Taxonomy & naming
Eigenmannia matintapereira was described in 2015 by Peixoto, Dutra & Wosiacki and placed in the family Sternopygidae, the glass and rattail knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Gymnotiformes are bony fish, but unusual ones: they are defined less by outward shape than by an electric organ running the length of the body and a single long anal fin that does almost all the swimming, since there is no dorsal fin and no functional tail fin in the ordinary sense.
The species belongs to the Eigenmannia trilineata species-group, a cluster of externally similar glass knifefishes distinguished mainly by fine details of dentary tooth counts and body-stripe pattern rather than by anything visible at a glance; a 2016 taxonomic monograph on the group (describing seven further new species) confirms its placement here. Despite the shared common name "knifefish", Eigenmannia and its Gymnotiform relatives are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus) — the resemblance is convergent body shape, not kinship. The Catalog of Fishes is the authority for the valid name.
Morphology
Like other members of its genus, Eigenmannia matintapereira has the classic glass-knifefish build: a long, laterally compressed, semi-transparent body that tapers smoothly to a fine point at the tail, with no dorsal fin and no separate caudal fin. Locomotion comes from a single elongated anal fin running along most of the underside, thrown into a continuous travelling wave that lets the fish glide forward or reverse direction without turning around — useful in narrow cover where an ordinary fish would have to reverse blind.
Across the genus, adult total length ranges roughly from about 5.5–19.5 in depending on species, with FishBase recording this species at up to about 8.5 in TL. Running the length of the body is the electric organ, paired with sensory pores across the skin that read the field it produces; the fish is scaleless, a detail that matters for how sensitive its skin is to water quality and medication. Sexes are not reliably told apart by outward appearance.
Habitat
The species is known from the Rio Uneiuxi and Rio Urubaxi, both tributaries of the Rio Negro in Amazonas, Brazil — a narrow, Amazonian range typical of many recently described Eigenmannia. These are tropical freshwater channels of the classic blackwater type: warm, softly acidic, and stained dark with dissolved tannins from decaying leaf litter, conditions in which eyesight counts for little and an electric sense is a considerable advantage.
Glass knifefishes generally favour flowing water among submerged roots, leaf litter and marginal vegetation rather than open, brightly lit water, and are active mainly after dark. No fine-grained temperature or pH data specific to this species has been published; the surrounding Neotropical blackwater habitat implies the same warm, soft, dim conditions favoured by its better-studied relatives.
Feeding
Eigenmannia matintapereira is, by inference from its genus and its FishBase trophic level (around 3.1, consistent with a low-level carnivore), a nocturnal micro-predator. Glass knifefishes hunt small aquatic invertebrates — insect larvae, worms and zooplankton-sized crustaceans — locating them in the dark largely through electrolocation rather than sight, sensing the way prey distorts the fish's own electric field at close range.
No species-specific feeding account has been published for E. matintapereira, but the genus is consistent enough in its habits that the pattern can be stated with confidence: small, soft-bodied invertebrate prey, taken at night, hunted by electric sense rather than vision. In the aquarium, relatives in the genus take standard small meaty foods readily, generally fed after dark when the fish becomes active.
Mating
Courtship in Eigenmannia is bound up with its electricity. The genus produces a wave-type electric organ discharge — a continuous, near-sinusoidal signal in roughly the 200–600 Hz range — that differs in frequency between males and females and carries information used in social recognition and courtship. This is also the genus in which the jamming avoidance response was discovered and remains a textbook example in neuroscience: when two individuals with similar discharge frequencies come close together, each shifts its own frequency away from the other's to avoid the interference, or "jamming", of their overlapping signals.
No courtship behaviour has been documented specifically for E. matintapereira; the species was described from a small number of specimens with data on reproduction sparse. What can be said with confidence, extrapolated from the genus, is that any courtship approach is likely to be conducted substantially through electric signalling exchanged in the dark, alongside physical proximity, rather than through visual display.
Breeding
Breeding has not been documented for Eigenmannia matintapereira specifically — data on this species remains sparse since its 2015 description. Within the genus more broadly, Sternopygidae are egg-layers that scatter adhesive eggs among plant roots or floating vegetation, typically triggered by warm, soft, higher-water conditions echoing the wet season, with little to no parental care given afterwards. Eigenmannia as a genus is bred in captivity somewhat more readily than most gymnotiforms, which is part of why the glass knifefish is one of the more approachable members of the order for hobbyists — but that experience comes from the widely traded E. virescens, not from this narrow-range species.
For E. matintapereira the honest position is that no breeding protocol exists in the literature; any aquarium spawning would be unreported territory rather than a documented method to follow.
In the aquarium
Eigenmannia matintapereira itself is essentially unknown in the hobby — a narrow-range Amazonian endemic described from just two tributary rivers, not a fish in general trade. What follows is the genus-level care picture, since Eigenmannia glass knifefishes as a group are among the more accessible gymnotiforms and worth outlining. They do best in small groups rather than singly: several individuals together space out their electric discharge frequencies and behave more naturally than a lone fish. A dim, tannin-tinted tank with plenty of cover — fine root tangles, dense planting, soft substrate — suits their electrolocation and their preference for the dark.
Being scaleless, glass knifefishes need great care with medications: copper-based treatments should be avoided entirely and other treatments used at reduced doses, and the fish are sensitive to ammonia and nitrate, demanding mature filtration and stable, warm, soft to moderately mineralised water. As nocturnal micro-predators they are not reliably safe with very small fish or shrimp, which risk being hunted after dark despite the knifefish's placid daytime appearance. Given the total absence of trade history for E. matintapereira specifically, any specimen reaching a tank would almost certainly be a wild-collected scientific voucher rather than an ornamental import.
Conservation
Eigenmannia matintapereira is assessed by the IUCN as Least Concern, an assessment made in January 2021. The species is known only from the Rio Uneiuxi and Rio Urubaxi, tributaries of the Rio Negro in Amazonas, Brazil, and like many recently described, narrow-range Amazonian fishes it sits in a position where a reassuring Red List category coexists with a genuinely small known range and limited survey effort.
No specific threats have been documented for this species. The broader pressures on its home basin are the general ones facing Amazonian blackwater tributaries — deforestation, land-use change and, in some Rio Negro-basin rivers, mercury contamination linked to small-scale gold mining — but nothing published ties these directly to E. matintapereira's population status. Given how recently it was described and how little has been surveyed since, its Least Concern rating should be read as a reflection of no known cause for alarm rather than confirmation of a large, well-studied population.