Taxonomy & naming
Sternarchella rex was described by Evans, Crampton and Albert in 2017 as part of a taxonomic revision of the deep-channel electric fish genus Sternarchella, published in Neotropical Ichthyology. It belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes — the New-World electric knifefishes, a group only distantly related to other freshwater aquarium fish and defined by their electric organs and long ribbon-like anal fin. Within Apteronotidae, the electric organ arises from modified nerve cells rather than muscle, giving these fish an unusually fast, high-frequency discharge compared with other gymnotiforms.
The species falls in what the describing authors term Clade D, the S. calhamazon clade, and is distinguished from its relatives by a combination of characters: a notably wide distance between the parietal bones (59-61% of head length), a broad mouth (20-27% of head length), a pale to whitish base body colour, two rows of bones along the caudal peduncle, and a continuous membrane along the base of the anal fin. The specific epithet rex is Latin for "king", chosen because S. rex is the largest species so far recognised in its genus. As with all New-World knifefishes, it is unrelated to the superficially similar Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to an entirely different order; the resemblance is convergent, not a sign of kinship. The Catalog of Fishes is the authority for the valid name.
Morphology
Sternarchella rex reaches a maximum recorded total length of about 16 in, making it markedly larger than most of its congeners and the largest species so far described in the genus. Like other apteronotids it has a compressed, blade-shaped body, no dorsal fin, and no distinct tail fin; locomotion comes from a long anal fin run as a travelling wave along the underside of the body, letting the fish move forward or backward with equal control. The body is scaleless, and the diagnostic features that separate it from other Sternarchella — the unusually wide skull across the parietal bones, the broad mouth, and the pale body tone — reflect a genus whose members are specialised, unusually among gymnotiforms, for preying on the tails and fins of other electric fishes rather than on typical benthic invertebrates.
Along the flanks runs the electric organ and the array of sensory pores that read the field it produces; live specimens have been recorded producing a wave-type electric organ discharge with a fundamental frequency in the range of roughly 945-1096 Hz (mean around 1023 Hz), among the faster discharges recorded in the family. Data on sexual differences are sparse; museum specimens examined in the description were not in full reproductive condition, so external sexual dimorphism has not been established.
Habitat
Sternarchella rex is known from the western Amazon basin, with its type locality at Tefé in the Mamirauá Lake System along the Paraná Maiana in Brazil, and additional specimens recorded from the Iquitos region of Peru. It is a deep-channel species, taken from depths of roughly 2 to 50 metres in the main channels of large South American rivers rather than from shallow margins or floodplain habitats — a niche that keeps it almost entirely out of reach of collectors for the aquarium trade.
Water at the type locality was recorded at about 81 °F with very low conductivity (around 55 microsiemens per centimetre), consistent with the soft, dilute blackwater and whitewater mixing typical of the western Amazon. Living in deep, dark river channels where light barely penetrates, S. rex — like other ghost knifefishes — relies on electrolocation rather than eyesight to navigate and to find prey.
Feeding
The genus Sternarchella is unusual among the Gymnotiformes for specialising on the fins, scales and skin of other electric fishes rather than on the aquatic invertebrates most knifefish species take. Recorded diet data for S. rex point to a piscivorous or scale-and-skin-feeding habit consistent with this genus-wide pattern, presumably located in the dark, sediment-laden channel water through electrolocation rather than sight.
Beyond this genus-level pattern, little has been published on the specific feeding behaviour, prey selection, or foraging schedule of S. rex in the wild, and nothing is documented for the species in captivity — it is not established in the aquarium trade. Data sparse.
Mating
As in other Apteronotidae, the wave-type electric organ discharge in S. rex is very likely a channel for social and reproductive signalling as well as a sensing tool — the discharge frequency and waveform can carry information about a fish's identity, sex and condition, a pattern well documented in related ghost knifefishes such as Apteronotus. No courtship behaviour has been directly observed or described for S. rex itself, however; the species is known from a small number of museum specimens collected for taxonomic work, not from field or aquarium observation of live behaviour.
The type series was not in full reproductive condition when collected, so details of pairing, spawning site selection or any physical courtship display remain undocumented. Data sparse.
Breeding
There is no published breeding record for Sternarchella rex, in the wild or in captivity. As with the great majority of Apteronotidae, egg-laying with no substantial parental care is the pattern expected across the family, and any spawning is presumed to occur among cover such as roots or sediment in its deep-channel habitat, but this has not been confirmed for this species. Sternarchella rex has essentially no presence in the aquarium hobby, so there is no hobbyist breeding experience to draw on either.
Given how recently the species was described (2017) and how little of its natural history has been recorded even in the primary taxonomic literature, breeding biology should be considered entirely unknown rather than inferred. Data sparse — prefer no claim over a guessed protocol.
In the aquarium
Sternarchella rex is not a hobby fish. It lives in deep river channels 2-50 metres down and has not been reported in the aquarium trade; there is no husbandry record to draw on, and this article makes no attempt to invent care parameters that have not been observed. What can be said, by extension from the family, is that any apteronotid of this size and habitat would need a large, dimly lit aquarium with strong but gentle flow, soft and near-neutral to slightly acidic water, and secure cover, plus the general apteronotid caution: it is scaleless, so unusually sensitive to ammonia, nitrate and to copper-based or harsh medications, which should be avoided or used at reduced dose.
Given its genus's specialised diet — preying on the fins and scales of other electric fishes — and its large adult size, S. rex would in any case be a poor community fish even if it were ever imported. For now it belongs firmly in the category of scientifically documented but practically unavailable species.
Conservation
Sternarchella rex has not been formally assessed by the IUCN Red List; no entry exists for the species. As a recently described (2017), narrow-range endemic of the deep river channels between the Mamirauá Lake System near Tefé, Brazil, and the Iquitos region of Peru, it would reasonably be treated as Data Deficient pending assessment, and this article follows that precautionary status rather than assuming Least Concern.
Because it lives in the deep main channels of large Amazonian rivers, S. rex is not directly exposed to the habitat degradation that threatens many shallow-water and headwater Amazon endemics, but river-wide pressures — damming, mining-related pollution including mercury, and broader basin deforestation — can still affect deep-channel fish communities indirectly through changes in flow, sediment and water chemistry. With so little natural history recorded, any conservation judgement beyond "data deficient, narrow known range" would be speculation.