Taxonomy & naming
Hypopygus benoneae was described in 2013 by Peixoto, Dutra, de Santana and Wosiacki, bringing the genus Hypopygus to nine recognized species. It belongs to the family Rhamphichthyidae — the sand and bluntnose knifefishes — within the order Gymnotiformes, the New-World electric knifefishes, a group of bony fish set apart from almost everything else in the aquarium hobby by a true electric organ and a ribbon-like anal fin used for locomotion in place of a tail. The species is placed in a clade with H. hoedemani, H. lepturus and H. minissimus, and it was diagnosed from its closest relatives by the absence of a sixth infraorbital bone and by a high anal-fin ray count of 115–134.
The Asian featherback and clown knifefishes (Chitala, Notopterus) share only the name "knifefish" and a superficially similar shape; they are bonytongues from an unrelated order and have no close relationship to Hypopygus or any other gymnotiform. The species epithet benoneae honours Naraiana Benone, who collected the type specimens. The Catalog of Fishes is the authority for the valid name.
Morphology
Hypopygus benoneae is a small, elongate, laterally compressed fish reaching only about 3.5 in in total length, tapering to a fine point at the tail with no dorsal fin and no true caudal fin — propulsion instead comes from a long anal-fin ribbon rippling beneath the body, which lets the fish move backward as readily as forward. Like other gymnotiforms it is entirely scaleless, a soft, unarmoured skin that makes it sensitive to poor water quality and to many standard fish medications.
Running the length of the body is a weak electric organ generating a pulse-type discharge, measured at roughly 1 volt in the original description — enough for electrolocation and signalling in the dark leaf litter it lives in, far below the danger threshold of the unrelated, strongly electric electric eel. Sensory pores across the skin read the field the organ produces. No description of external sexual dimorphism has been published for this species.
Habitat
The type locality is the lower Rio Anapu, part of the lower Amazon basin in Brazil, where the species was collected from shallow rainforest stream margins around 2–3 ft deep and 8–62 ft wide, thick with submerged root tangles and leaf litter. It was found living alongside a close relative, H. lepturus, in the same stretches of water.
More broadly, the genus Hypopygus is confined to the Amazon, Orinoco and upper Paraguay basins and the Guianas, occupying both clearwater and blackwater streams across the Amazonian lowlands and the Guiana Shield. These are warm, tropical, often tannin-stained waters where visibility is poor and an electric sense does the work that eyes cannot — exactly the conditions the aquarium needs to reproduce for the fish to settle and feed.
Feeding
No dedicated dietary study of Hypopygus benoneae has been published, but its trophic level of about 3.1 on FishBase places it as a mid-level carnivore, consistent with the small invertebrate diet typical of leaf-litter-dwelling Rhamphichthyidae. Like its relatives it almost certainly forages at night among root tangles and sediment, using its pulse-type electric field to locate small aquatic insect larvae, worms and other invertebrates that eyes would struggle to find in the dark, silt-laden water it inhabits.
In the aquarium, small Hypopygus of this kind are best offered fine meaty foods after dark — bloodworm, micro-worm, and other small live or frozen foods sized for a fish under 4 in — mirroring the nocturnal, substrate-probing feeding style documented across the genus and family, though no specific husbandry account for this species exists in the literature.
Mating
Courtship and spawning behaviour have not been documented for Hypopygus benoneae. Across the pulse-type Gymnotiformes, the electric organ discharge doubles as a social signal alongside its sensing role — species- and sex-specific pulse characteristics are thought to feature in courtship and rival interactions in related genera — but no study has confirmed this for H. benoneae itself, and it would be speculation to describe a courtship sequence for this species.
Breeding
Breeding in Hypopygus benoneae is essentially undocumented; data are sparse even at the genus and family level. Where reproduction has been recorded in Hypopomidae and Rhamphichthyidae relatives, it takes the form of egg-scattering among leaf litter, roots or sand, generally timed to the wet season, with little to no parental care afterward — but no aquarium or field account of spawning exists for this species. There is no known aquarium breeding record.
In the aquarium
Hypopygus benoneae is not established in the aquarium trade and is essentially unknown outside scientific collections, so any husbandry notes here are inferred from its close relatives and family rather than from direct experience with the species. Being scaleless, it would need the same caution given to all gymnotiforms — no copper-based medications, reduced dosing generally, and stable water free of ammonia and nitrate spikes.
If kept, a small, dimly lit tank with a soft sand base and plenty of leaf litter or root-wood cover to hide in by day would suit its nocturnal, cryptic habits, alongside gentle, warm, softwater conditions matching its blackwater and clearwater stream origins. At under 4 in it poses little threat to similarly sized tankmates, but as a nocturnal predator it should not be trusted with anything small enough to be mistaken for food after dark.
Conservation
Hypopygus benoneae is assessed by the IUCN as Least Concern, consistent with its FishBase status and its occurrence across multiple river systems in the genus's broader Amazon, Orinoco, Guiana Shield and upper Paraguay range rather than a single isolated locality. Described only in 2013, it remains a poorly studied species with no population trend data published.
As with other small, narrow-range Amazonian stream fish, ongoing deforestation and habitat disturbance in the lower Amazon basin are the general pressures worth watching, though nothing in the available literature indicates a specific threat to this species. It has no presence in the aquarium trade to speak of, so collection pressure is not a relevant concern at this time.