Taxonomy & naming
Hypopygus hoedemani was described by de Santana & Crampton, with the species account published in 2013 covering material collected some years earlier (the species is dated to 2011 in some catalog listings). It belongs to the genus Hypopygus within the family Hypopomidae, one of several families of weakly electric fish making up the order Gymnotiformes, the New-World electric knifefishes. Older literature sometimes grouped Hypopygus under a broader Rhamphichthyidae, but the modern classification restricts Rhamphichthyidae to the sand- and tubesnout knifefishes proper and keeps Hypopygus in Hypopomidae, the bluntnose knifefishes.
The species epithet honours J. J. Hoedeman, who made significant contributions to the study of Neotropical fishes. As with all Gymnotiformes, Hypopygus hoedemani is only superficially similar to the unrelated Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance is convergent body shape, not shared ancestry. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name.
Morphology
This is a genuinely small fish, reaching a maximum of about 2.5 in total length, which places it among the smallest of all electric knifefishes. It has a short, notably broad and blunt head relative to its slender body — the trait that gives the bluntnose knifefishes their common name — and, like every gymnotiform, it lacks a dorsal fin and a conventional tail fin. Locomotion comes from a long anal fin that runs beneath most of the body, rippled in travelling waves that let the fish move forward or backward with equal ease.
The body carries a bold pattern of 12–15 dark crossbands that extend up onto the back, a striking contrast against the paler ground colour and a useful field mark among the more cryptically coloured bluntnose species. The anal fin itself is long, typically carrying somewhere in the range of 140–160 rays. Like other Gymnotiformes it is entirely scaleless, and it carries an accessory electric organ in the humeral (shoulder) region that produces its weak electric organ discharge (EOD), used alongside an array of skin sensory pores for electrolocation. As a pulse-type gymnotiform, its EOD is delivered as discrete, intermittently spaced pulses rather than the near-continuous wave produced by ghost knifefishes.
Habitat
Hypopygus hoedemani is known from the Rio Negro and Rio Preto da Eva drainages of the Amazon basin in Brazil, living in small terra firme streams — the shaded, black-water tributaries that drain unflooded forest and run tannin-stained and acidic from decaying leaf litter. These are quiet, structurally complex habitats: shallow, slow-flowing water threaded with submerged roots, branches, and thick beds of fallen leaves that the fish hides among by day.
As with other small nocturnal gymnotiforms, darkness and cover are central to how this species lives. Its narrow known range in a black-water tributary system of the Rio Negro fits the broader pattern among the bluntnose knifefishes, several of which are recently described, poorly surveyed, and restricted to a handful of similar habitats.
Feeding
Hypopygus hoedemani is nocturnal, becoming active after dark and feeding on small invertebrates it finds among the leaf litter and sediment of its stream habitat. Like the rest of the bluntnose and sand knifefishes, it almost certainly relies heavily on electrolocation to find this prey, since visual cues are of little use in the tannin-dark, often turbid water it inhabits and after nightfall when it forages.
By day it is reported to hide, often in groups of several dozen individuals sheltering together within the same patch of cover — a social, aggregating habit that is somewhat unusual among the generally solitary bluntnose knifefishes and may reduce individual risk from predators while the fish are inactive and vulnerable.
Mating
Courtship and spawning in Hypopygus hoedemani have not been documented in the literature available for this account. Across the bluntnose and sand knifefishes generally, electric organ discharges are understood to carry information used in social and reproductive contexts — each species, and often each sex, produces a characteristic pulse pattern, and it is reasonable to expect that pulse-type signalling plays some part in how individuals of this species locate and assess one another in the dark. Beyond that inference from the wider group, specific courtship behaviour in H. hoedemani is data sparse.
Breeding
Breeding biology for Hypopygus hoedemani is poorly documented, which is typical for the Hypopomidae and Rhamphichthyidae as a whole. Where breeding has been observed in related bluntnose and sand knifefishes, eggs are scattered or deposited among leaf litter, roots, or sand rather than guarded in a nest, generally without a strong pair bond, and with little or no parental care afterward. There is no aquarium spawning record for this species to draw on, and nothing further can be stated with confidence.
In the aquarium
Hypopygus hoedemani is rarely, if ever, seen in the aquarium trade, and this account should be read as a description of the species rather than a settled care guide. Should it appear, its needs would follow the pattern of the bluntnose and sand knifefish group: soft, acidic, tannin-stained water in the range of roughly 73–82 °F, a dim tank with a sandy or fine substrate, and generous leaf litter or root cover to replicate the black-water streams it comes from. Being scaleless, it would need careful handling of medications — no copper-based treatments, and reduced doses of anything else — along with pristine water quality, since scaleless fish are unusually sensitive to ammonia and nitrate.
Given its very small adult size and reported habit of sheltering in groups, it would likely do best kept in a quiet species tank with several companions rather than alone, and would need bite-size tankmates avoided given its nocturnal, predatory habits. Any specimens that do reach hobbyists are effectively certain to be wild-caught, given the complete absence of an aquarium breeding record.
Conservation
Hypopygus hoedemani is assessed by the IUCN as Least Concern. This reflects available knowledge of the species within its known Rio Negro basin range in Brazil rather than any indication that it is common or its populations well studied; like many small, cryptic gymnotiforms restricted to black-water tributary systems, it is not a well-surveyed fish.
As with other narrow-range terra firme stream specialists in the Amazon basin, the practical pressures on this kind of habitat are deforestation and associated changes to water quality and flow in small forest streams, though no species-specific threat assessment is available in the sources consulted for this account.