Taxonomy & naming
Hisonotus nigricauda was originally described by George Albert Boulenger in 1891 in the Proceedings of the Zoological Society of London (part 2, page 234, plate 25 figure 3), from material collected in Rio Grande do Sul, Brazil. The original combination was Otocinclus nigricauda; a subsequent synonym placed it as Microlepidogaster nigricauda before the current valid combination Hisonotus nigricauda was established. The Catalog of Fishes (Eschmeyer, CAS) recognises the valid name as Hisonotus nigricauda (Boulenger, 1891).
The genus Hisonotus was for many years a catch-all for small hypoptopomatin loricariids of the La Plata basin. A thorough systematic revision by Carvalho and Reis (2011), published in Neotropical Ichthyology, substantially clarified species limits within the Laguna dos Patos clade; that work documents H. nigricauda as widely distributed across the Laguna dos Patos coastal drainage system, fixing its standard length at a maximum of 1.5 in SL.
Within Loricariidae, Hypoptopomatinae is the basal subfamily housing the small, often depressed-bodied suckermouth catfishes sometimes called otocinclus relatives — though the group is considerably more diverse than the aquarium trade suggests. Hisonotus is the largest genus in the subfamily, with multiple species spread across subtropical South America.
Morphology
Hisonotus nigricauda is a tiny fish, reaching approximately 1.5 in standard length (SL) as an adult — equivalent to roughly 2.5 in total length (TL). The body is compressed and moderately depressed, typical of small hypoptopomatin loricariids, and is covered in rows of keeled bony scutes. Colouration is cryptic: a brownish-olive ground tone with fine darker mottling, providing excellent camouflage against the organic-matter substrate the species favours.
The most distinctive marking is the darkened caudal peduncle and tail base from which the specific epithet is derived (nigricauda = black tail). The caudal fin itself is typically dusky rather than intensely black, with the pigmentation concentrated near the base.
The mouth is positioned ventrally, forming the characteristic suckermouth disc common to all loricariids. The teeth are small, numerous, and bicuspid, suited to scraping fine algae and biofilm from substrate surfaces and plant stems rather than gouging wood. The eyes are laterally positioned and set relatively high on the head, a common orientation in Hypoptopomatinae.
Sexual dimorphism is not well documented for this species specifically. In closely related hypoptopomatin genera, females tend to be slightly more robust through the abdomen when gravid; males may develop fine odontodes on the pectoral spines and interopercular region, though these are not as elaborate as in some other loricariid subfamilies.
Habitat
Hisonotus nigricauda is endemic to the Laguna dos Patos coastal drainage system in southernmost Brazil, with a range extending into the lower Uruguay River basin and bordering regions of Argentina and Uruguay. The IUCN assessment (Carvalho and Reis, 2011) records the species as widely distributed in lower portions of tributaries near the lagoon, absent from the upper Jacuí basin, and also found in the lower Uruguay basin, headwaters of the Rio Negro, Rio Quaraí, and Rio Ibicuí — apparently absent above the mouth of the Rio Ibicuí.
Microhabitat, as described in the IUCN assessment, is characterised by slow-flowing, brown-water streams with dark organic-matter substrate or sand, and individuals are associated with marginal and aquatic vegetation. The tea-coloured, humic waters reflect leaf-litter tannins and low light penetration — a typical subtropical Atlantic-slope softwater environment.
FishBase lists the species as subtropical, freshwater, benthopelagic, and records it as a facultative air-breather — a characteristic shared across Hisonotus and related genera, enabling survival in poorly oxygenated, stagnant conditions that typify warm-season shallow stream pools. The species is recorded from within the Reserva Biológica do São Donato and the Área de Proteção Ambiental do Banhado Grande.
Feeding
Hisonotus nigricauda is an aufwuchs and biofilm grazer, using its ventrally positioned suckermouth to rasp fine periphyton — the mixed community of algae, diatoms, bacteria, detritus, and micro-invertebrates — from plant stems, submerged roots, leaf surfaces, and fine organic substrate. This feeding mode is consistent across the Hypoptopomatinae and distinct from the wood-eating xylophage strategy of some other loricariid groups.
No species-specific feeding studies are on record. By analogy with Otocinclus and other small hypoptopomatin genera, the diet in the wild is dominated by soft algae, periphytic films, and fine detrital particles. In the aquarium, small-bodied aufwuchs grazers of this type do best with a steady supply of surface algae on glass and décor, supplemented by algae wafers, spirulina-based foods, and blanched soft vegetables such as courgette (zucchini) or cucumber. Protein foods can be offered occasionally but should not dominate the diet.
Mating
No specific courtship observations are on record for Hisonotus nigricauda. The broader Hypoptopomatinae are understudied in this respect. Among the few hypoptopomatin loricariids for which breeding behaviour has been documented, courtship appears to involve males following females and investigating spawning sites on plant leaves or flat surfaces — behaviour consistent with the plant-associated, aufwuchs-grazer lifestyle of these small species.
Given the species' habitat in densely vegetated, slow-stream environments, spawning likely occurs on aquatic plant surfaces or submerged leaf debris. Males likely lack the elaborate interopercular odontode development seen in cave-spawning plecos, as surface-spawning loricariids typically show less secondary sexual elaboration of this kind.
Breeding
No captive breeding reports are on file for Hisonotus nigricauda. PlanetCatfish (record last updated December 2010) carries no breeding accounts, and the species has no registered keepers in that database. Breeding information is therefore absent from the cache and is not documented in the available literature at the species level.
By comparison with closely related small loricariids, this genus likely deposits adhesive eggs on plant leaves or other flat surfaces, with limited or absent parental care — the surface-spawning mode common in the Hypoptopomatinae, which contrasts with the paternal cave-guarding seen in cave-spawning plecos. Hobbyists attempting to breed this species should provide densely planted or heavily decorated tanks with clean, soft, slightly acidic subtropical water and a varied diet to condition adults.
In the aquarium
Hisonotus nigricauda is an uncommon aquarium fish with no established hobbyist following documented in the English-language trade as of the most recent data available. Its extremely small adult size (under 1.5 in SL) makes it suitable only for nano and species tanks or carefully chosen community set-ups with similarly tiny, peaceful inhabitants — it would be overwhelmed or outcompeted in a typical community tank.
Aquarium conditions should replicate the subtropical, slow-flowing, humic-water streams of the Laguna dos Patos region: temperature approximately 64–75 °F (the subtropical range appropriate for this species, which comes from climates with seasonal cooling rather than a true tropical thermal regime), soft to moderately soft water, slightly acidic to neutral pH, and significant plant cover or leaf litter to provide grazing surfaces and refuge.
Facultative air-breathing (recorded at the genus level by FishBase) means the fish can tolerate reduced dissolved oxygen to a degree, but clean, well-maintained water remains important for long-term health. Dense planting with fine-leaved plants or Anubias/Java fern provides both grazing substrate and shelter. The species is entirely peaceful and should not be kept with any fish large enough to view it as prey.
Conservation
The IUCN Red List assessed Hisonotus nigricauda as Least Concern in November 2018 (published 2022; DOI: 10.2305/IUCN.UK.2022-1.RLTS.T186753A1817864.pt). The assessment, conducted by the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio) and reviewed by A. Carapeto, notes that the species is abundant and frequent across its range, with no measurable threats identified. Population trend is listed as unknown.
The species is recorded within at least two protected areas: the Reserva Biológica do São Donato and the Área de Proteção Ambiental do Banhado Grande in southern Brazil. Its distribution across the coastal drainages of the Laguna dos Patos system, the lower Uruguay basin, and bordering areas of Argentina and Uruguay represents a reasonably wide range for a small, subtropical catfish.
The main potential long-term threats to this and related species in the Laguna dos Patos system are the intensification of agricultural land use, riparian vegetation clearance, and hydrological modifications to the slow-flowing, vegetated streams the species inhabits — none of which currently place it at risk of listing but which warrant continued monitoring.