — a continuous, quasi-sinusoidal wave-type signal rather than the pulsed discharge of families such as Gymnotidae. The skin, as in all gymnotiforms, is scaleless.
Habitat
Sternarchorhynchus gnomus is known only from the lower Río Caroní in Venezuela, a tributary of the Orinoco that drains the Guiana Shield. This is a single-river endemic in the strictest sense: the only documented locality for the species lies in this stretch of the Caroní, and it has not been recorded in the wider Orinoco basin or elsewhere.
As with other Apteronotidae, it is presumed to inhabit flowing river channels rather than still water, in the warm, typically soft and often turbid or shaded conditions where an electric sense outperforms vision. The lower Caroní has been substantially altered by hydroelectric dam construction, and the specific habitat from which the species was originally described has since been affected by these changes — the central conservation concern for the species.
Feeding
Detailed dietary data specific to Sternarchorhynchus gnomus are not available in the literature reviewed. By analogy with the wider genus and family, it is reasonable to infer a nocturnal, benthic-foraging habit: Apteronotidae generally hunt small invertebrates — aquatic insect larvae, worms and small crustaceans — locating them in the dark or in turbid water largely through electrolocation rather than sight. The elongated snout characteristic of Sternarchorhynchus is thought to assist this kind of close-range probing into crevices and sediment.
Because this species has essentially no presence in the aquarium trade and little targeted field study, its feeding ecology should be treated as inferred from the genus and family rather than directly documented for S. gnomus itself.
Mating
No species-specific courtship data exist for Sternarchorhynchus gnomus. What is well established for Apteronotidae generally — and for the family's best-studied member, the brown ghost knifefish Apteronotus leptorhynchus — is that the wave-type electric organ discharge doubles as a social signal. Discharge frequency differs by sex and individual, and in laboratory studies of related apteronotids, males and females shift their discharge frequencies toward one another during courtship interactions, an exchange sometimes described as an electric duet, while rival fish signal with distinctive discharge interruptions.
It is reasonable to infer that S. gnomus, sharing the same neurogenic wave-type electric organ, uses similar electrical signalling in courtship, but this has not been directly documented in the species itself. This is inference from the family, not a confirmed observation for S. gnomus.
Breeding
Reproduction in Sternarchorhynchus gnomus is essentially undocumented, and the species is not known to have been bred in captivity. Apteronotidae as a family are egg-layers that spawn among cover such as roots, plants and crevices, and are only rarely bred even in aquaria, with most trade specimens of the better-known genera being wild-caught or produced on commercial farms under hormone induction rather than spawned naturally by hobbyists. No parental care is documented in the family.
For S. gnomus specifically — a fish with no established aquarium presence and known from a single, now-altered locality — breeding biology should be considered data sparse rather than described. Its reproductive mode is inferred as egg-laying based on family-level biology, with no confirmed parental care.
In the aquarium
Sternarchorhynchus gnomus is not a fish of the aquarium trade. Unlike the black ghost knifefish and a handful of other Apteronotidae that are established, if niche, aquarium subjects, this species is a narrow-range Venezuelan endemic with no meaningful history in the hobby, and it should not be sought or collected given its conservation status.
Where any Sternarchorhynchus or related tube-snouted apteronotid is kept, the general husbandry principles for the family apply: these are scaleless fish, sensitive to ammonia, nitrate, and to copper-based or harsh medications, which must be used at reduced doses; they are nocturnal and do best with dim lighting and secure cover such as tubes or root tangles; and given their electrolocation-driven hunting behaviour, they are unlikely to be safe with small or bite-size tankmates. None of this is established specifically for S. gnomus, which remains, for practical purposes, outside the hobby.
Conservation
Sternarchorhynchus gnomus is assessed as Critically Endangered. The species is known from a single locality in the lower Río Caroní, Venezuela, and that locality has been affected by hydroelectric dam construction on the river — habitat alteration that directly threatens the only known population of a fish with no documented occurrence anywhere else.
As a single-river endemic with an extremely restricted range, S. gnomus exemplifies the vulnerability of narrow-range Gymnotiformes more broadly: unlike widely distributed relatives such as the black ghost knifefish, species confined to one river system or one stretch of one river have no refuge if that specific habitat is degraded or impounded. There is no aquarium trade in this species to offset with captive breeding, and its conservation depends entirely on the condition of the lower Caroní itself.