Taxonomy & naming
Tenebrosternarchus preto was described in 2007 by de Santana and Crampton as Sternarchogiton preto, based on a holotype collected at Toco Preto in the municipality of Tefé, on the Rio Tefé in Amazonas, Brazil. In 2020 it was moved into a new, monospecific genus, Tenebrosternarchus, in a redescription by de Santana and Crampton published alongside a broader revision of the Sternarchogiton-type ghost knifefishes; it remains the only species placed in that genus. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) lists de Santana & Crampton (2007) as the describing authority and records the same type locality and holotype data.
The species sits in the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes — the New-World electric knifefishes, a bony-fish lineage defined by a weak electric organ discharge (EOD) used for electrolocation and communication, and by a long ribbon-like anal fin that does almost all the swimming. Apteronotidae are distinguished within the order by an electric organ built from modified nerve cells rather than muscle, giving them an unusually fast, high-frequency discharge. These New-World fish are unrelated to the Asian featherback and clown knifefishes (Chitala, Notopterus); the shared "knifefish" name reflects similar body shape, not common ancestry.
Morphology
Tenebrosternarchus preto is a laterally compressed, blade-shaped fish reaching about 13 in in total length, based on the largest known specimens; the holotype measured 9.5 in TL. Living fish are described as strikingly dark — close to black overall, consistent with the species name preto. As in other Apteronotidae it has no dorsal fin and no functional tail fin; locomotion comes from a very long anal fin that runs along most of the underside of the body, undulating in travelling waves that let the fish move forward or backward with equal control.
Running the length of the body is the electric organ that generates the species' weak electric field, together with an array of skin sensory pores (electroreceptors) that read distortions in that field caused by nearby objects, prey or other fish. The genus name Tenebrosternarchus combines a reference to darkness with sternarchus, a name applied to several related ghost-knifefish genera. Secondary sexual differences have been documented in the species, though the cache does not specify which external features differ between the sexes.
Habitat
This species is known from the western Amazon basin, the Rio Negro (from Manaus to Barcelos), the Orinoco basin in Venezuela, and the Madeira drainage — a wide range across several major South American river systems, spanning both blackwater rivers such as the Rio Negro and Rio Nanay and whitewater channels such as the mainstem Amazon, the Rio Apure and the Rio Madeira. The type locality itself is a specific stretch of the Rio Tefé near Tefé, Amazonas, Brazil.
Tenebrosternarchus preto is described as an obligate deep-channel dweller: unlike shallow-water and marginal-habitat knifefishes, it is a benthic species found on or near the floor of large river channels rather than among bankside roots or floating vegetation. That deep, dark, sediment-level environment is exactly the setting an electric sense is built for — sight is of little use there, and electrolocation lets the fish navigate and forage over the bottom in conditions no aquarium can easily replicate.
Feeding
The known diet of Tenebrosternarchus preto is dominated by benthic invertebrates: chironomid (non-biting midge) larvae, beetle (coleopteran) larvae, and ceratopogonid (biting midge) larvae, all organisms that live in or on river-bottom sediment. The species has also been recorded feeding on freshwater sponges of the genus Drulia, an unusual prey item that underscores how specialised its deep-channel, bottom-foraging lifestyle is.
Larvae of the species have been found mixed with sand and detritus, further supporting a benthic foraging ecology in which the fish works close to or within the substrate. As in other ghost knifefishes, prey is almost certainly located primarily through electroreception rather than vision, given the depth and darkness of its river-channel habitat.
Mating
No observations of courtship behaviour in Tenebrosternarchus preto itself are available; this is a deep-channel Amazonian species known mainly from museum specimens rather than from field or aquarium observation. What can be said is grounded in the biology shared across the ghost knifefishes: the wave-type electric organ discharge documented for this species (a continuous, fast, two-phase signal with a fundamental frequency in the range of roughly 1,266–1,922 Hz) is, in related and better-studied Apteronotidae, a channel for identity, sex and readiness-to-spawn information, and courtship in the group is understood to involve exchanges of electric signals alongside physical approach.
Unlike some relatives such as the brown ghost knifefish, no sexual dimorphism in discharge frequency or waveform has been reported for T. preto, so if electric signalling plays a courtship role here it does not show up as an obvious male/female split in the EOD itself. This remains a data-sparse area for the species specifically.
Breeding
There is no documented breeding record for Tenebrosternarchus preto, in captivity or otherwise; it is not established in the aquarium hobby, and the species is known almost entirely from wild-collected specimens used for taxonomic study. As in other Apteronotidae, it is presumed to be an egg-layer, but no spawning site, egg description or developmental account has been published for this species, and no parental care has been documented.
Given how poorly studied its reproduction is, the honest position is that breeding biology in T. preto is essentially unknown rather than merely unbred in aquaria — there is no protocol to report, only the general expectation, drawn from its family, that eggs would be scattered among cover with no substantial parental guarding.
In the aquarium
Tenebrosternarchus preto is not a hobby fish. It has no established presence in the aquarium trade, and as an obligate deep-channel forager from large Amazonian, Rio Negro, Orinoco and Madeira river systems, it would be an exceptionally demanding species to keep even if it appeared: dim, soft, warm blackwater or whitewater conditions, large open channel-like space, and a benthic sediment environment are not easily reproduced in a home tank.
As in all Apteronotidae, any hypothetical care would need to reckon with the fact that these fish are scaleless and correspondingly sensitive to ammonia, nitrate, and to copper-based or harsh medications, which should be avoided or used at reduced doses. Any specimen encountered in trade would almost certainly be a scientific misidentification or a byproduct of mixed wild collection rather than a targeted, sustainable fishery, and captive-bred stock does not exist for this species.
Conservation
Tenebrosternarchus preto is assessed on the IUCN Red List as Least Concern. It is known from a wide span of the western Amazon, Rio Negro, Orinoco and Madeira systems, consistent with a distribution wide enough to support that listing, though as a poorly known deep-channel specialist there are no detailed population trend data or abundance estimates available.
In general terms, deep-channel Amazonian and Orinoco fishes of this kind face broad regional pressures shared with other freshwater species in these basins — damming, mining-related mercury pollution, deforestation-driven sediment and water-quality change, and habitat disruption from navigation and dredging — but no source in hand ties a specific pressure to this species.