Taxonomy & naming
Tembeassu marauna was described by Triques in 1998 from three specimens collected decades earlier, and it is the only species placed in the genus Tembeassu. It belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Like all Gymnotiformes it is a bony fish but an unusual one, defined by an electric organ and a long ribbon-like anal fin rather than by the fin arrangement of typical aquarium fish; the Apteronotidae in particular generate their electric organ discharge from modified nerve tissue rather than muscle, giving them a fast, continuous wave-type signal used for electrolocation in the dark.
Despite the shared common name, knifefishes of the New World are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to an entirely different order and arrive at a similar body shape by convergence, not descent. A subsequent anatomical reappraisal of the type series using micro-computed tomography, published in PLOS One, re-examined the only three known specimens in detail, underscoring how little material exists for this species. The Catalog of Fishes is the authority for the valid name Tembeassu marauna.
Morphology
The type series measures roughly 7–7.5 in to the end of the anal fin, with the holotype at 7.5 in; FishBase gives a maximum total length of about 7.5 in, based on the same specimens. As in other ghost knifefishes, the body is blade-like and laterally compressed, tapering toward the tail, with no dorsal fin and propulsion generated by an elongated anal fin running along the underside — the trait that gives the whole order its ability to swim backward as readily as forward. The skin is scaleless, as in all Gymnotiformes.
What distinguishes Tembeassu marauna from its relatives is a set of conspicuous fleshy extensions at the tips of the jaws, each carrying a patch of extra teeth, along with an enlarged fleshy lobe on the chin — features documented in the SciELO taxonomic reappraisal of the species. Their function is not established; researchers have speculated they may play a role in feeding, but this remains unconfirmed. As an apteronotid, the fish carries its electric organ and electroreceptive pores along the body, the sensory system it would have relied on in the dim water of its deep-channel habitat; the specific wave- or pulse-type character of its discharge has not been documented for this species.
Habitat
Tembeassu marauna is known only from the upper Paraná River in Brazil, in the vicinity of Ilha Solteira, spanning the states of Mato Grosso do Sul and São Paulo — a single-river endemic in the strictest sense. FishBase and the SciELO reappraisal both describe it as an inhabitant of deep riverine environments, a specialist of the deeper portions of large rivers rather than the shallow margins or floodplain habitats used by many other Neotropical fishes.
The only known specimens were taken from a coffer-dam during the construction of the Ilha Solteira hydroelectric reservoir in 1965 — an incidental collection made possible by the temporary drawdown and disturbance of the riverbed during dam works, rather than a targeted survey. No living population has been documented since, and nothing further is known about the specific microhabitat, water chemistry or seasonal behaviour of the species in the wild.
Feeding
No diet has been documented for Tembeassu marauna. The SciELO reappraisal notes that stomach-content analysis was in progress at the time of publication but not yet reported, and FishBase likewise records the diet as data sparse.
By analogy with other ghost knifefishes, which are nocturnal carnivores that hunt small invertebrates and occasionally small fish using electrolocation in low-visibility water, it is plausible that Tembeassu marauna fed similarly on benthic prey in its deep-river habitat — but this is inference from relatives, not an observed fact, and the unusual jaw extensions and extra teeth raise the possibility of a more specialised feeding mode that has not been characterised.
Mating
Nothing is recorded about courtship or spawning behaviour in Tembeassu marauna; with only three preserved specimens ever collected, no living individual has been observed in the wild or in captivity. As in other Apteronotidae, the electric organ discharge likely serves a signalling role between individuals — carrying cues to identity, sex and readiness to spawn — a pattern documented in better-studied relatives such as the brown ghost knifefish, but this has never been examined in Tembeassu marauna itself.
Breeding
There is no breeding data for this species, in the aquarium or in the wild. It has never been kept or bred in captivity, and given that it is known from only three museum specimens collected over sixty years ago, no eggs, larvae or juveniles of Tembeassu marauna have ever been described. As with other ghost knifefishes it is presumed to be an egg-layer with no substantial parental care, but this is an inference from the family rather than a documented fact for the species.
In the aquarium
Tembeassu marauna is not and has never been part of the aquarium trade. It is not known to exist as a living population anywhere, and the entire scientific record for the species consists of three preserved specimens collected in 1965. There is nothing to recommend or caution about in husbandry terms; this is a species for the taxonomic and conservation literature, not the fishroom.
Conservation
Tembeassu marauna is assessed as Critically Endangered on the IUCN Red List. Its entire known range is a short stretch of the upper Paraná River near Ilha Solteira, and it has not been recorded since the three type specimens were collected in 1965 during construction of the Ilha Solteira dam — a project that permanently altered the river's deep-channel habitat by impounding it into a reservoir. FishBase notes explicitly that the species may already be extinct as a consequence of that dam construction.
More than forty years of subsequent collecting in the region turned up no further specimens, per the SciELO reappraisal, and the species' apparent dependence on deep, flowing river habitat makes it especially vulnerable to impoundment. Tembeassu marauna is a clear case of a narrow-range Neotropical endemic that may have been lost to a large infrastructure project before it was ever properly studied alive.