Taxonomy & naming
Sternarchella sima was described by Starks in 1913 and belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Like all Gymnotiformes it is a bony fish but an unusual one, defined by an electric organ and a long ribbon-like anal fin rather than by the ordinary fin plan of most aquarium fish; the Apteronotidae are further distinguished within the order by a neurogenic electric organ, derived from nerve tissue rather than muscle, which gives them the fastest and highest-frequency discharges of any knifefish family.
The genus Sternarchella was revised taxonomically by Evans, Crampton & Albert (2017), a study that described two new species alongside the existing ones and clarified the morphological groupings within the genus. Sternarchella sima is not related to the Asian featherback or clown knifefishes (Chitala, Notopterus), which only superficially resemble it through convergent body shape. The Catalog of Fishes is the authority for the valid name; data on this species beyond its taxonomy and basic biology are sparse.
Morphology
Sternarchella sima has the compressed, blade-like body typical of the ghost knifefishes, propelled by a long anal fin run as a travelling wave along the underside rather than by a tail. The species reaches a maximum total length of around 15.5 in. A distinguishing feature noted in the taxonomic literature is its mouth: Sternarchella sima has a subterminal to ventral (underslung) mouth, a trait shared with its closest relative S. orinoco and interpreted as a trophic adaptation, in contrast to species with more terminal mouths elsewhere in the genus.
As in other Apteronotidae, an electric organ runs the length of the body and the skin carries the sensory pores (tuberous and ampullary electroreceptors) that read the field it produces. The species is scaleless, as is typical of the order. Detailed data on external sexual differences are not documented in the sources available; dimorphism in this genus, where studied at all, tends to appear in the frequency of the electric discharge rather than in obvious external features.
Habitat
Sternarchella sima is found in the Amazon and Orinoco river basins of tropical South America, where it occupies the main channel of large rivers rather than tributaries, backwaters or floodplain habitat. It is a deep-channel specialist, recorded at depths of roughly 10 to 50 metres, living a benthopelagic existence over the riverbed in conditions of total or near-total darkness far below the light-penetration zone.
That depth and darkness make an electric sense essential rather than optional: at 33–164 ft in a silt-laden Amazonian river, vision is of little practical use, and Sternarchella sima — like the rest of its family — navigates, forages and likely communicates almost entirely through its self-generated electric field. This deep main-channel lifestyle also explains why the species is essentially absent from the aquarium trade: it is not a marginal, easily netted fish of shallow creeks but an inhabitant of strong, deep river currents that are difficult to sample and impossible to replicate casually in captivity.
Feeding
Detailed dietary study of Sternarchella sima specifically is limited, but the taxonomic literature on the genus notes an unusual and citable trophic habit: species in this deep-channel clade, including S. sima, are recorded preying on the tails of other electric fishes — a specialised, almost parasitic style of predation distinct from the more generalised invertebrate-hunting typical of shallower-water ghost knifefishes such as the black ghost. This fits the subterminal, underslung mouth shape described for the species, which is interpreted as an adaptation for this kind of feeding.
As an Apteronotid living in permanent darkness, any prey detection is presumably accomplished largely through electrolocation rather than vision, sensing the electrical signatures and water displacement of other fish and small animals around it in the deep channel. Beyond this, specific data on foraging behaviour, feeding frequency or diet breadth in the wild are data sparse, and there is no aquarium feeding record because the species is not kept.
Mating
As in other Apteronotidae, electric organ discharge is understood to function not just in electrolocation but as a social signal, carrying information about a fish's identity, sex and physiological state to other knifefish sharing the same water. In the best-studied member of the family, the brown ghost knifefish (Apteronotus leptorhynchus), males and females shift their discharge frequency during courtship in a kind of electrical duet, and rival fish produce characteristic interruptions when they encounter one another. It is reasonable to expect broadly similar electric-signalling courtship across the family, including in Sternarchella, given the shared wave-type discharge mechanism.
No species-specific courtship or spawning behaviour has been documented for Sternarchella sima in the sources available. Living permanently in deep, dark river channels far from observation, its reproductive behaviour is essentially unstudied outside of the general electrophysiology known for the family — this is genuinely data sparse territory rather than a gap to fill with inference.
Breeding
Sternarchella sima is presumed, on the basis of its family, to be an egg-layer, but no breeding record — wild or captive — is documented for this species in the available literature. Apteronotidae generally are rarely bred even in captivity; wave-type electric signalling plays a role in courtship in the family's better-studied genera, but that has not been directly observed or reported for Sternarchella sima itself.
Given its deep-channel habitat, far outside any aquarium or field-observation setting typically used to document fish reproduction, no parental care behaviour has been recorded. The honest statement for this species is that its breeding biology is essentially unknown and unbred; there is no protocol to report, only the general pattern shared across its family.
In the aquarium
Sternarchella sima is not a fish of the aquarium hobby. It lives in the deep main channels of the Amazon and Orinoco, 10 to 50 metres down, and is not collected for the trade; no husbandry record exists for the species. Any general Apteronotidae care principles — scaleless skin sensitive to copper-based and harsh medications, sensitivity to ammonia and nitrate, a need for warm, soft, dimly lit water and secure daytime cover — would apply if the species were ever kept, but none of this has been tested for S. sima specifically.
For the aquarist, the honest note is that this is a genuinely wild, scientifically documented species rather than a hobby fish, and any claim of aquarium suitability would be speculation. Prospective keepers of Apteronotidae are better served by species with an actual husbandry record, such as the black ghost or brown ghost knifefish.
Conservation
Sternarchella sima is assessed as Least Concern on the IUCN Red List, with the assessment dated 7 November 2018. This reflects its occurrence across the large, still relatively intact main channels of two major river systems, the Amazon and the Orinoco, rather than any specific study of population trend.
As a deep-channel main-river specialist, the species is not directly threatened by the habitat conversion and small-stream degradation that pressure many shallow-water Neotropical fish, though large-scale threats to the Amazon and Orinoco basins overall — damming, mining-related pollution and deforestation-driven changes to river hydrology — are relevant to the wider ecosystem it depends on. There is no evidence of trade-driven collection pressure on this species, since it is not part of the aquarium trade.