Taxonomy & naming
Hisonotus paulinus was originally described by Charles Tate Regan in 1908 on the basis of material from the Rio Piracicaba, São Paulo, Brazil — a tributary of the Tietê River and part of the upper Paraná drainage. Regan placed it in the genus Microlepidogaster, giving the combination Microlepidogaster paulinus. It was subsequently transferred to Otocinclus and circulated for some decades as Otocinclus paulinus before Eigenmann and Eigenmann's broader Hisonotus framework was refined and the species was moved to its current valid combination, Hisonotus paulinus (Regan, 1908).
The Catalog of Fishes (Eschmeyer, CAS) recognises Hisonotus paulinus as the valid name, with Microlepidogaster paulinus and Otocinclus paulinus both treated as junior synonyms. Hisonotus belongs to the subfamily Hypoptopomatinae — the smaller, often slender otocinclus-allied loricariids — which is well represented in São Paulo's Atlantic Forest drainages and the upper Paraná system. The name Hisonotus depressinotus is treated as a synonym of this species in Eschmeyer's Catalog of Fishes.
Morphology
Hisonotus paulinus is a small fish, reaching approximately 1.5 in standard length (SL) — placing it firmly among the diminutive end of the Hypoptopomatinae. Like all members of the subfamily, it is encased in the characteristic loricariid armour of keeled bony scutes rather than scales. The mouth is ventral and disc-like, adapted for rasping attached algae and biofilm from hard substrates.
Detailed colour pattern and meristic data for this species are sparse in the accessible literature. The general hypoptopomatine body plan applies: a somewhat flattened ventral profile suited for clinging to rock and wood surfaces in flowing water, with eyes positioned dorsally for surveying the water column above while the fish feeds against the substrate. Beyond size and the meristic details in Regan's original description, morphological information specific to H. paulinus is not well documented in recent literature.
Habitat
The type locality of Hisonotus paulinus is the Rio Piracicaba, a tributary of the Tietê River in São Paulo state, Brazil. The Tietê drains eastward across the São Paulo plateau before bending west to meet the Paraná River, placing H. paulinus within the upper Paraná basin — one of South America's major river systems and a region of exceptionally high loricariid diversity.
The Rio Piracicaba and the broader Tietê catchment encompass a range of habitat types, from forested headwater streams on the Serra da Mantiqueira slopes to slower lowland reaches. Small hypoptopomatines typically favour shallow, rocky or cobble-bottomed stretches of clear to slightly turbid streams with moderate current, where algae and periphyton colonise exposed surfaces. Specific temperature, pH, or conductivity data for H. paulinus collection sites are not available in the accessible literature; given the subtropical São Paulo plateau (elevation roughly 1312–2625 ft), ambient water temperatures are likely cooler and more seasonally variable than in Amazonian lowlands.
Feeding
Hisonotus paulinus is an aufwuchs grazer — a micro-algae and biofilm feeder in the tradition of the Hypoptopomatinae more broadly. Using its disc-like ventral mouth, it rasps attached algae, diatoms, detrital films, and associated invertebrate matter from rock, root, and wood surfaces in flowing water. This guild of feeding behaviour is characteristic of the otocinclus alliance and is strongly tied to well-lit, current-swept substrates where periphyton accumulates.
No species-specific feeding or gut-content studies for H. paulinus are documented in the accessible literature. Inferences about diet are drawn from the general Hypoptopomatinae feeding mode and the type of microhabitat the species likely occupies in the Tietê system.
Mating
No courtship or mating observations for Hisonotus paulinus have been reported in the scientific or hobbyist literature. Data are sparse for this species. The general pattern in Hypoptopomatinae involves males defending spawning sites and displaying to females, but no species-specific information can be reported for H. paulinus without risking fabrication.
Breeding
No breeding records for Hisonotus paulinus in captivity or in the wild are known from the accessible literature. The species does not appear to have been kept or bred in the hobby in documented numbers. By analogy with other small hypoptopomatines, the reproductive mode is likely egg-scattering or deposition on substrate surfaces, possibly with short-term parental association, but no data specific to H. paulinus are available.
In the aquarium
Hisonotus paulinus does not appear to be established in the aquarium hobby. It is not listed in commercial availability records and has no documented keeping or breeding reports. Given its small adult size (approximately 1.5 in SL), it would in principle be suited to a modest-sized planted or biotope stream aquarium mimicking the upper Paraná / São Paulo plateau habitat — clean, well-oxygenated, slightly cool water with smooth rocks and driftwood for algae grazing.
In the absence of any husbandry data, no specific tank size, water parameter, or compatibility recommendations can be offered honestly. If specimens were ever to become available, the keeper would be doing novel work, and careful documentation of conditions, feeding response, and behaviour would be valuable to the wider community.
Conservation
The IUCN Red List assessed Hisonotus paulinus as Data Deficient (DD) in 2018, reflecting insufficient information on its range, population size, and trends — not a confirmed determination of rarity or abundance. The Data Deficient designation is itself a signal of concern: the species has not been surveyed recently enough to determine its status with confidence.
The Tietê River basin, including the Rio Piracicaba type locality, sits within and around the greater São Paulo metropolitan region — one of the most densely urbanised river catchments in South America. The Tietê mainstream has suffered severe water quality degradation from industrial and domestic effluents, though restoration programmes over recent decades have improved conditions in some stretches. Smaller tributaries like the Piracicaba system face ongoing pressures from urban expansion, agricultural runoff, flow regulation, and riparian deforestation. For a small-bodied loricariid with an apparently restricted range tied to this catchment, these stressors represent a plausible threat — but without current survey data, the actual impact on H. paulinus cannot be quantified.