Taxonomy & naming
Hisonotus montanus was described by Carvalho and Reis in 2009 in Zootaxa (volume 2113, pages 1–28), a monograph that described four new Hisonotus species from the upper Rio Uruguay and reviewed the genus across the entire drainage. The three companion new species are H. iota, H. leucophrys, and H. megaloplax; the paper also transferred Epactionotus aky to Hisonotus at the same time. The type specimens were collected from the Rio Canoas drainage, upper Rio Uruguay basin, Santa Catarina state, Brazil, at approximately 2789 ft above sea level.
The Catalog of Fishes (Eschmeyer, CAS) recognises the valid name Hisonotus montanus Carvalho & Reis, 2009. The genus Hisonotus belongs to the subfamily Hypoptopomatinae, a diverse assemblage of small to minute armoured catfishes distributed across South America east of the Andes. In 2022, Reis and Lehmann Albornoz erected the genus Rhinotocinclus and transferred 18 species from Hisonotus and allied genera; H. montanus was not among those transferred and remains in Hisonotus.
The species is morphologically distinguished from most Hisonotus by a specific character of the sensory canal system: the infraorbital canal enters the infraorbital series via the compound pterotic — a feature shared with only H. aky and H. iota within the genus. From those two species, H. montanus is separated by its wider light stripes on the dorsal surface of the head, which extend from the snout tip posteriorly without interruption.
Morphology
Hisonotus montanus is a small loricariid reaching approximately 2 in SL, placing it among the smaller members of a genus that itself runs small. The body follows the standard Hypoptopomatinae plan: strongly depressed and fully armoured in interlocking scutes, with a ventrally positioned suctorial mouth equipped for scraping hard surfaces. The overall form is compact and oval in dorsal view.
The diagnostic colour pattern feature — relatively wide pale stripes running from the tip of the snout posteriorly along the dorsal surface of the head — distinguishes this species from its two closest relatives within the upper Uruguay fauna. The infraorbital canal configuration (entering the infraorbital series via the compound pterotic) unites H. montanus, H. aky, and H. iota in a character state that is absent in most other Hisonotus.
Full morphometric details are documented in the 2009 type description (Carvalho & Reis, Zootaxa 2113), which provides holotype measurements and comparisons with all congeners known at that time. Sexual dimorphism has not been reported for this species.
Habitat
Hisonotus montanus is known exclusively from the Rio Canoas drainage, a headwater tributary of the upper Rio Uruguay in Santa Catarina state, southern Brazil. Its collection at approximately 2789 ft above sea level makes it the highest-elevation Hisonotus yet documented in the Uruguay basin, occupying montane stream habitats where the river system is still narrow, fast-flowing, and heavily shaded by gallery forest.
No direct water chemistry measurements from the type locality have been published in accessible sources. Montane headwaters of the upper Rio Uruguay in Santa Catarina are characterised by cool temperatures — southern Brazil's upland streams at this elevation typically range approximately 59–72 °F seasonally — soft water with low mineral content, and pH values from mildly acidic to circumneutral (approximately 6.0–7.5). Substrates in these fast headwater reaches are predominantly cobble, pebble, and exposed bedrock, providing the hard surfaces on which Hisonotus species graze periphyton and algae.
Feeding
No stomach content or direct dietary data have been published for H. montanus. The feeding mode is consistent with the genus: Hisonotus species are algivores and periphyton grazers, using their suctorial ventral mouth to scrape microalgae, diatoms, and associated biofilm from hard surfaces. The fine, multi-cusped teeth characteristic of Hypoptopomatinae are adapted for this scraping mode rather than the wood-rasping seen in Loricariinae or the large vegetation consumption of Hypostominae. At 2789 ft elevation, fast-flowing, well-lit headwater riffles support productive diatom and algal communities on exposed rocks, providing the dietary base this species almost certainly relies upon.
Mating
No mating behaviour observations exist for H. montanus, and no captive specimens have been maintained. Hypoptopomatinae show broadly consistent reproductive behaviour at the genus level: males are typically the parental sex, and seasonally conditioned males develop odontodes on the snout and cheeks. No specific mating observations or secondary sexual characters have been documented for this species.
Breeding
Breeding biology is entirely undocumented for H. montanus. No captive spawning has been reported. Based on the genus-level inference available for Hisonotus and close Hypoptopomatinae relatives, breeding likely involves adhesive egg deposition on a hard substrate surface, followed by brief parental attendance, but this remains speculative in the absence of any direct data for this species or close relatives in captivity.
In the aquarium
Hisonotus montanus has not been maintained in captivity and is entirely absent from the ornamental trade. Its restricted distribution, apparent rarity (data limited to a single drainage and a single scientific description), and montane specialist ecology mean it is unlikely to become available to hobbyists. Were specimens ever obtained, the high-elevation origin implies a preference for cool, highly oxygenated water (temperatures possibly as low as 59–64 °F), excellent current and filtration, and hard surfaces for algae grazing. These requirements are speculative and based entirely on habitat inference; no first-hand aquarium accounts exist.
Conservation
The IUCN Red List assessed Hisonotus montanus as Data Deficient (DD) in 2018. The Data Deficient designation reflects the genuine scarcity of information: the species is known from a single drainage system (Rio Canoas, upper Rio Uruguay) at a single high-elevation site in Santa Catarina, and no population surveys, abundance estimates, or ecological monitoring data have been published since the 2009 description.
The upper Rio Uruguay basin in Santa Catarina faces real conservation pressure. Expansion of agriculture — particularly soybean cultivation and cattle ranching — drives riparian deforestation and sedimentation of headwater streams. The Rio Uruguay drainage has also seen significant hydroelectric development along its main channel. Headwater streams at 2789 ft elevation are exposed to land-use change on the plateau above them, and the loss of gallery forest cover directly affects the water temperature, sediment load, and substrate quality of the small streams these catfishes depend on. Until baseline surveys establish the species' full range and population status, the DD assessment is the most honest available summary: there is not enough information to determine whether H. montanus is threatened.