Taxonomy & naming
Hisonotus ringueleti was described by Aquino, Schaefer, and Miquelarena in 2001 in the American Museum Novitates (No. 3333), based on material from a creek at kilometre 18 of the road from Santana do Livramento, Brazil, to Rivera, Uruguay — contiguous border cities on the Quaraí River system, which drains into the Rio Uruguay. The holotype (CFA-IC 10886, ex ILPLA 886) is lodged at the Centro de Recursos Naturais of Argentina, with paratypes distributed across AMNH, ANSP, FMNH, MCP, MLP, MZUSP, ULPLA, and USNM.
Hisonotus ringueleti was the first species of its genus recorded from the Rio Uruguay basin. A second species from the same basin, Hisonotus candombe, was described by Casciotta, Azpelicueta, Almirón, and Litz in 2006. Carvalho and Reis (2009), in a Zootaxa revision that added four further Uruguay-basin species, formally synonymised H. candombe with H. ringueleti, reducing the two names to one valid taxon.
The Catalog of Fishes (Eschmeyer, CAS) confirms the current valid name as Hisonotus ringueleti Aquino, Schaefer & Miquelarena, 2001. The genus Hisonotus belongs to the subfamily Hypoptopomatinae, a clade of predominantly small, algae-grazing loricariids distributed across Atlantic coastal drainages and major river basins of South America.
Morphology
Hisonotus ringueleti reaches a maximum of 1.5 in standard length — small even by cascudinho standards, placing it among the more diminutive loricariids. The body is dorsoventrally depressed and fully armoured with the overlapping bony scutes characteristic of the family, though the scute series in Hypoptopomatinae are notably fine-textured compared to the large plates of some other subfamilies.
The ventral surface is naked (unplated) in the abdominal region, a feature shared with most Hypoptopomatinae and useful in distinguishing these from the closely related Otothyrinae. The mouth is small, terminal-inferior, with fine multicuspid teeth adapted to scraping periphyton from hard surfaces. The caudal fin is gently forked without elongated filaments.
Specific colour pattern data for H. ringueleti is limited in available sources. Like most Hisonotus, the fish is presumably cryptically marked with a pale-brown or olive ground colour and darker spotting or reticulation that provides camouflage against the rocky and leaf-littered stream beds it inhabits. Dimorphism is not specifically documented for this species.
Habitat
The type locality lies at approximately 31°00'S, 55°30'W on the Brazil–Uruguay border, within the headwaters of the Quaraí (Cuareim) River drainage — a tributary of the Rio Uruguay. This is subtropical rather than tropical terrain, and the water chemistry reflects that: FishBase records pH 7.2–7.7 and a temperature range of 54–75 °F for the species, consistent with the seasonally variable climate of the southern cone of South America where summer and winter temperatures diverge substantially.
Cascudinho habitats in the Uruguay basin, as described by Carvalho and Reis (2009), are shallow, fast-flowing montane to piedmont streams with rocky or sandy substrate and associations with leaf litter and submerged wood. These are clear, well-oxygenated streams where scouring currents maintain open rock surfaces suitable for periphyton growth. H. ringueleti, as the founding member of the genus in this basin, presumably occupies this same microhabitat type.
Feeding
Hisonotus ringueleti is an algae and periphyton micro-grazer, consistent with the feeding mode of all known Hypoptopomatinae. The small, multi-cusped teeth and ventrally placed mouth allow the fish to rasp biofilm, diatoms, and soft algae from rock surfaces, submerged wood, and leaf litter. The species likely also ingests fine organic detritus incidentally as it grazes.
No aquarium feeding records exist for H. ringueleti specifically; the species is too obscure to have a meaningful hobby history. By analogy with other small cascudinhos, it would likely accept algae wafers and fine-grained sinking foods in captivity, particularly if the tank provides natural periphyton growth on rocks and glass surfaces.
Mating
No breeding or courtship observations for Hisonotus ringueleti are reported in available literature. The species has not been maintained in aquaria by hobbyists to any documented degree, and no field observations of reproductive behaviour have been published.
Broader Hypoptopomatinae biology suggests that males of this group may develop interopercular odontodes that are reduced or absent outside the breeding season, and that spawning likely occurs in association with rocky substrates during periods of elevated flow. Given the subtropical range of H. ringueleti, breeding may be triggered by the cooler wet-season conditions characteristic of the Uruguay basin's spring flood pulse, but this remains inferential.
Breeding
No captive breeding records exist for Hisonotus ringueleti, and published field data on its reproductive biology are absent. The species is too rarely encountered in aquaria and too recently described for a breeding dossier to have accumulated.
Within Hypoptopomatinae more broadly, some species have been bred in aquaria under conditions of clean, well-oxygenated, moderately cool water with rocky substrate and strong current. Whether H. ringueleti would follow this pattern is plausible but unconfirmed. Data are too sparse to describe clutch size, incubation period, or parental behaviour.
In the aquarium
Hisonotus ringueleti is not an established aquarium species. It does not appear in the regular ornamental trade, and no significant captive husbandry literature exists for it. Its small adult size (under 1.5 in) would make it suitable for a modestly sized aquarium if it were available, but the subtropical temperature requirements (54–75 °F) are atypical by tropical aquarium standards and would need to be accommodated deliberately.
A tank maintained at 64–72 °F with strong filtration, significant current, and a rocky substrate grown with algae and biofilm would best approximate the natural environment. Soft to moderately hard, near-neutral water matching the recorded pH range of 7.2–7.7 is appropriate. Given the general cascudinho temperament, the species would be expected to be peaceful toward other fish of comparable size. Without captive experience to draw from, detailed recommendations on stocking, compatibility, or diet beyond these broad inferences are not appropriate to state.
Conservation
The IUCN Red List assessed Hisonotus ringueleti as Least Concern in October 2020, reflecting adequate population size and no evidence of significant range decline across the Rio Uruguay drainage shared by Argentina, Brazil, and Uruguay.
The Uruguay basin faces moderate anthropogenic pressure from agricultural intensification, riparian clearing, and water extraction, which can degrade the clear, fast-flowing stream microhabitats occupied by small cascudinhos. These threats are not considered to have risen to a level that imperils H. ringueleti at a global scale. The species' range spans a trinational watershed, and portions of the Uruguay basin in southern Brazil and northern Uruguay retain reasonable habitat quality. Ongoing monitoring of stream habitats in this region would be prudent given the sensitivity of small, substrate-specialist loricariids to sedimentation and altered flow regimes.