Taxonomy & naming
Hisonotus iota was formally described by Carvalho and Reis in 2009 as one of four new species of Hisonotus from the upper rio Uruguay basin, published in Zootaxa 2113 (pages 1–40). The type locality is the rio Chapecó at Vila São Miguel, on the road from Coronel de Freitas to Quilombo, rio Uruguay basin, Santa Catarina State, Brazil (26°51'26"S, 52°44'29"W). The species has not been transferred to any other genus; PlanetCatfish records it as Hisonotus iota without a genus redirect, in contrast to some congeners that have been moved to Rhinotocinclus following molecular work.
Hisonotus belongs to subfamily Hypoptopomatinae, one of the morphologically and speciologically diverse subfamilies within Loricariidae. The genus is characterised among Hypoptopomatinae by its small body size, complete lateral plate coverage, and details of the cheek and pectoral girdle armour. Carvalho and Reis erected iota alongside three sister species from the same river system, distinguished on fin ray counts, plate row numbers, and body proportions. The Catalog of Fishes (Eschmeyer, CAS) treats Hisonotus iota Carvalho & Reis, 2009 as the valid name.
Morphology
Hisonotus iota is the smallest Hisonotus described from the upper Uruguay basin, reaching only 1.5 in standard length (SL) in the largest known female specimen. The body follows the typical Hypoptopomatinae blueprint: dorsoventrally flattened, encased in bony scutes, with a subterminal mouth bearing fine, bifid teeth adapted for scraping epilithic algae and biofilm from hard substrates.
The species name itself encodes the most salient morphological fact: iota, from the Greek for 'anything very small', was chosen by the authors specifically to denote the diminutive stature of this species relative to its congeners. No detailed colour description from live specimens is available in the published literature, and the species has not been photographed in life under aquarium conditions. Meristic and morphometric data — fin ray counts, plate row numbers, body proportions — are the primary characters distinguishing it from co-occurring Hisonotus species, and these are documented in the original 2009 Zootaxa description.
Habitat
Hisonotus iota is known only from the rio Chapecó, a tributary of the upper rio Uruguay basin in Santa Catarina State, southern Brazil. The type locality coordinates (26°51'26"S, 52°44'29"W) place it in the subtropical uplands of the Paraná Plateau, where small, clear, fast-flowing streams drain over basalt bedrock. The upper Uruguay and its headwater tributaries in this region are characterised by relatively cool water temperatures compared to tropical Brazilian rivers, moderate to low conductivity, and seasonally variable flow regimes tied to subtropical rainfall patterns.
No published water chemistry data exist for the type locality specifically. Based on the general properties of upper Uruguay headwater streams and the broader biology of Hypoptopomatinae, the micro-habitat is likely to involve firm substrates — rocks, leaf litter, submerged woody debris — in areas with moderate current and good oxygenation, which are the conditions under which small, armoured algae-grazers in this clade consistently occur. The species has not been recorded from any locality outside the rio Chapecó drainage.
Feeding
Like other Hisonotus and the broader Hypoptopomatinae, H. iota is inferred to be an epilithic algae and biofilm grazer. The group's fine bifid teeth are well-suited to scraping microscopic algae, diatoms, and associated organic matter from hard surfaces such as rocks, submerged wood, and leaf litter. FishBase records a trophic level of 2.3 (± 0.1 se) for the species — consistent with an herbivore-detritivore diet dominated by plant-derived material and microorganisms rather than invertebrate prey.
No direct feeding observations in the field or in captivity have been published. The inference of micro-algae grazing is supported by the anatomy of the clade and the trophic data from relatives with similar body plans. Whether the species also ingests fine sediment or detritus as part of its natural foraging, as some Hypoptopomatinae do, is not known from the available literature.
Mating
No information on courtship or mating behaviour in Hisonotus iota has been published, and the species has no recorded breeding reports in the aquarium hobby. In related Hypoptopomatinae for which breeding observations exist, spawning typically involves adhesive eggs deposited on firm substrate surfaces and some degree of parental attendance, but these details have not been confirmed for H. iota or for Hisonotus specifically. The reproductive biology of this species remains entirely undescribed.
Breeding
Hisonotus iota has never been bred in captivity as far as published records indicate. PlanetCatfish records no registered keepers, no breeding accounts, and no wishlists for the species. The combination of its very small adult size, restricted natural range, and complete absence from the ornamental trade means that reproductive data — clutch size, incubation period, fry development — are unknown. Any account of breeding would necessarily be extrapolated from related genera, which is not appropriate given the species-level variation documented across Hypoptopomatinae.
In the aquarium
Hisonotus iota does not appear in the aquarium trade and has no documented captive husbandry record. No pH or temperature parameters have been published for the species, and no aquarists are known to maintain it. Given its type locality in the subtropical uplands of Santa Catarina — a region of cool, clear streams — the species would be expected to prefer cooler water temperatures than many tropical plecos, and good oxygenation would likely be important. Its tiny adult size (1.5 in SL) would make it suitable for small aquaria in principle, but the absence of any hobby record means nothing reliable can be stated about its care requirements beyond what is inferred from the ecology of its headwater-stream habitat and the general biology of small Hypoptopomatinae.
Conservation
The IUCN Red List assessed Hisonotus iota as Data Deficient (DD) in November 2018. This status reflects the reality that the species is known only from its type locality in the rio Chapecó, and too little collection data exist to make a population assessment possible. Being known from a single stream drainage with no confirmed records elsewhere places H. iota in a category of loricariids where the precautionary concern is real but the data are too sparse to support a threat category.
The upper Uruguay basin in Santa Catarina faces pressures familiar across southern Brazil's subtropical Atlantic Forest zone: agricultural intensification, stream sedimentation from soy and cattle farming, water extraction, and small hydroelectric infrastructure. Whether H. iota's population is stable, reduced, or expanding is simply not known. The DD listing should not be read as reassurance; it reflects an absence of information rather than an absence of risk. Targeted surveys of the rio Chapecó and neighbouring tributaries would be needed to evaluate whether the species has a wider range than currently documented or whether its distribution is genuinely as narrow as the type series implies.