Taxonomy & naming
Hisonotus brunneus was described by Carvalho and Reis in 2011 as part of a taxonomic review of Hisonotus species from the laguna dos Patos basin of southern Brazil, published in Zootaxa. The Catalog of Fishes (Eschmeyer, CAS) recognises Hisonotus brunneus Carvalho & Reis, 2011 as a valid species. The type locality is the rio Passo Novo, an affluent of the rio Ingaí within the upper rio Jacuí drainage, near the municipality of Cruz Alta, Rio Grande do Sul, Brazil.
The genus Hisonotus belongs to subfamily Hypoptopomatinae of the family Loricariidae — one of the smaller-bodied loricariid lineages, sometimes informally grouped with the otocinclus-like dwarf suckermouth catfishes. Hypoptopomatinae is characterised by small adult size, a broadly plated body, and an ecological specialisation toward biofilm grazing in flowing waters.
The genus name Hisonotus derives from the Greek isos (equal) combined with noton (back), a reference to a feature of the dorsal scute arrangement. The species epithet brunneus is straightforward Latin for brown, referring to the ground colour of the body — a modest but accurate descriptor for a small catfish that blends readily into sand and leaf-litter substrates.
Morphology
Hisonotus brunneus is a very small loricariid, reaching a maximum recorded standard length of 1.5 in. This places it firmly in the dwarf suckermouth category alongside the otocincluses and other Hypoptopomatinae. The body is dorsoventrally flattened and armoured with bony scutes in the loricariid fashion, though the overall form is more compact and less elongated than twig catfishes or whiptails.
As the epithet implies, the colouration is brown — the original description from Carvalho and Reis notes the brown body coloration as a diagnostic character of the species, though details of the precise pattern and any markings visible in life are not available in the sources consulted here.
Sexual dimorphism in Hisonotus broadly parallels that of related hypoptopomatines: females may be broader-bodied when gravid, and males may develop minor odontodes on the snout or body in breeding condition. Species-specific dimorphism data for H. brunneus is not available from the sources consulted.
Habitat
Hisonotus brunneus is known from tributaries of the rio Jacuí drainage within the laguna dos Patos system, Rio Grande do Sul — the southernmost major river drainage of Brazil and one that drains a subtropical rather than tropical landscape. The specific habitat recorded at the type locality on the rio Passo Novo is described as a median to fast-flowing watercourse with clear to brown water over sandy or stony substrate, with submersed marginal grasses.
This is a cool-to-warm subtropical stream environment, subject to genuine seasonal temperature fluctuation — Rio Grande do Sul experiences cool winters with temperatures that may approach 59 °F in stream water, and warm summers. The presence of clear to brown water suggests variable tannin and dissolved organic matter input depending on local substrate and vegetation.
The upper rio Jacuí is an agricultural landscape; Cruz Alta municipality sits in a region of soy and wheat cultivation, meaning the riparian corridors and water quality of these streams face significant anthropogenic pressure.
The species is not known to have been collected outside the upper rio Jacuí basin.
Feeding
Hisonotus species are biofilm and periphyton grazers, applying their ventral suckermouth to stream substrates — sand, fine gravel, stones, and submerged plant stems — and rasping the thin layer of algae, bacteria, diatoms, and fine detritus that colonises these surfaces. The small body size of H. brunneus is consistent with a feeding niche in fine interstitial microhabitats and on the surfaces of submersed grasses, which the type locality habitat description specifically notes.
In the aquarium, related dwarf hypoptopomatines accept soft algae on glass and decor, supplemented by fine-particulate foods such as powdered spirulina, algae wafers, and biofilm allowed to develop on mature surfaces. Unlike larger plecos, Hisonotus species do not rasp driftwood significantly and do not require wood as a dietary component.
Mating
Reproductive biology of Hisonotus brunneus specifically is unknown from the published record. Hypoptopomatinae generally reproduce in a manner consistent with other small loricariids: males likely establish small territories and females deposit eggs on clean surfaces, with some degree of male guarding.
The subtropical origin of H. brunneus suggests that seasonal temperature cycling — cooler winter, warmer summer — may play a role in triggering reproduction, as is common in fish from climatically variable temperate and subtropical regions. This would differentiate it from tropical loricariids that may breed year-round under stable conditions.
Breeding
No captive breeding records for Hisonotus brunneus are available. The species is not established in the hobby trade, and it is unclear whether any individuals have been maintained in aquaria outside of scientific collection.
General Hypoptopomatinae breeding, where documented, involves deposition of small adhesive eggs on flat surfaces or plant leaves, with the male providing at minimum some degree of territorial guarding. Fry are miniature adults capable of grazing biofilm shortly after hatching.
Data on this species remains too sparse to describe its breeding biology with any confidence.
In the aquarium
Hisonotus brunneus is not in the aquarium trade as of 2026. No hobbyist accounts or care records are available; the following notes are generalised from the species' known biology and habitat.
If encountered, the species would require a setup suited to a subtropical southern-Brazilian stream catfish: moderate temperatures reflecting seasonal variation (perhaps 64–79 °F), clean, well-oxygenated water with gentle to moderate current, a fine sandy or gravel substrate, and marginal plant growth. The very small adult size (1.5 in SL) means it could be maintained in a modest aquarium, but its likely sensitivity and obscure biology would make it a specialist undertaking rather than a casual community fish.
Its absence from trade means tank-raised stock is not available, and wild collection from the upper rio Jacuí would face regulatory and practical obstacles. This is not a species the average hobbyist is likely to encounter.
Conservation
The IUCN Red List assessed Hisonotus brunneus as Data Deficient (DD) in November 2018, reflecting insufficient information on population size, distribution extent, and threats to assign a threat category. Data Deficient is not a reassurance — it means the species is poorly known, not that it is secure.
The known range of H. brunneus is restricted to the upper rio Jacuí basin in Rio Grande do Sul, an intensively farmed region of Brazil where soy, wheat, and livestock agriculture dominate the landscape. Riparian forests in this region have been substantially cleared, reducing the quality and connectivity of small stream habitats. Sedimentation and pesticide runoff from surrounding farmland affect stream communities throughout the laguna dos Patos basin.
Given the restricted known range and the pressured landscape it inhabits, a precautionary approach to conservation planning for this species is warranted. A reassessment incorporating targeted field surveys to determine the actual extent of its distribution would be valuable.