Taxonomy & naming
Hypopygus varii was described by Ricardo Campos-da-Paz in 2018 (Zootaxa 4410(1):190-200), based on specimens collected from igarapé Saracá, a small stream in the lower rio Trombetas system of Pará State, Brazil — the fish's type locality and, so far, its only confirmed locality. It belongs to the genus Hypopygus, a group of small, cryptic knifefishes now placed in the family Rhamphichthyidae (the sand and longnose knifefishes) rather than the Hypopomidae where the genus was historically classified; a phylogenetic treatment of the description places H. varii within a broadly monophyletic Hypopygus, alongside all congeners except H. neblinae.
Hypopygus sits within the order Gymnotiformes, the New-World electric knifefishes — a group of bony fish defined by a weak electric organ and a single long anal fin used for locomotion, entirely unrelated to the superficially similar Asian featherback and clown knifefishes (Chitala, Notopterus) of the order Osteoglossiformes. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name Hypopygus varii.
Morphology
The original description characterises Hypopygus varii by a slender, laterally compressed body and a long anal fin bearing roughly 140 to 160 rays, which runs almost the full length of the underside and is the fish's primary means of propulsion — it has no dorsal fin and no distinct tail fin in the ordinary sense. The body carries 8 to 10 dark bands, with pale markings along the back that distinguish it from its closest congeners; these banding and count characters are the features used to separate H. varii from other Hypopygus species in the original diagnosis.
Like all Gymnotiformes, it is scaleless, with an electric organ running along the body and a covering of electroreceptive pores that sense the field the organ produces. The description is based on the holotype and a small series of paratypes, and beyond these diagnostic characters, size, growth and other morphological detail have not been documented in the literature; a close relative, H. lepturus, tops out around 4 in total length, which gives a rough sense of scale for the genus, though H. varii itself has not been measured at that resolution in print.
Habitat
Hypopygus varii is known only from igarapé Saracá, a small forest stream in the lower rio Trombetas drainage — itself a major right-bank tributary of the Amazon in Pará State, Brazil. Congeners in the genus, such as H. lepturus, are typically found in shallow creeks over sandy substrate, in low-conductivity blackwater or clearwater with root mats and accumulated leaf litter, and the type locality fits that same pattern of small, structured, tannin-stained Amazonian streams.
As with other bluntnose and sand knifefishes, this is a nocturnal, cryptic fish of dim, structurally complex water, where an electric sense compensates for the near-uselessness of vision in the dark or turbid conditions typical of these habitats. No dedicated ecological survey of H. varii's habitat beyond the type collection has been published; its broader range within the Trombetas system, if any, is undocumented.
Feeding
No diet study specific to Hypopygus varii has been published. Based on the ecology of its genus and family, it is presumed to be a small nocturnal predator of invertebrates — aquatic insect larvae, worms and other small benthic prey — picked out of leaf litter and sediment largely by electrolocation rather than sight, in keeping with the general pattern for small Rhamphichthyidae and Hypopomidae feeding in dark, structured habitats.
This is an inference from the family rather than a documented fact for the species: data on H. varii's feeding behaviour, prey items or foraging pattern in the wild is sparse to non-existent in the available literature.
Mating
Courtship and mating in Hypopygus varii have not been documented. In pulse-type weakly electric knifefishes generally — the group to which Hypopomidae and Rhamphichthyidae, including Hypopygus, belong — the electric organ discharge functions as a social signal as much as a sensing tool, with discharge waveform and timing carrying information about species identity and, in some species, sex; courtship in related pulse-type gymnotiforms is understood to involve an exchange of these electric signals alongside physical approach in the dark.
Whether this pattern holds for H. varii specifically is not established; no electrophysiological or behavioural study of this species has been published, and any account of its courtship must be read as inference from family-level biology rather than direct observation.
Breeding
Breeding biology in Hypopygus varii is undocumented. Across the Hypopomidae and Rhamphichthyidae generally, reproduction is poorly studied; where it is known in related genera, these are egg-layers that scatter or deposit eggs among leaf litter, root mats or sand, typically in the wet season, with little or no parental care afterward.
There is no record of H. varii being kept, bred, or observed spawning in captivity, and given its description from a single small collection, its reproductive behaviour in the wild remains essentially unknown. Data sparse is the honest summary rather than any specific breeding protocol.
In the aquarium
Hypopygus varii is not established in the aquarium hobby; it is a recently described, narrow-range species known from a single Amazonian stream, and there is no record of it being kept or traded. Any care guidance must be drawn from its genus and family rather than direct experience with this species.
Small Hypopygus and related bluntnose and sand knifefishes are delicate, nocturnal, scaleless fish that fare best in dim, quiet tanks with a soft sand substrate and generous cover — root tangles, leaf litter and fine-leaved plants — mimicking the structured blackwater streams they come from. Being scaleless, they are sensitive to ammonia, nitrate and to copper-based or harsh medications, which should be avoided or heavily reduced. They are not community-safe with very small or bite-size tankmates, which a nocturnal electrolocating predator will hunt after dark, and captive-bred stock would be strongly preferable to wild collection given how little is known about the species' population.
Conservation
Hypopygus varii is assessed by the IUCN Red List as Vulnerable. It is known from a single type locality in the lower rio Trombetas drainage, and no detailed population or range-extent data has been published since its 2018 description.
The rio Trombetas basin has faced pressure from damming, mining and deforestation elsewhere in its watershed, and a narrow-range single-stream endemic like H. varii is inherently vulnerable to habitat change at its only known site — consistent with the Vulnerable listing, even without a species-specific threat assessment. Further survey work in the Trombetas system would help clarify its status in more detail.