Taxonomy & naming
Brachyhypopomus gauderio was described by Giora & Malabarba in 2009 (Zootaxa 2093) from central, southern and coastal Rio Grande do Sul in Brazil, with further material from Uruguay and Paraguay. It had previously been confused with and identified as Brachyhypopomus pinnicaudatus, from which it is distinguished by body coloration (a yellowish dorsal ground colour with brown reticulate markings), anal-fin ray counts, the position of the anal-fin origin, and the shape of the caudal filament in mature males.
The genus Brachyhypopomus sits in the family Hypopomidae, one of several families within the order Gymnotiformes — the New-World electric knifefishes. Like its relatives it produces a weak electric organ discharge (EOD) used for electrolocation and communication, and in this genus the discharge is of the PULSE type: brief, discrete pulses rather than a continuous wave. The species epithet 'gauderio' is a direct reference to the gaúcho or pampas-dweller of Rio Grande do Sul, tying the name to the fish's home range rather than to any anatomical feature. Catalog of Fishes is the authority for the valid name.
Morphology
Brachyhypopomus gauderio is a small, soft-bodied, laterally compressed fish reaching a recorded maximum of around 6 in standard length. Like other bluntnose knifefishes it has a blunt, rounded snout rather than the elongated probing snout of the sand knifefishes, no dorsal fin, and no separate tail fin — locomotion comes from a long anal fin run as a travelling wave along the underside of the body, letting the fish move forward and backward with equal control. The body is entirely without scales.
A weakly electric organ runs along the body and drives the pulse-type discharge that is central to how this fish senses its surroundings. Mature males show a distinctive hypertrophy of the caudal filament associated with electrical signalling, one of the external features used to tell the sexes apart and to distinguish this species from its close relative B. pinnicaudatus. Sampled adults ranged from roughly 1.5–7.5 in in males and 2–7 in in females, underlining how small and delicate the species stays throughout life.
Habitat
The species is known from Laguna dos Patos and the Rio Tramandaí system on the coastal plain of Rio Grande do Sul in southern Brazil, extending into the Rio Uruguay basin in Brazil and Uruguay and into the Rio Paraguay basin in Paraguay — a relatively confined, subtropical range at the southern edge of where Gymnotiformes occur in South America.
Field data describe warm-to-mild water, with recorded temperatures spanning roughly 59–85 °F, near-neutral pH around 7.1–7.3, and often quite low dissolved oxygen (as little as 0.25–1.7 mg/l) — the kind of still, vegetated, oxygen-poor margin habitat typical of small bluntnose knifefishes, which tend to shelter among submerged plants and leaf litter by day and become active at night, relying on electrolocation rather than eyesight to find their way in the dark and in turbid water.
Feeding
Brachyhypopomus gauderio is omnivorous-carnivorous, with a trophic level around 3.1 consistent with a diet dominated by small invertebrates. Studied populations feed on autochthonous aquatic insects, chironomid (midge) larvae, cladocerans and copepods gleaned from among vegetation and sediment — prey it most likely locates in large part by electrolocation, sensing the weak electrical signatures and water disturbances of small animals hidden in cover after dark.
As a small, delicate nocturnal predator it is best matched in the aquarium with correspondingly small, soft-bodied foods: bloodworm, daphnia, brine shrimp, chopped blackworm and similar frozen or live fare offered after lights-out, echoing the insect- and crustacean-heavy diet documented in the wild.
Mating
Courtship and spawning cues in Brachyhypopomus gauderio are tied to its electric sense, as in other weakly electric knifefishes: the pulse-type discharge carries information about identity, sex and condition, and is likely used by both sexes to locate and assess potential mates in the dark, tannin-stained or turbid water they inhabit. The pronounced caudal-filament development seen in mature males is thought to relate to this electrical signalling rather than to any visual display.
Field studies of a Rio Grande do Sul population found the species to be a fractional spawner, releasing eggs in batches rather than as a single clutch, with breeding activity concentrated in the Southern Hemisphere spring and summer (roughly October to February) and apparently triggered by photoperiod. First maturation was recorded at approximately 4.5 in in males and 4 in in females.
Breeding
This species is an egg-layer, and unusually for a small hypopomid its reproductive biology has been documented in some detail in the wild: fecundity in the studied population ranged from roughly 299 to 799 oocytes per female (average around 589), a relative fecundity of about 0.20 oocytes per milligram of body weight. Distinctively, paternal care has been observed — males guard the larvae under vegetation after spawning, rather than the 'little or no care' typical of many small Gymnotiformes.
None of this field-documented biology has been translated into a known home-aquarium breeding protocol; the species is essentially unrecorded in captive breeding, and its wild reproductive cycle (photoperiod-triggered, seasonal, fractional spawning) would be difficult to replicate without close attention to season and water chemistry. Prospective breeders should treat this as data sparse in the aquarium context and expect, at best, to observe courtship behaviour rather than a repeatable spawning method.
In the aquarium
Brachyhypopomus gauderio is rarely if ever seen in the aquarium trade and almost nothing has been written about keeping it in captivity, so any husbandry approach has to be extrapolated cautiously from its wild ecology and from the general needs of small bluntnose knifefishes. Field conditions point to a warm-temperate to warm setup, soft and near-neutral water, dim lighting, and dense cover of fine-leaved plants, roots or leaf litter for a fish that is most active at night and uses electrolocation rather than sight.
Being scaleless, it should be treated with the same caution as other Gymnotiformes regarding water quality and medication: no copper-based treatments, reduced dosing generally, and clean, low-nitrate water maintained with a mature filter. Its small size and soft body make it unsuited to boisterous tankmates; a peaceful, similarly-scaled community with no fin-nippers and no fish large enough to see it as food is the safer approach. Given how little is documented, this is a species for the specialist prepared to accept considerable uncertainty rather than a fish with an established care sheet.
Conservation
Brachyhypopomus gauderio is assessed as Least Concern on the IUCN Red List (assessed September 2020), reflecting a range that spans multiple river systems and lagoons across southern Brazil, Uruguay and Paraguay rather than a single vulnerable population.
That said, its core range on the coastal plain of Rio Grande do Sul — Laguna dos Patos and the Rio Tramandaí system — sits in an area subject to ongoing agricultural development, wetland drainage and water-quality pressure, and low-oxygen-tolerant, vegetation-dependent species of this kind are generally sensitive to habitat degradation even where the formal IUCN status is not (yet) elevated. As with other small, poorly documented Gymnotiformes, continued field monitoring across its Brazilian, Uruguayan and Paraguayan range would be needed to catch any future decline early.