Taxonomy & naming
Eigenmannia macuxi was formally described in 2024 by Dutra, Peixoto, Donin, de Santana & Menezes in the Journal of Fish Biology, in a study that used integrative taxonomy — combining morphology with molecular data — to show that what had been treated as a single wide-ranging glass knifefish was in fact several distinct species, E. macuxi among them. It belongs to the family Sternopygidae, the glass and rattail knifefishes, within the order Gymnotiformes, the New-World electric knifefishes. Gymnotiformes are bony fish, but unusual ones: nearly every species produces a weak electric organ discharge (EOD) around its body, used both to sense the environment in the dark — electrolocation — and to communicate with other knifefish, each species and sex carrying something close to a signature waveform.
The species epithet honours the Macuxi, an Indigenous people of the Rio Branco basin and the wider Guiana Highlands region where the type material was collected, a naming convention increasingly used for South American freshwater fish described from Indigenous territory. Despite the shared common name, Gymnotiformes are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus) — the resemblance is convergent body shape, not kinship. Within Sternopygidae, Eigenmannia — the glass knifefishes — are wave-type 'hummers' that hold a near-constant discharge frequency, in contrast to the pulse-type signals of families such as Gymnotidae. The Catalog of Fishes is the authority for the valid name.
Morphology
As in other Eigenmannia, the body of E. macuxi is expected to be laterally compressed, elongate and semi-transparent — the "glass" in glass knifefish — tapering to a fine point at the tail, with no dorsal fin and no distinct caudal fin. Locomotion comes from a single long anal fin running most of the length of the underside, undulated in travelling waves that let the fish move forward or backward with equal ease and hover in place, a hallmark of the group. FishBase records a maximum total length of 5 in for E. macuxi, distinctly smaller than the largest individuals of its well-known congener E. virescens, which can approach 17.5 in in males.
The formal description separates E. macuxi from its relatives on a combination of external measurements and molecular markers rather than any single dramatic feature, which is typical of this genus — Eigenmannia species are notoriously difficult to distinguish by eye alone, and the 2024 study is explicit that morphology plus DNA were both needed to resolve the new species. Like all sternopygids it is scaleless, and it carries the electric organ running along the body and the associated electroreceptor pores across the skin that give the group its sensory edge in dark water.
Habitat
Eigenmannia macuxi is known from the Rio Branco basin in Roraima state, northern Brazil — a tributary system of the Rio Negro and, in turn, the Amazon, draining part of the Guiana Highlands. It is a single-basin endemic as currently understood, with no records reported outside the Rio Branco drainage. The basin includes slow-flowing channels, river margins and floodplain habitat typical of the region, and glass knifefish generally are associated with submerged roots, aquatic plants and floating vegetation along quieter water rather than fast, open channels.
Water in this part of the Guiana Highlands drainage tends to be tropical, often tannin-stained and acidic to soft, conditions that favour electrolocation over vision in fish that are active mainly after dark. Data specific to E. macuxi's microhabitat is sparse — the description is recent and based on limited collections — so the habitat picture here leans on what is known of Eigenmannia and the Rio Branco basin more broadly rather than confirmed field notes for this species alone.
Feeding
No feeding study exists for Eigenmannia macuxi specifically. By strong analogy with E. virescens and other glass knifefish, it is expected to be a nocturnal micro-predator, foraging after dark for small aquatic invertebrates — insect larvae, worms and tiny crustaceans — located largely through electrolocation rather than sight, probing among roots and plant cover where its prey hides during the day.
Glass knifefish are not ambush hunters; they work over cover and substrate methodically, using the distortions their own electric field produces around nearby objects and animals to find food in water too dark, turbid or vegetated for vision to be much use. Nothing in the cache record speaks to captive feeding for this particular species, since it is not yet established in the aquarium trade.
Mating
Courtship in Eigenmannia is inseparable from its electric signalling. The genus is the classic subject of the jamming avoidance response in neuroscience — when two individuals with similar discharge frequencies come close, each shifts its frequency apart from the other's to avoid electrosensory interference — and discharge frequency itself differs consistently between males and females in well-studied species such as E. virescens, functioning as a channel for sex and individual recognition during encounters. It is reasonable to expect E. macuxi shares this wave-type signalling architecture, since it is conserved across the genus, but no species-specific courtship observations exist yet.
Nothing beyond that inference is documented for this species. As a very recently described, single-basin endemic not yet in the aquarium trade, its courtship behaviour in the wild has not been directly studied or published.
Breeding
There is no breeding record for Eigenmannia macuxi, in the wild or in captivity — it has not been kept or bred by aquarists. Its close relative E. virescens, however, is bred in captivity more readily than most gymnotiforms: it is a fractional spawner that scatters adhesive eggs, on the order of 100–200 per spawning, among aquatic plants and fine cover, typically in warm, soft water, with no parental care given afterwards.
If E. macuxi follows the pattern of its genus — a reasonable but unconfirmed assumption — its breeding biology would look similar: egg-scattering among vegetation with little to no parental investment. For now this section is necessarily written from congener data rather than direct evidence for the species itself.
In the aquarium
Eigenmannia macuxi is not currently established in the aquarium hobby; it was described in 2024 from a limited number of Rio Branco basin specimens, and there is no record of it being collected or traded for the ornamental market. Any aquarium guidance here is therefore drawn from the genus in general rather than from keeping this species.
Glass knifefish as a group are considered one of the more approachable gymnotiforms: Eigenmannia species do well in a small shoal, are largely peaceful toward tankmates too large to eat, and are active, engaging fish once acclimated to dim, sheltered conditions with plenty of cover. As with every member of the order, an eventual E. macuxi in captivity would need the standard precautions for scaleless electric fish — no copper-based medication, reduced dosing generally, clean and stable water, and a tank kept dim with roots or pipe-like hides so the fish can retreat by day. Given how little is known about the wild population and its tolerances, captive husbandry recommendations for this specific species should be treated as speculative until real aquarium experience accumulates.
Conservation
Eigenmannia macuxi has not yet been assessed by the IUCN Red List; as a species described in 2024, its conservation status is Not Evaluated. There is no population trend data, and its range is currently known only from the Rio Branco basin in Roraima, Brazil.
As with many recently described, single-basin Amazonian and Guiana Highlands endemics, the practical conservation concern is exposure to a comparatively small range: habitat change, mining, agricultural runoff and hydrological alteration anywhere in the Rio Branco drainage could disproportionately affect a species with no known populations elsewhere. No specific threats have been documented for E. macuxi in the literature to date, and nothing here should be read as an assertion that the species is currently at risk — only that narrow-range basin endemics of this kind warrant future assessment as more is learned.