Taxonomy & naming
Hisonotus laevior was described by the American palaeontologist and herpetologist-turned-ichthyologist Edward Drinker Cope in 1894, in the Proceedings of the American Philosophical Society (vol. 33, p. 95), from material collected along the Rio Jacuhy (Jacuí), Rio Grande do Sul, Brazil, by the Naturalist Expedition. The Catalog of Fishes (Eschmeyer, CAS) recognises Hisonotus laevior Cope, 1894 as the valid combination.
The species has accumulated several synonyms across its nomenclatural history. It was originally placed in Otocinclus as O. laevior, later transferred to Microlepidogaster as M. laevior, and another associated taxon passed through the synonyms Hisonotus leptochilus, Microlepidogaster leptochilus, and Otocinclus leptochilus before all were united under H. laevior in the comprehensive taxonomic review of Hisonotus from the laguna dos Patos system by Carvalho & Reis (2011, Neotropical Ichthyology 9(1): 1–48), which remains the primary reference for this species.
Molecular studies (Cramer, Bonatto & Reis 2011) have shown that Hisonotus as circumscribed is not monophyletic, and some workers place the Laguna dos Patos lineage in tribe Hisonotini within Hypoptopomatinae. Pending a formal revision of generic limits, Hisonotus laevior remains the current valid CoF name. No L-number has been assigned, as the species is essentially absent from the ornamental trade.
Morphology
Adults of H. laevior reach approximately 2 in standard length (documented maximum 2 in SL; FishBase lists up to 3 in TL for the species). The body is dorsoventrally compressed and encased in the overlapping keeled bony scutes characteristic of Loricariidae. Overall colouration is a muted brownish or olive-tan with fine darker speckling dorsally; the belly is pale to cream. The suedamerikafans.de database records the common name 'cascudinho-pardo' (brown little armoured catfish) in reference to this subdued colouration.
The genus name Hisonotus derives from the Greek isos (equal) and noton (back), referring to the relatively level or even dorsal profile of the head and body — contrasting with genera that have a more abruptly raised predorsal region. Fins are moderately developed: the dorsal fin lacks an adipose fin, which is one of the diagnostic characters separating this clade from Parotocinclus; the caudal fin has approximately 14 soft rays. The pectoral spine is strong and slightly serrated.
Like all Hypoptopomatinae, the genus is capable of facultative aerial respiration via a modified suprabranchial chamber. Sexual dimorphism is subtle; gravid females are visibly broader in the body when viewed dorsally. Breeding males of related species develop more pronounced interopercular odontodes, and papilla morphology provides a reliable secondary indicator of sex in conditioned adults.
Habitat
Hisonotus laevior is endemic to the Laguna dos Patos drainage system of southern Brazil and adjacent Uruguay, one of the most species-rich freshwater systems in the Southern Cone. Its range spans the tributaries of the southern and central portions of the system — including the Rio Jacuí (the type locality), canal São Gonçalo, tributaries of laguna Mirim, and various arroios (small streams) of Rio Grande do Sul — but it is absent from the headwaters of the Rio Jacuí basin (Carvalho & Reis 2011, FishBase).
The suedamerikafans.de database documents confirmed localities across a remarkable breadth of waterbodies: arroio Inhuquipá, arroio Dom Marcos, arroio Gil, rio São Sepé, rio Piratini, rio dos Sinos, and others, as well as distribution extending into Canelones and San José departments of Uruguay. The climate is subtropical — a key distinction from tropical Orinoco congeners — and FishBase lists the species as subtropical, with recommended temperatures as low as 59 °F and up to about 72 °F per specialist databases (suedamerikafans.de recommends 59–72 °F for captive care).
Microhabitat data from Carvalho & Reis (2011) describe the species as occurring in slow to moderately flowing watercourses with clear to brown water, running over sandy substrate, in marginal or submerged aquatic vegetation — a weed-bed and grass-edge specialist rather than a riffle-rock grazer.
Feeding
Hisonotus laevior is a periphyton grazer, scraping biofilm — algae, diatoms, bacteria, and fine organic detritus — from plant stems, leaf surfaces, submerged roots, and occasionally sand grains. Its microhabitat in marginal and submerged vegetation places it in close association with the periphyton communities that colonise aquatic macrophytes in southern Brazilian streams, a food resource that is abundant year-round in slower, vegetated reaches.
The multi-cusped bicuspid teeth of the premaxillary and dentary are adapted for scraping soft biofilm from smooth or moderately rough surfaces rather than rasping hard wood or stone. In the aquarium, the diet should consist primarily of algae wafers, spirulina tablets, and blanched vegetables (courgette, cucumber, spinach). A mature, algae-bearing tank with some plant growth provides the most biologically appropriate environment. The species is strictly herbivorous-periphytivorous in the wild and does not require or benefit from meaty foods.
Feeding competition with other grazer species should be monitored — small otocinclus-type fishes can out-compete this species at wafer feeding stations if kept in insufficient numbers with inadequate food availability.
Mating
Mating behaviour in Hisonotus laevior has not been documented in the published literature, and PlanetCatfish records no breeding report for the species. Extrapolating from closely related Hypoptopomatinae, courtship likely involves males patrolling the submerged vegetation and margins of their microhabitat, displaying to females and competing with rival males through lateral posturing and fin display.
The species' subtropical distribution and seasonal water temperature variation — with winter temperatures potentially dropping to 59 °F or below in Rio Grande do Sul — likely plays a role in triggering reproductive condition, as is common in temperate and subtropical fish. Gravid females are recognisably broader than males when viewed from above, particularly in the posterior body. Aquarists keeping subtropically distributed loricariids report that allowing a winter cool-down (dropping temperature by 39–43 °F over several weeks) followed by gradual warming in spring can successfully trigger spawning; this likely applies here.
Breeding
No captive breeding of Hisonotus laevior has been reported to PlanetCatfish, and no field study has published detailed reproductive data — exact spawning site, egg adhesion substrate, clutch size, or incubation period are all undescribed. Based on the genus and subfamily, eggs are likely deposited on firm submerged surfaces — plant stems, leaf undersides, smooth stones — and are adhesive. Male guarding is expected based on congener behaviour.
Any attempt to breed H. laevior in the aquarium would benefit from mimicking the natural seasonal cycle: a cooler, slightly drier phase in autumn and early winter, with reduced water changes and somewhat lower temperature (59–63 °F), followed by a spring increase in temperature (to 68–72 °F), water changes with cooler, softer water, and increased feeding on green food. The subtropical character of the species means it can tolerate temperatures well below those expected for tropical loricariids, and this temperature gradient is probably the primary spawning cue. A well-planted tank with dense marginal vegetation and a sandy bottom is likely to be the most conducive environment.
In the aquarium
Hisonotus laevior is essentially unknown in the commercial aquarium trade, and hobbyists are unlikely to encounter it except through specialist importers or field-collected material. Nevertheless, its care requirements are inferrable with reasonable confidence from its ecology and from the broader Hypoptopomatinae literature.
The species is subtropical, not tropical — a critical distinction. Temperature should be maintained between 15 and 72 °F; temperatures above 77 °F for extended periods are likely to cause stress, reduced immunity, and shortened lifespan. A standard tropical community tank at 79–82 °F would be inappropriate. A temperate or subtropical setup — common in the US for native-fish or South Brazilian biotope tanks — is far more suitable.
A tank of 15 US gal is adequate for a small group; the species benefits from being kept in groups of five or more, as it is social and will be stressed in isolation. Water chemistry should be soft to moderately hard and slightly acidic to neutral: pH 6.5–7.2, up to 10 °dH. A planted tank with fine-leaved or broad-leaved aquatic vegetation and marginal plants above the water line provides the most natural environment. Sandy substrate is preferred. Tankmates should be other subtropical species: temperate Corydoras (e.g., C. paleatus, C. aeneus), small subtropical tetras, or loaches comfortable at lower temperatures. The species is peaceful and should not be combined with aggressive or highly active tropical fish.
Conservation
The IUCN Red List assessed Hisonotus laevior as Least Concern in November 2018, noting a reasonably wide distribution across the Laguna dos Patos drainage and the absence of documented significant declines. The FishBase IUCN link confirms this status (IUCN species page 186927).
Despite the LC assessment, the Laguna dos Patos system faces considerable anthropogenic pressure. The Rio Grande do Sul lowland catchments are among Brazil's most heavily farmed landscapes — soy and rice cultivation dominate — and diffuse agricultural runoff (fertilisers, pesticides, sedimentation) degrades water quality in many smaller arroios within the species' range. Urbanisation in the greater Porto Alegre metropolitan area affects several sub-basins. The 2024 Rio Grande do Sul floods, one of the most severe in recorded history, caused extraordinary damage to the hydrological infrastructure and riparian zones of the region; the long-term effects on aquatic biodiversity in affected sub-basins remain to be assessed.
The species does not appear in the ornamental trade, so commercial collection pressure is negligible. Targeted conservation effort is lacking, as is for most small, non-charismatic freshwater fish in southern South America.