Taxonomy & naming
Hisonotus carreiro was described by Carvalho and Reis in 2011 as part of a comprehensive taxonomic revision of Hisonotus from the laguna dos Patos system of southern Brazil (Neotropical Ichthyology 9(1):1–48). The holotype was collected from the headwaters of the rio Taquari drainage, Rio Grande do Sul, with paratypes from arroio Guabiju, municipality of Guabiju, Rio Grande do Sul (MCP 40943). The Catalog of Fishes (Eschmeyer, CAS; species ID 71779) recognises the valid combination Hisonotus carreiro Carvalho & Reis, 2011.
The species belongs to the subfamily Hypoptopomatinae within Loricariidae, the same grouping that includes the popular otocinclus catfishes. The 2011 revision by Carvalho and Reis substantially reorganised Hisonotus diversity in the laguna dos Patos basin, separating several previously confused or undescribed lineages based on plate counts, abdominal coverage, and other meristic characters.
Morphology
Hisonotus carreiro reaches a maximum recorded length of 1.5 in standard length (SL). The body plan follows the typical Hypoptopomatinae layout: strongly depressed and dorsoventrally flattened, covered in overlapping bony scutes, with a ventral sucking mouth adapted for grazing biofilm from hard surfaces.
Diagnostic morphological details specific to H. carreiro relative to its congeners in the laguna dos Patos system are established in the original 2011 revision by Carvalho and Reis, which provides meristic comparisons among the species treated. No published photographic record or aquarium observation data are available to supplement the type series description. Trophic level is estimated at 2.3 based on data from relatives in the subfamily, consistent with herbivorous or detritivorous micro-grazing.
Habitat
The species is known from the headwaters of the rio Taquari drainage, which forms part of the laguna dos Patos hydrological system in Rio Grande do Sul state, Brazil. Type and paratype localities lie in the Serra Gaúcha uplands at approximately 29°S, 51°W — a region of subtropical highland streams characterised by cool, clear water and well-oxygenated conditions.
The laguna dos Patos system drains a large portion of Rio Grande do Sul, the southernmost state of Brazil, and supports a distinctive fish fauna with many narrow-range endemics. Stream headwaters in this system typically run over rocky or sandy substrate with moderate to high gradient and seasonal fluctuation in flow. No site-specific water chemistry data (pH, temperature, conductivity) have been published for the collection localities of H. carreiro.
Feeding
Hisonotus carreiro is presumed to be an aufwuchs and biofilm micro-grazer, consistent with its mouth morphology, ventral positioning, and the feeding ecology documented across the subfamily Hypoptopomatinae. The species would graze the thin layer of algae, diatoms, bacteria, and fine detritus from hard substrate surfaces in its native streams.
No direct dietary observations exist for this species, either from the field or from captivity. A trophic level of 2.3 estimated from congeners is consistent with primary consumption at the base of the food web rather than invertebrate predation.
Mating
No mating behaviour has been described or observed for Hisonotus carreiro. The species is not present in the aquarium hobby, and no field observations of courtship have been published. Reproductive biology is expected to resemble that of related Hypoptopomatinae in general terms, but specifics remain undocumented.
Breeding
No breeding data exist for Hisonotus carreiro. The species has not been maintained or bred in captivity, and reproductive details are absent from the published literature. The subfamily Hypoptopomatinae includes species that spawn adhesive eggs on hard substrates; whether H. carreiro follows this pattern is not established by available sources.
In the aquarium
Hisonotus carreiro is not present in the ornamental fish trade and has no documented captive history. No reliable husbandry information can be offered beyond what can be inferred from the subfamily.
At 1.5 in SL, it is among the smaller loricariids. Any future attempt to maintain this species would logically follow protocols for small, subtropical Hypoptopomatinae: cool, clean, well-oxygenated water consistent with upland southern Brazilian streams, abundant hard surfaces colonised by biofilm, and a calm community. Its subtropical origin suggests that lower temperatures — perhaps 64–75 °F — may be tolerated or even preferred, but this is inference rather than documented fact.
Conservation
The IUCN Red List assessed Hisonotus carreiro as Least Concern in May 2022, noting its endemism to the headwaters of the rio Taquari in the laguna dos Patos drainage of Rio Grande do Sul. The Least Concern assessment does not imply the species is abundant — it reflects the absence of evidence of significant population decline at the time of assessment. As a narrow-range endemic restricted to highland stream headwaters, the species is potentially vulnerable to land-use change, sedimentation, and water extraction in the Serra Gaúcha agricultural region, though such pressures had not triggered a threatened listing as of 2022.