Taxonomy & naming
Hisonotus armatus was formally described by Carvalho, Lehmann A., Pereira, and Reis in 2008 in Copeia (volume 2008, number 3, pages 510–516), based on type specimens collected from the Laguna dos Patos basin in Rio Grande do Sul state, southern Brazil. The Catalog of Fishes (Eschmeyer, CAS) recognises the valid combination Hisonotus armatus with CoF species ID 70456.
The species belongs to the subfamily Hypoptopomatinae within the family Loricariidae — the armoured suckermouth catfishes. Hisonotus is a diverse Neotropical genus of small rheophilic loricariids distributed primarily in Atlantic-slope drainages of southern and eastern South America.
H. armatus is distinguished from most congeners by a defining characteristic that gives rise to its epithet: the anterior margin of the snout is completely covered by odontodes. Only H. notatus and H. leucofrenatus share this feature, and H. armatus can be separated from those two species by the pattern on the caudal fin — a series of light hyaline spots arranged into a vertical bar, rather than the rounded hyaline blotch seen in the other pair. Additional characters include large plates in the abdominal median series.
Morphology
H. armatus reaches a maximum recorded length of 2.5 in total length (TL) in unsexed adults, placing it firmly among the smaller Hypoptopomatinae. The body is covered in the characteristic bony scutes of loricariids, forming a rigid armour over the head and trunk. As its name implies, the anterior tip of the snout is densely covered in odontodes (small hook-like denticles), a feature visible with close inspection and useful for separating this species from most congeners at a glance.
The caudal fin bears a pattern of light hyaline spots arranged into a transverse bar — a useful field mark distinct from the rounded blotch of the closely related H. notatus and H. leucofrenatus. Large plates characterise the abdominal median series. The ventral surface is largely unarmoured in the typical Hypoptopomatinae fashion, and the ventrally positioned mouth is adapted for scraping periphyton and aufwuchs from submerged surfaces.
Sexual dimorphism has not been formally described in the literature for this species. Like other loricariids, males may develop interopercular odontodes during the breeding season, but this has not been specifically documented for H. armatus.
Habitat
H. armatus is known from the Laguna dos Patos basin in Rio Grande do Sul, southernmost Brazil — a subtropical drainage that includes major rivers such as the Jacuí, Camaquã, and Gravataí. The Laguna dos Patos itself is the largest coastal lagoon in South America, and the catchment rivers drain through rolling highlands and lowlands of the Pampas and Atlantic Forest transition zone. The geographic range is restricted to the state of Rio Grande do Sul, at approximately 29–33°S latitude.
FishBase notes the species inhabits slow to moderately flowing watercourses with clear to brown water over sandy bottoms, often in or around marginal or submerged aquatic vegetation. The subtropical latitude imposes seasonal temperature variation that is significant by tropical loricariid standards. H. armatus is reported to occur sympatrically with Hisonotus laevior throughout its range, suggesting overlapping but not necessarily identical microhabitat preferences.
No water chemistry parameters (pH, conductivity) are recorded in the published literature or FishBase for this species. Given the southern latitude and Atlantic-slope origin, soft, slightly acidic to neutral water is probable, but any specific figures would be speculative.
Feeding
Like the majority of Hypoptopomatinae, H. armatus is presumed to be a periphyton and aufwuchs grazer — rasping algal films, diatoms, and associated organic matter from submerged surfaces using its ventrally placed, scraping mouth. The completely odontode-covered snout tip may facilitate feeding in microcrevices or on rough substrates.
No gut-content studies or specific dietary analyses have been published for this species in the available literature. By analogy with the broader subfamily, the diet is expected to comprise algae, biofilm, fine detritus, and microinvertebrates encountered while grazing. Genus Hisonotus members are also noted to be facultative air-breathers, a trait shared with several Loricariidae and likely of use in warmer, oxygen-depleted shallow-water microhabitats.
Mating
No courtship or mating observations have been published for H. armatus specifically. By comparison with other small Hypoptopomatinae, males likely defend small territories around shelter sites and display odontode development during the breeding season. The subtropical origin suggests possible seasonal reproductive cues tied to temperature cycles across the Laguna dos Patos catchment, where winter temperatures in Rio Grande do Sul can fall markedly.
Given the absence of data, further detail would be speculative.
Breeding
Breeding behaviour and reproductive biology have not been documented for H. armatus in the scientific literature, and the species is essentially unknown in the ornamental hobby, leaving no aquarium breeding accounts available. Small Hypoptopomatinae are generally cave or crevice spawners with paternal brood guarding, but applying that pattern directly to H. armatus remains an inference from family-level biology rather than observation.
Data on clutch size, incubation period, or larval development are not available.
In the aquarium
Hisonotus armatus is not an established aquarium fish. It does not appear in the ornamental trade under any recognised common name, and there are no husbandry accounts on record. Any aquarist obtaining specimens — likely through specialist scientific or conservation channels — would be working without a practical road map.
General Hypoptopomatinae husbandry guidelines suggest that small species in this group do best in mature, well-oxygenated tanks with moderate current, biofilm-covered surfaces for grazing, and appropriate temperature ranges reflecting their wild climate. For H. armatus, the subtropical origin would require accommodation of cooler seasonal temperatures not typical for tropical loricariid setups — perhaps 64–79 °F based on the latitudinal range, though no confirmed figures exist.
Given the restricted endemic range and absence of trade activity, this species is best regarded as a fish for specialist researchers rather than the general hobby.
Conservation
The IUCN Red List assessed Hisonotus armatus as Least Concern (LC) on 7 November 2018, reflecting no evidence of significant population decline and a range within a relatively large hydrographic basin. The Laguna dos Patos catchment, though subject to agricultural intensification and riparian degradation across Rio Grande do Sul, remains a functioning drainage system supporting diverse loricariid fauna.
The species is not commercially collected, which removes trade pressure as a concern. However, endemic subtropical loricariids restricted to a single river system remain inherently vulnerable to catchment-scale changes — sedimentation from agriculture, channelisation of watercourses, and habitat degradation of marginal vegetation could all affect local populations. Long-term monitoring of H. armatus and its congeners in the Laguna dos Patos basin would strengthen the evidence base for future assessments.