Taxonomy & naming
Eigenmannia muirapinima was described by Peixoto, Dutra & Wosiacki in 2015 in the Zoological Journal of the Linnean Society, placing it in the family Sternopygidae — the glass and rattail knifefishes — within the order Gymnotiformes, the New-World electric knifefishes. The holotype, 5.5 in total length, was collected from the Igarapé Santo Antônio, a tributary of the Rio Amazonas in Pará state, Brazil, which is also the type locality for the species.
As a member of Gymnotiformes, E. muirapinima belongs to a group of bony fish defined by their electric organs and their long, ribbon-like anal fin rather than by any resemblance to the Asian featherback and clown knifefishes (Chitala, Notopterus), which despite the shared common name are unrelated bonytongues. Within the order, Sternopygidae are the wave-type electric knifefishes — they emit a continuous, near-sinusoidal electric organ discharge (EOD), in contrast to the pulse-type discharge of families like Gymnotidae. The Catalog of Fishes is the authority for the valid name.
Morphology
Eigenmannia muirapinima follows the classic glass-knifefish body plan: strongly compressed side to side, tapering smoothly from a modest head to a fine point at the tail, with no dorsal fin and no distinct caudal fin. Locomotion comes from a very long anal fin running most of the length of the underside, thrown into continuous travelling waves that drive the fish forward, backward, or hold it hovering in place — a mode of swimming shared across the genus. FishBase records 170–198 soft rays in that anal fin, and a maximum size of about 5.5 in total length, modest even by the standards of the glass knifefishes.
Like its relatives, the species is semi-transparent in life, with an olive-to-silvery cast that gives the genus its "glass knifefish" name, and it is scaleless, its skin exposed and permeable. Running the length of the body is the electric organ that produces its wave-type EOD; congeneric work on the genus (in the related species E. sirius) records discharge frequencies in the roughly 100–780 Hz range, monophasic and continuous — a signal both for electrolocation in the dark and for signalling to other Eigenmannia. Data on external sexual dimorphism in E. muirapinima specifically are sparse.
Habitat
Eigenmannia muirapinima is known only from the Rio Amazonas basin in Pará state, Brazil, recorded from the Igarapé Santo Antônio and the Lago Jará tributary system near Santarém. It is a freshwater, benthopelagic fish of clearwater streams and river margins rather than open channel or still lake habitat.
Work on the wider genus Eigenmannia describes the group's typical habitat as narrow clearwater streams a few metres wide, running over sand, pebbles and dead leaves with a noticeable current, and thick with submerged roots and riparian vegetation that the fish shelter and hunt among. That combination of moving, low-turbidity, root-tangled water suits a wave-type electric fish well: visibility is limited, but the electric field is not, and cover among roots gives daytime shelter from predators. As with other Sternopygidae, dim, tannin-influenced water and a gentle current are likely central to how this species lives, though a formal ecological account of E. muirapinima itself has not been published beyond its original description.
Feeding
No feeding study specific to Eigenmannia muirapinima has been published, but FishBase estimates its trophic level at roughly 3.1, consistent with a small insectivore/omnivore rather than a specialist predator — in line with the wider genus, which is generally described as omnivorous with a strong insectivorous leaning. Glass knifefish typically forage over sediment and among root tangles and leaf litter for aquatic insect larvae and other small invertebrates, locating them chiefly through electrolocation in water where vision offers little advantage after dark.
In the aquarium, congeners such as the well-studied Eigenmannia virescens take small meaty foods readily — bloodworm, brine shrimp, chopped invertebrates and quality flake or pellet — and are considerably easier to feed than many of the order's larger, more strictly carnivorous species. Given the close relationship and shared body plan, E. muirapinima would be expected to feed similarly, though this remains an inference from the genus rather than a documented fact for this species.
Mating
As in other wave-type gymnotiforms, courtship in Eigenmannia is thought to run substantially through the electric organ discharge. The genus is the classic study animal for the jamming avoidance response, in which two fish with similar EOD frequencies shift their discharges apart to avoid interference — a phenomenon discovered in Eigenmannia and now a textbook case in sensory neuroscience — and EOD frequency in the genus differs consistently between males and females, making the discharge itself a likely channel for sex recognition and readiness signalling in the dark.
No courtship account specific to Eigenmannia muirapinima has been published. What can be said with confidence is drawn from the genus and family: approach and recognition between glass knifefish likely proceed by electric signal before any physical contact, with the details of pairing and spawning behaviour for this particular species still undocumented.
Breeding
Breeding has not been documented for Eigenmannia muirapinima specifically — FishBase records no reproductive data for the species. Within the family Sternopygidae, and notably in the well-studied congener Eigenmannia virescens, glass knifefish are egg-layers that scatter several dozen to a couple of hundred small adhesive eggs among plant roots or fine floating vegetation, typically triggered by conditions that mimic the onset of the wet season: a rise in water level, a softening and slight acidification of the water, and often a modest drop in temperature. Parental care is minimal to absent — eggs are left among vegetation to develop unguarded, and adults have been reported to become notably more aggressive around spawning.
Eigenmannia as a genus is considered one of the more readily bred groups of gymnotiforms in captivity, in contrast to species like the black ghost knifefish that are almost never spawned at home. Whether E. muirapinima itself has ever been bred in an aquarium is not documented; the responsible statement for this species is that its breeding biology is inferred from its genus rather than observed directly.
In the aquarium
Eigenmannia muirapinima is not established in the aquarium trade, and virtually nothing has been written about keeping this particular species — any care approach has to be extrapolated from its genus. Glass knifefish in general are considered among the easier-going gymnotiforms: modest adult size, a peaceable temperament toward fish too large to eat, and a tendency to do well in a small same-species shoal rather than alone, since group living appears to suit their electric social signalling.
As with every scaleless gymnotiform, care should assume sensitivity to ammonia and nitrate, and to copper-based or otherwise harsh medications, which should be avoided or used at reduced dose. A dim, quietly flowing tank with root-wood or plant cover for daytime shelter, gentle filtration, and warm, soft, near-neutral water would suit the genus's known habitat preferences. Given how little is documented about E. muirapinima, any specimens in the hobby should be treated cautiously and any husbandry notes as provisional rather than settled.
Conservation
Eigenmannia muirapinima is assessed by the IUCN Red List as Data Deficient, reflecting how recently the species was described (2015) and how narrowly its distribution has been documented — so far restricted to a small number of Rio Amazonas tributaries near Santarém, Pará, Brazil.
A range this narrow, if it holds up as more of the basin is surveyed, would make the species vulnerable to the same pressures facing other small-range Amazonian stream fish: deforestation and land-use change along headwater tributaries, water quality decline, and damming or channel modification. At present there is no evidence of population decline or targeted collection pressure, and no aquarium trade to speak of; the responsible conservation statement is that too little is known to assess risk, not that the species is safe.