Taxonomy & naming
Hisonotus charrua was described by Almirón, Azpelicueta, Casciotta, and Litz in 2006, in the Revue suisse de Zoologie (volume 113, number 1, pages 87–94). The type locality is Cañada de Los Peña, Departamento Tacuarembó, República Oriental del Uruguay, within the Río Uruguay basin. The Catalog of Fishes (Eschmeyer, CAS) recognises the valid combination Hisonotus charrua Almirón, Azpelicueta, Casciotta & Litz, 2006.
The genus Hisonotus belongs to the tribe Otothyrini within the subfamily Hypoptopomatinae, one of the five subfamilies of Loricariidae. The original authors placed the species explicitly in Otothyrini, the group that also contains Otothyris and closely related miniature loricariids of the subtropical and temperate South American drainages. These fishes are ecologically analogous to the more widely kept Otocinclus but represent a distinct evolutionary lineage.
Within Hisonotus, H. charrua is distinguished by a caudal-fin pattern that is among its most reliable external characters: a conspicuous pale transversal band composed of hyaline spots against a dark background, with a second posterior band visible in larger individuals. This feature, along with head proportions and scale counts, separates it from congeners occurring elsewhere in the Río de la Plata system.
Morphology
Hisonotus charrua is a very small catfish: maximum recorded size is 1.5 in standard length (SL). Like all hypoptopomatines, it is dorsoventrally flattened, with a broad, depressed head and a ventrally placed suctorial mouth ringed by fleshy lips bearing fine, multicuspid teeth well-suited to rasping algae and periphyton from hard surfaces. The body is covered in bony scutes arranged in longitudinal series; the armoured plating is complete from head to caudal peduncle, giving the fish a rigid, tank-like appearance at odds with its tiny size.
Ground colour is brown to greyish-brown with variable darker mottling. The most diagnostic colour character is the caudal fin: a prominent pale (hyaline) transversal band crossing the middle of the fin against a dark background; in larger individuals a second, more posterior pale band may be visible. The fins are otherwise plain or faintly spotted. The eyes are positioned dorsally on the head, giving the fish excellent overhead vision while pressed against a substrate.
Sexual dimorphism is subtle in small loricariids. Gravid females may show a slightly broader body profile when viewed from above, and differences in papilla shape between sexes are likely present but have not been formally documented for this species.
Habitat
Hisonotus charrua is a subtropical stream fish with a distribution centred on the middle and lower portions of the Río Uruguay basin. The type locality in Departamento Tacuarembó, Uruguay, represents one of the more northerly records; the confirmed range extends south through the Río Negro drainage in Uruguay, and east into the coastal streams of Uruguay and at least one headwater tributary of the Laguna dos Patos system in Rio Grande do Sul, Brazil. A significant first Argentine record comes from Parque Nacional El Palmar, Provincia de Entre Ríos, extending the known range into northeastern Argentina.
Microhabitat data from the original description indicate small to medium creeks with loose stones and gravel substrates and median to rapid water flow. The fish rests and forages on the surfaces of loose stones and gravel, gripping with the suctorial mouth to resist current. These are clear-water, well-oxygenated streams; the subtropical latitude means pronounced seasonal temperature variation compared to the more stable Amazonian rivers where most loricariids occur. The genus is noted as a facultative air-breather, a useful adaptation in shallow, seasonally warm or low-flow conditions.
Feeding
Hisonotus charrua is an algae and periphyton micro-grazer. The fine-toothed ventrally positioned mouth is adapted for scraping thin layers of biofilm — diatoms, green algae, cyanobacteria, and associated detritus — from the surface of stones, gravel, and other hard substrates. This feeding mode is characteristic of the tribe Otothyrini and is closely analogous to that of the popular aquarium genus Otocinclus.
In aquaria, the species would be expected to graze continuously on glass, rock surfaces, and broad plant leaves. Supplementary diet should reflect the natural biofilm-grazing habit: high-quality algae wafers, spirulina-based sinking foods, and blanched soft vegetables such as courgette (zucchini) or cucumber. Because of the small size and correspondingly small gut capacity, frequent access to grazing surfaces is more important than large discrete feedings.
Mating
No specific observations of courtship behaviour in Hisonotus charrua are available in the literature. By analogy with other members of the tribe Otothyrini and with the closely related and better-studied Otocinclus, males are likely to pursue females actively as conditioning proceeds and may show some level of site attachment to preferred flat grazing surfaces.
The small body size of this species means that pairs or small groups can be accommodated in modest aquarium volumes, and conditioning on a varied diet with clean, well-oxygenated water would be the standard approach to encouraging reproductive behaviour in a loricariid of this type.
Breeding
No captive breeding record for Hisonotus charrua has been found in the available literature. The reproductive mode is not formally documented for this species. Based on what is known of related hypoptopomatine loricariids including Otocinclus species, flat-surface spawning — where adhesive eggs are deposited directly on broad plant leaves, smooth stones, or the aquarium glass — is the most likely mode; this is distinct from the cave-spawning typical of the Hypostominae.
Any aquarist attempting to breed H. charrua would be working with very sparse prior information, and detailed records of water parameters, egg numbers, incubation times, and fry development for this species are currently lacking. A conservative approach following Otocinclus breeding protocols — soft, slightly acid water; conditioning on live foods; a planted tank with broad-leaved plants — would be a reasonable starting point.
In the aquarium
At 1.5 in SL, Hisonotus charrua is among the smallest loricariids likely to be encountered, and its aquarium requirements reflect that scale. A small species tank or a planted nano aquarium would suit a group of three to six individuals; even a modestly sized 40-litre tank provides sufficient territory for multiple fish if clean water is maintained and algae are available.
Water quality is critical: small loricariids are sensitive to poor water conditions, and the subtropical stream habitat of H. charrua is well-oxygenated and chemically clean. Moderate current from a quality filter, regular partial water changes, and zero tolerance of ammonia or nitrite are essential. The subtropical origin suggests this species can tolerate cooler temperatures than many tropical loricariids — the seasonal streams of Uruguay experience temperatures well below 68 °F in winter — but the optimal range for captive maintenance is not established from the literature.
Decoration should include smooth rounded river stones and gravel as primary grazing surfaces. Dense planting with broad-leaved species (Anubias, Echinodorus) provides additional grazing area and natural cover. The fish is not known to damage plants and is peaceful toward dissimilar tankmates. Compatibility with small, peaceful fish from similar temperate-subtropical South American drainages — such as smaller tetra species, Corydoras, or other non-competing bottom dwellers — would be appropriate.
H. charrua is not an established aquarium trade species and is unlikely to be encountered in general retail. It has no L-number designation and does not appear in the major pleco trade catalogues. Specimens would typically reach hobbyists only through specialist importers or field collection.
Conservation
The IUCN Red List assessed Hisonotus charrua as Least Concern, with the assessment completed on 8 October 2020. The species has a documented presence across multiple drainage systems in Uruguay, Rio Grande do Sul (Brazil), and Entre Ríos (Argentina), and no major decline was identified at the time of assessment.
The Río Uruguay basin and its associated coastal drainages face moderate pressure from agriculture, riparian deforestation, water abstraction, and watershed modification. The gravel- and stone-bottomed streams that H. charrua occupies are susceptible to siltation from erosion and land clearing, which can smother the hard substrate on which these micro-grazers depend. The species' restricted body size and apparent association with fast-flowing, clear-water microhabitats means it could be locally sensitive to habitat degradation even if range-wide population levels remain stable.
As a recently described species (2006) with a limited published record, the biology and population structure of H. charrua remain understudied, and the Least Concern assessment reflects absence of evidence of decline as much as confirmed population health.