Taxonomy & naming
Eigenmannia waiwai was described in 2015 by Peixoto, Dutra and Wosiacki as one of seven new species carved out of the Eigenmannia trilineata species-group, a revision that untangled a complex of superficially similar glass knifefishes previously lumped together. It belongs to the family Sternopygidae, the glass and rattail knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. These are bony fish, but an unusual lineage: Gymnotiformes are defined by an electric organ and a long ribbon-like anal fin rather than a conventional tail, and they are only distantly related to the rest of the freshwater aquarium fauna.
The species epithet honours the Waiwai, an indigenous people of the border region between Brazil and Guyana, reflecting the type locality near their territory on the Rio Mapuera. Despite the shared common name "knifefish", Eigenmannia and its Sternopygidae relatives are not related to the Asian featherback or clown knifefishes (Chitala, Notopterus) — those are bonytongues from an entirely different order, and the resemblance is convergent body shape rather than kinship. The Catalog of Fishes is the authority for the valid name.
Morphology
Eigenmannia waiwai has the classic glass-knifefish build: a long, slender, laterally compressed body that tapers 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 along most of the belly, thrown into travelling waves that let the fish move forward or backward with equal facility — useful for backing out of the roots and crevices it hides among. The species reaches a recorded maximum of about 9 in in total length. As in other Sternopygidae, the skin is entirely without scales, semi-transparent in life, and carries the electroreceptor pores and the electric organ that define the group.
Peixoto and colleagues distinguished E. waiwai from its close relatives in the trilineata group using fine anatomical characters — maxilla proportions, scale-row counts (vestigial in this scaleless body plan) and skeletal details — rather than anything obvious to the naked eye, which is typical of the group; several Eigenmannia species are effectively indistinguishable in the field and must be told apart in the hand or by their electric signal. Anal-fin ray counts in the species run roughly 167–195, a fin long enough to be the fish's only real means of propulsion.
Habitat
The species is known from the Rio Mapuera, part of the Rio Trombetas basin in Pará, Brazil — a tributary system draining into the Amazon from the north. Its recorded range is narrow: a single Amazonian sub-basin rather than the continent-spanning distribution seen in some wider-ranging Sternopygidae. It is described as benthopelagic, occupying the water column over the bottom in flowing tropical freshwater rather than open water or still swamp.
As with other glass knifefishes, this is warm, soft, often tannin-stained river water where daylight vision counts for little and an electric sense counts for a great deal. The genus is associated with slower channel margins, submerged roots and vegetation, and — where community data exist for related Eigenmannia — with a preference for cover and dim conditions over open, brightly lit water.
Feeding
Specific dietary data for Eigenmannia waiwai have not been published; what is known comes from the ecology of the genus and family. Glass knifefishes are nocturnal micro-predators, foraging after dark for small invertebrates — aquatic insect larvae, worms, small crustaceans and similar prey — located largely through electrolocation rather than sight, reading the way prey items distort the fish's own electric field.
They are generally peaceful toward anything too large to be swallowed, hunting only what fits their small mouths, and forage steadily over cover and substrate rather than actively chasing prey in open water. Data are sparse for this particular species, and no aquarium feeding record could be confirmed from available sources.
Mating
As in other Eigenmannia, courtship in this genus is thought to run largely through the electric organ discharge. Eigenmannia produces a continuous, wave-type EOD — a steady electrical hum rather than the intermittent pulses of some other gymnotiform lineages — and the frequency of that hum differs between the sexes and carries information used in the well-studied jamming-avoidance response, in which two fish with similar discharge frequencies shift them apart to avoid interference with each other's electrolocation. That same signalling system is presumed to underlie mate recognition and approach in the genus, though no species-specific courtship account exists for E. waiwai.
No detailed observations of courtship behaviour in Eigenmannia waiwai itself have been published; the electric-signalling framework above is inferred from the genus rather than documented for this species specifically.
Breeding
Sternopygidae as a family are egg-layers, scattering adhesive eggs among plant roots and floating vegetation, typically triggered by warm, soft, rising water of the kind associated with the wet season, and offering little or no parental care afterwards. Eigenmannia as a genus is reported to be bred more readily in captivity than most other gymnotiforms, which has made the common glass knifefish (Eigenmannia virescens) a modestly attainable captive-breeding project for experienced keepers.
No breeding record specific to Eigenmannia waiwai has been published, and given its narrow known range and recent description, it should be treated as unbred and poorly documented in aquarium conditions rather than assumed to follow the pattern of its better-known relatives.
In the aquarium
Eigenmannia waiwai is not an established aquarium fish — it is a recently described, narrow-range species with essentially no hobby track record — but its family's care requirements are well understood through Eigenmannia virescens and other glass knifefishes, and would apply if it were ever offered. Being entirely scaleless, glass knifefishes need careful attention to water quality and to medication: no copper-based treatments, and reduced doses of anything else, along with low ammonia and nitrate in mature, well-filtered water. As nocturnal electroreceptive predators they want dim lighting and secure cover — plants, roots or pipe — and are not reliably safe with fish or shrimp small enough to be hunted after dark.
Given the lack of any documented trade or captive history, the honest position for this species is that it should not be assumed available, and any specimen encountered should be treated with the same caution given to other little-known, single-basin Amazonian endemics: captive-bred stock, where any exists for the genus, is always preferable to wild collection.
Conservation
Eigenmannia waiwai is assessed by the IUCN Red List as Data Deficient, reflecting how recently it was described and how little targeted survey work exists across its known range in the Rio Mapuera and wider Rio Trombetas basin. It is not flagged as being of commercial fisheries interest, and there is no evidence it poses any threat to people.
As a narrow-range endemic apparently restricted to a single Amazonian sub-basin, it belongs to the category of gymnotiform most exposed to habitat change — damming, mining-related pollution and deforestation pressures affect Trombetas-basin tributaries as they do much of the Amazon — but no basin-specific threat assessment has been published for this species, and its Data Deficient status should be read as a call for more survey work rather than as evidence of decline.