Taxonomy & naming
Hisonotus vireo was formally described by Carvalho and Reis in 2011, in their paper 'Taxonomic review of Hisonotus Eigenmann & Eigenmann from the laguna dos Patos system, southern Brazil', published in Neotropical Ichthyology (volume 9, number 1, pages 1–48). The species is recognised as valid by the Catalog of Fishes (Eschmeyer, CAS).
The genus Hisonotus belongs to the subfamily Hypoptopomatinae, the group of small, slender suckermouth catfishes within the family Loricariidae. Hisonotus comprises numerous diminutive species distributed across southern and southeastern South America, several of which were clarified or newly described in the 2011 revision that introduced H. vireo.
Morphology
Hisonotus vireo is a very small loricariid, reaching a maximum of about 1.5 in standard length (SL). Like other members of the subfamily Hypoptopomatinae, it has the slender, somewhat depressed body and ventral suckermouth typical of the algae-grazing dwarf catfishes, with the body armoured in rows of bony plates.
Detailed morphological diagnostics distinguishing this species from its congeners are set out in the original description by Carvalho & Reis (2011). No data on sexual dimorphism specific to this species has been published in the sources available here.
Habitat
The species is endemic to the rio Jacuí basin within the laguna dos Patos system, in the state of Rio Grande do Sul, southern Brazil. It is a freshwater, demersal fish of subtropical streams.
Detailed water-chemistry measurements (temperature, pH, hardness) for the type localities are not published in the sources available here. The laguna dos Patos drainage is a coastal system of southernmost Brazil, and the genus as a whole is associated with flowing, vegetated stream margins.
Feeding
Like other members of the genus Hisonotus, H. vireo is understood to be an aufwuchs grazer, rasping the film of algae, biofilm, and associated micro-organisms from submerged surfaces with its suckermouth. Specific dietary studies for this species have not been published in the sources available here.
Members of the genus are also reported to show facultative air-breathing, an adaptation that allows survival in warm, oxygen-poor stream conditions.
Mating
No data on the courtship or mating behaviour of Hisonotus vireo has been published in the sources available here.
Breeding
No data on the reproduction or breeding of Hisonotus vireo has been published in the sources available here. The genus is noted for high resilience, with an estimated population doubling time of under 15 months, but this is a demographic inference rather than a documented breeding account.
In the aquarium
Hisonotus vireo is not a species established in the aquarium trade, and no husbandry data specific to it has been published in the sources available here. As a tiny, algae-grazing stream fish of subtropical southern Brazil, any care would by analogy with related Hypoptopomatinae require well-oxygenated, clean water with abundant grazing surfaces, but this remains untested for the species.
Conservation
The IUCN Red List assessed Hisonotus vireo as Least Concern (LC), with the assessment dated 7 November 2018. The species is restricted to the laguna dos Patos system of Rio Grande do Sul, but the assessment did not identify a level of threat warranting a higher category. The genus is regarded as highly resilient. CITES and CMS listings have not been evaluated for the species.