Taxonomy & naming
Hypostomus borellii was described by George A. Boulenger in 1897 on the basis of specimens collected during Alberto Borelli's zoological expeditions to northwestern Argentina. The species belongs to the large, polyphyletic genus Hypostomus Lacépède, 1803, which encompasses well over 100 valid species and has long been a focus of revisionary work. The borellii species group — loosely delimited by its combination of spot patterning and moderate odontode armature — is recognised by several researchers but formal phylogenetic circumscription of the group remains incomplete. Synonyms that have appeared in the literature include Plecostomus borellii and Hypostomus aurantiacus in some older regional checklists, but Catalog of Fishes treats H. borellii as a valid species distinct from both. The authority parentheses indicate the species was originally placed in a different genus before being moved to Hypostomus. The specific epithet honours the Italian naturalist and explorer Alberto Borelli (1858–1942), who made multiple collecting expeditions to Argentina and Paraguay in the 1890s and provided type material for several South American vertebrate species.
Morphology
Hypostomus borellii is a moderately built loricariid, typically reaching 8–10 in standard length in the wild, though aquarium specimens rarely exceed 8 in. The body is depressed and covered in bony scutes bearing fine odontodes; the dorsal surface is brownish to olive-grey, heavily marked with dark spots or reticulations that vary in density across individuals and populations. The ventral surface is paler, often cream to buff. The large, subterminal mouth is armed with numerous bicuspid teeth adapted for scraping; the lips are broad and papillose, expanding into a sucker disc. The dorsal fin is sail-like when fully erect, with 7 branched rays, and the pectoral fins bear robust, odontode-covered spines that can inflict minor wounds when handling. As in most Hypostomus, the caudal fin is forked and the lower lobe is slightly longer. No striking sexual dimorphism is reported, though breeding males may develop enlarged genital papillae and slightly thickened pectoral spines with more prominent odontodes.
Habitat
The species is associated with the lowland reaches of the Río Pilcomayo and the middle and upper Río Paraguay, including tributaries draining the Chaco lowlands of Argentina, Paraguay, and extreme southern Bolivia. These are typically warm, slow to moderately flowing rivers with high turbidity, soft to moderately hard water, and substrates of sand, silt, and submerged woody debris. The Pilcomayo in particular undergoes dramatic seasonal fluctuations in flow and chemistry driven by Andean melt and Chaco rains; during the low-water dry season, fish may be concentrated in remnant pools. Water temperatures in the range habitat span roughly 72–86 °F (72–86 °F) seasonally, with pH tending toward neutral to slightly alkaline (7.0–8.0) reflecting the calcareous nature of much Chaco runoff. Hypostomus borellii is benthic and cryptic, resting against hard substrates, wood, or undercut banks during the day and foraging more actively after dusk.
Feeding
Like the majority of Hypostomus species, H. borellii is predominantly an aufwuchs and biofilm grazer. The robust scraping dentition and large sucker mouth allow it to rasp algae, diatoms, and associated organic material from submerged wood, rock, and compacted mud. Gut content studies on ecologically similar Hypostomus species from the Paraguay-Paraná system document algae (particularly filamentous green algae and diatoms), fine detritus, fungal hyphae, and small amounts of aquatic invertebrate fragments as incidental inclusions. Woody material (lignocellulose) is ingested in variable amounts — the species does not appear to be a specialised wood-eater in the manner of Panaque or Panaqolus, but opportunistic consumption of submerged wood and leaf litter likely supplements its dietary intake of carbohydrates. In captivity the species accepts sinking algae wafers, blanched vegetables, and driftwood, and will opportunistically take protein-rich foods, though a diet weighted toward plant and algal matter is appropriate.
Mating
Breeding behaviour in Hypostomus borellii has not been described in detail in the scientific literature, and dedicated aquarium breeding accounts are sparse. Based on the biology of related Hypostomus species from the same drainage system — including H. commersoni and H. albopunctatus — the species is inferred to be a cavity spawner. Males of Hypostomus typically excavate or occupy burrows dug into earthen riverbanks or beneath submerged root masses, and defend these sites against rival males. Courtship involves the male guarding an excavated chamber and enticing or trapping a gravid female inside. Competition among males for suitable cavities can be intense during the breeding season. In seasonal Chaco rivers, spawning is likely correlated with rising water levels and warming temperatures during the spring-summer flood pulse (October–January in the Southern Hemisphere). Data specific to H. borellii are absent from published studies consulted.
Breeding
No confirmed captive breeding accounts for Hypostomus borellii were located in the hobbyist or scientific literature. By analogy with the reproductive biology of other large Hypostomus from the Río de la Plata basin — a group that has been studied in rivers of Brazil, Uruguay, and Argentina — the spawning mode is almost certainly substrate cavity guarding. The male prepares a burrow and provides parental care to the clutch, fanning the eggs with his fins and protecting them from predators and fungal infection. Egg numbers in large Hypostomus can range from several dozen to a few hundred per clutch, and incubation periods of 5–10 days have been recorded in related species at tropical temperatures. The larvae are large and yolk-laden at hatching, remaining in the nest cavity until free-swimming. Any aquarist wishing to attempt breeding would require a large aquarium with robust clay or foam cave structures, soft substrate for excavation, and water parameters mimicking the warm, slightly alkaline conditions of the Chaco lowlands.
In the aquarium
Hypostomus borellii is an uncommon species in the aquarium trade; it is rarely exported commercially and most hobbyist encounters are with specimens collected incidentally during trade runs from Argentina or Paraguay. When kept, it requires a spacious aquarium — a minimum of 80 US gal (approximately 80 US gallons) for a single adult, and preferably larger to accommodate its active nocturnal foraging and territorial tendencies. Water temperature should be maintained between 22 and 82 °F (72–82 °F); the species tolerates cooler conditions better than many tropical loricariids, reflecting its subtropical origin. Neutral to slightly alkaline pH (7.0–8.0) and moderate hardness are appropriate. Filtration should be robust, as large plecos produce considerable waste. The tank should include driftwood for grazing and shelter, flat stones, and cave structures. Lighting can be subdued, as the species is most active at night. Diet in captivity: sinking algae wafers, spirulina tablets, blanched courgette, cucumber, and occasional protein treats such as sinking pellets or bloodworm. Tankmates should be robust species that can tolerate the same subtropical temperatures; avoid delicate or thin-bodied fish that may be harassed or accidentally damaged by the pleco's odontode-covered fins.
Conservation
Hypostomus borellii has not been formally assessed by the IUCN Red List as of 2026, and its status is listed as Not Evaluated (NE). The species inhabits the Chaco lowland river system, which is subject to multiple anthropogenic pressures. The Río Pilcomayo has experienced severe degradation over recent decades: upstream damming and diversion in Bolivia have dramatically reduced water and sediment flow, leading to progressive desiccation of the middle and lower reaches in Argentina and Paraguay and causing catastrophic fish kills. Deforestation of the Chaco for cattle ranching and soy agriculture has increased sedimentation and altered the hydrological regime of tributary streams. The upper Río Paraguay faces pressures from navigation channelisation (the Hidrovia Paraguay-Paraná project) and wetland conversion. Despite these regional threats, H. borellii appears to be a broadly tolerant, moderately widespread species, and there is no evidence of population collapse. Formal range-wide assessment and population monitoring would be required before conservation status can be reliably assigned.