Taxonomy & naming
Sternarchorhynchus retzeri was described by de Santana & Vari in 2010 and placed in 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: Gymnotiformes are defined by an electric organ and a ribbon-like anal fin used for propulsion, traits shared with no other South American freshwater group. The Apteronotidae are distinguished within the order by an electric organ built from modified nerve cells rather than muscle, giving them the fastest, most refined electrosensory system of any knifefish.
The genus Sternarchorhynchus is the "tube-snouted" branch of the ghost knifefishes — a large assemblage of externally similar species distinguished mainly by snout proportions, tooth counts and other fine anatomical detail rather than obvious external markings. The epithet retzeri honors Michael Retzer, an ichthyologist with the Illinois Natural History Survey. As with the Asian featherback and clown knifefishes (Chitala, Notopterus), which are unrelated bonytongues, the shared common name "knifefish" describes body shape only, not kinship. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name.
Morphology
Sternarchorhynchus retzeri has the classic tube-snouted ghost-knifefish build: a laterally compressed, blade-like body tapering to a fine tail, with an elongated, downward-curved snout used to probe into crevices and sediment for prey. As in other Apteronotidae there is no dorsal fin and no functional 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. Females reach at least 15 in in total length, the largest verified size on record for the species.
The skin is scaleless and unprotected, a trait shared across the order that has real consequences for how sensitive fish are to disease and to medication. Running the length of the body is the electric organ, paired with rows of electroreceptor pores in the skin that read the field it generates — the sense organ this genus relies on far more than sight, especially fishing along a dark river bed.
Habitat
The species is recorded from the Amazon basin in Brazil and Peru, where it lives as a benthopelagic fish — foraging near and over the river bed rather than in open water. Congeners in the tube-snouted Sternarchorhynchus group are documented across whitewater, blackwater and clearwater rivers of the lowland Amazon, from smaller tributary streams into the main river channels, which gives a reasonable picture of the habitat type even where records for this exact species are sparse.
These are warm, Neotropical river waters, and like other Apteronotidae the species is best understood as a fish of the dark: dim or turbid conditions where electrolocation, not vision, does the navigating. Data on its precise micro-habitat and depth preferences within the basin are sparse.
Feeding
Sternarchorhynchus retzeri forages along the river bed for aquatic insect larvae and other small invertebrates, using its long, curved snout to probe into the substrate and crevices among roots and debris. As in other tube-snouted ghost knifefishes, prey is located largely by electrolocation — sensing the faint electrical signature of a buried or hidden animal — rather than by sight, an approach suited to a nocturnal, bottom-foraging lifestyle in dark water.
No aquarium feeding record exists for this species specifically. Related Apteronotidae kept in captivity take meaty foods such as bloodworm, chopped worms and small frozen invertebrates, and it is reasonable to expect a similar diet would be accepted, but this is inference from the genus rather than a documented fact for S. retzeri.
Mating
No courtship behaviour has been documented for Sternarchorhynchus retzeri itself. Across the ghost knifefishes generally, the electric organ discharge doubles as a social signal alongside its sensing role, encoding a fish's identity, sex and reproductive state; in wave-type Apteronotidae studied in the laboratory, males and females shift discharge frequency during encounters in what is effectively an electrical duet, and the same signalling is presumed to structure courtship across the genus. Wild Sternarchorhynchus show clear seasonal gonadal development — enlarged eggs and testes at certain times of year — indicating a defined breeding season in the wild, but the courtship sequence that leads up to spawning has not been described for this species.
Breeding
Sternarchorhynchus species are egg-layers, but no successful spawning of this genus has been documented in the aquarium. What evidence exists comes from wild-caught specimens showing seasonal reproductive readiness, not from observed spawning events, and there is no record of parental care of the kind seen in cichlids or other guarding fish. For the aquarist the honest summary is that breeding biology in this species is essentially undocumented — rarely bred, poorly understood, and not something to expect or attempt to induce.
In the aquarium
Sternarchorhynchus retzeri is not an established aquarium fish; it belongs to a genus of deep-channel Amazonian species rarely if ever seen in the trade, and almost nothing specific to its captive care has been published. What can be said follows from its family and its scaleless, electrogenic biology: it would need warm, soft, near-neutral to slightly acidic water, dim lighting and secure cover to feel settled, given the species' likely nocturnal, electrolocating habits. Being scaleless, Apteronotidae in general are sensitive to ammonia and nitrate and cannot tolerate copper-based medications, which should be assumed to apply here too.
Given the near-total absence of husbandry data, this is a fish for the specialist prepared to work from first principles and family-level knowledge, not a species with an established aquarium track record. Its elongated, curved snout and specialized bottom-probing feeding style also suggest it would need a soft substrate and undisturbed foraging areas rather than a bare or heavily decorated show tank.
Conservation
Sternarchorhynchus retzeri is assessed by the IUCN as Least Concern, an assessment made in 2018. This reflects its occurrence across a substantial stretch of the Amazon basin in Brazil and Peru rather than any detailed population study, which is typical for a Data Deficient-adjacent group of small, poorly surveyed electric fishes.
No specific threats have been documented for this species. As with other Amazon basin fish, the general pressures on its habitat are deforestation, water pollution and the cumulative effects of dam construction altering river flow, though none of these has been tied to this species individually in the available literature.