Taxonomy & naming
Procerusternarchus pixuna was described by Cox Fernandes, Nogueira & Alves-Gomes in 2014, in a paper establishing both the new genus and the species from material collected in the Rio Negro basin of Amazonas State, Brazil. It belongs to the family Hypopomidae, the bluntnose knifefishes, within the order Gymnotiformes — the New-World electric knifefishes, a group of bony fish defined not by scales or fin shape but by an electric organ and a weak electric sense found nowhere else among freshwater aquarium fish. Within Hypopomidae the species was placed in the tribe Microsternarchini, alongside the genera Microsternarchus and Racenisia, on the strength of an enlarged, myogenic (muscle-derived) electric organ with a reduced negative phase to its discharge.
The describers distinguished the new genus from its relatives by a combination of features: a fully scaled body (many hypopomids are partly naked), a uniformly darker overall coloration, and the dark stripe running above the lateral line that gives the species its common thread with its Nheengatu name. The genus name Procerusternarchus combines the Latin procerus ("tall" or "elongate") with sternarchus, a classical root used across several electric-knifefish genera for the ribbon-bodied, keel-chested shape; pixuna, the species epithet, is simply the Nheengatu word for dark or black, describing the fish's coloration. As with all Gymnotiformes, these are not related to the superficially similar Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to an entirely different order of bony fish; the resemblance is convergent, not a sign of kinship.
Morphology
Procerusternarchus pixuna is a small knifefish, reaching a maximum recorded total length of about 5.5 in — modest even by the standards of the bluntnose and sand knifefish group, most of which stay compact throughout life. Like other Gymnotiformes it lacks a dorsal fin and has no true tail fin; locomotion comes from a long anal fin run as a travelling wave along the underside of the body, letting the fish move forwards or backwards with equal ease and hover in place while it probes for food. The body is uniformly dark, distinguished from close relatives by being fully scaled rather than partly naked, and by a darker line running above the lateral line — the feature, along with its overall coloration, that gave rise to the species name.
As in all gymnotiforms, an electric organ runs the length of the body and the skin carries the sensory pores (tuberous and ampullary electroreceptors) that read the electric field it produces. The discharge is pulse-type — delivered as discrete, separated pulses rather than the continuous wave-type signal of families like Apteronotidae or Sternopygidae — a pattern typical of Hypopomidae and well suited to a fish that spends its time investigating close-range structure in dark water rather than broadcasting continuously. The species is scaleless-skinned in the sense shared by all knifefishes at the level that matters to aquarists: no protective armour, and a corresponding sensitivity to poor water quality and to harsh medications.
Habitat
The species is known from the upper Rio Negro basin in Amazonas State, Brazil — a classic Amazonian black-water system, stained deep tea-brown by tannins leaching from decaying leaf litter, and correspondingly warm, soft and strongly acidic. Type material was collected from fast-flowing streams, small waterfalls and river margins with abundant submerged vegetation, roots and leaf litter, rather than from still backwaters or open channels.
That combination of structure and flow is where Procerusternarchus pixuna does its electrolocating: leaf litter, root tangles and margin vegetation give it cover and a rich hunting ground for small invertebrates, while the flow keeps the water oxygenated even as it stays warm. Black water of this kind is naturally very soft and acidic — cited water chemistry for the species runs to a pH as low as 3.0–4.5, among the most extreme tolerated by any aquarium fish — and dim, which is precisely the setting in which an electric sense earns its keep over eyesight. As with most narrow-range Hypopomidae, little has been published on the species beyond its original description and subsequent taxonomic notice, so its habitat use is inferred largely from the type series and its immediate surroundings rather than from extended field study.
Feeding
Procerusternarchus pixuna feeds on insect larvae, in keeping with the general diet of small Hypopomidae, which forage nocturnally over sediment, leaf litter and submerged root tangles for aquatic invertebrates. As with other pulse-type electric fish, prey is located largely by electrolocation: the fish's own field is distorted by anything nearby with different conductivity than the surrounding water, including the burrows and bodies of buried or hidden invertebrates, and the fish reads those distortions through electroreceptors covering its skin rather than relying on vision in habitats too dark or turbid for sight to be reliable.
Detailed dietary study of this particular species is sparse — most of what is documented comes from the original description rather than dedicated feeding studies — but the pattern is consistent with its relatives across Hypopomidae and Rhamphichthyidae: small, soft-bodied invertebrate prey, taken by careful nocturnal probing of cover rather than by open-water pursuit. In captivity, small hypopomids of this kind are generally kept on live and frozen foods sized to match — bloodworm, small blackworm, daphnia and similar — offered after dark when the fish become active.
Mating
As in other pulse-type Gymnotiformes, the electric organ discharge in Hypopomidae is understood to carry information beyond simple electrolocation — species, sex and individual identity can in principle be encoded in the pulse pattern, and courtship in electric fish of this kind generally involves an exchange of signals at close range as much as visual or tactile contact, particularly given the dim, often turbid water these fish inhabit. No species-specific study of courtship behaviour has been published for Procerusternarchus pixuna itself, however, and what can be said here is inferred from the wider pattern documented across small pulse-type knifefishes rather than from direct observation of this species.
Given how recently the genus was described (2014) and how little field or captive study has followed, courtship and mate choice in P. pixuna should be treated as effectively undocumented. This is typical of the bluntnose and sand knifefish group as a whole, where many species are known only from their type series.
Breeding
Breeding biology in Procerusternarchus pixuna is not documented. This is the norm rather than the exception for small Hypopomidae: most are known from limited field collections, and the reproductive habits of even long-recognised genera in this family are poorly recorded, let alone a genus described as recently as 2014. Where breeding has been observed in related bluntnose knifefishes, it generally follows a simple pattern — eggs scattered or deposited among leaf litter, roots or fine sediment, typically tied to seasonal high water, with no meaningful parental care afterwards — but this should be read as a general pattern for the group, not a confirmed account for this species.
There is no record of Procerusternarchus pixuna being bred in captivity, and given its narrow known range, small size and very recent description, it is not established in the aquarium trade at all. Any statement about its breeding requirements or protocol would be speculation; the honest position is that this remains an unstudied area of the species' biology.
In the aquarium
Procerusternarchus pixuna is not a fish encountered in the aquarium trade — it is a recently described, narrow-range species known from a small number of black-water localities in the upper Rio Negro, and there is no record of it being collected or exported for the hobby. Anything said about its aquarium care is therefore necessarily inferred from its biology and from the general husbandry of small Hypopomidae, not from direct keeping experience.
Were it ever to appear in the trade, the guidance for its relatives would apply: very soft, strongly acidic, warm black water; dim lighting and abundant cover in the form of leaf litter and fine root tangles to replicate the structure it forages through; a nocturnal schedule with feeding after dark; and, because it is scaleless like all Gymnotiformes, careful attention to water quality and a wariness of copper-based or harsh medications, which this group tolerates poorly. Given the extremely acidic water reported at its type locality (pH as low as 3.0–4.5), it would demand a specialist black-water setup well beyond typical community-tank parameters — not a beginner's fish even by the standards of its own family.
Conservation
Procerusternarchus pixuna is assessed by the IUCN Red List as Least Concern. This reflects, in part, that black-water tributaries of the upper Rio Negro remain relatively intact habitat, but it should be read alongside the fact that this is a narrow-range species known from a limited area within a single Amazonian basin, described only in 2014 and with no subsequent broad survey establishing its full distribution or population trend.
The general pressures facing Amazonian black-water fish — deforestation altering leaf-litter input and water chemistry, mining-related mercury and sediment pollution, and small-scale habitat disturbance from development along tributaries — are relevant to the wider Rio Negro system this species inhabits, even though none has been documented as an active threat to Procerusternarchus pixuna specifically. As with many recently described, single-basin gymnotiforms, its Least Concern status is best understood as reflecting limited evidence of decline rather than a confirmed picture of abundance across its range.