Hisonotus is a genus of small armored catfishes in the family Loricariidae, part of the otocinclus-like subfamily Hypoptopomatinae. Its species live in slow to moderate-flow streams and rivers across southeastern South America, mainly the La Plata basin and nearby Atlantic drainages, with records from Brazil, Argentina, Uruguay, and Paraguay. Like their relatives, Hisonotus have flattened, bony-plated bodies, a ventral sucker-mouth used to rasp algae and biofilm from rocks and submerged wood, and cryptic brown or mottled patterning that blends into leaf litter. Most species are genuinely tiny: the atlas covers 33 species with adult sizes spanning roughly 1–2.5 in, placing several among the smallest catfishes known.
Hisonotus is traded far less than its better-known cousin genus Otocinclus, and most of its species reach the aquarium hobby rarely if at all; many of the names in the atlas carry no established common name, a sign of how obscure the genus stays outside ichthyology. A few do have trade names, such as the Emerald Oto (Hisonotus aky) and the Brown Dwarf Oto (H. brunneus), and they hint at the appeal: peaceful, algae-grazing dwarf catfish suited to mature, densely planted tanks with gentle current and a steady film of algae or biofilm to browse. The genus's conservation picture leans reassuring overall, with most assessed species rated Least Concern, though a meaningful share are Data Deficient and at least one species, Hisonotus depressicauda, is listed as Endangered — a reminder that individual species inside an otherwise unremarkable genus can still be narrowly range-restricted.