Taxonomy & naming
Hisonotus aky was originally described as Epactionotus aky by Azpelicueta, Casciotta, Almirón, and Koerber in 2004. The species was subsequently transferred to Hisonotus by Carvalho and Reis (2009), after examination of the type specimens revealed the absence of most diagnostic features of Epactionotus. The Catalog of Fishes (Eschmeyer, CAS) recognises the current valid combination Hisonotus aky, with the original Epactionotus aky as a junior synonym.
Within Loricariidae, Hisonotus belongs to the subfamily Hypoptopomatinae — the small, largely algae-scraping 'oto' clade. The species shares characters with H. iota and H. montanus, and phylogenetic analysis places it within a group that includes a derived feature: the infraorbital canal entering the infraorbital series via the compound pterotic. Members of the genus are facultative air-breathers, a trait shared widely among hypoptopomins inhabiting oxygen-variable shallow streams.
Morphology
Adults are tiny, reaching a maximum of approximately 1.5 in standard length (SL) — among the smallest loricariids documented. Body shape is typical of the hypoptopomins: laterally compressed, covered in overlapping bony scutes, with a ventral suckermouth. In freshly collected or well-conditioned individuals the ground colour is a striking deep green — the colour that inspired both the Guaraní species name and the hobbyist common name 'Emerald Oto'. In aquaria, this vivid green fades to brownish but periodically reappears, particularly in fish that are well fed and experiencing stable, high-quality water conditions.
Sexual dimorphism is subtle. Females tend to grow slightly larger than males overall. Males show proportionally longer fins and, when breeding, less intense coloration compared to gravid females. Body depth viewed from above provides a reliable indicator in conditioned pairs, with mature females noticeably broader.
Habitat
Hisonotus aky is native to the upper Río Uruguay drainage in South America, documented from Misiones Province (Argentina) and the states of Rio Grande do Sul and Santa Catarina (Brazil). The type locality is Arroyo Garibaldi, Río Uruguay basin, Misiones, Argentina (approximately 26°38'46"S, 53°59'55"W). The species has also been collected from Arroyo Fortaleza in the same drainage.
At the type locality in February, water temperature was recorded at 69 °F and pH at 6.7 — cool, slightly acidic, clear-water conditions characteristic of the southern Uruguayan piedmont. The fish inhabit clear, swift, well-oxygenated streams, and current is a meaningful requirement both for oxygen uptake and for maintaining the biofilm communities on which the fish feed. Substrates are typically sandy with smooth rocks, submerged wood, and broad-leaved aquatic plants such as Echinodorus. Fish cling to the undersides of plant leaves and smooth rock surfaces using their pelvic fins in a grip-like posture.
Feeding
Hisonotus aky is an algae micro-grazer and biofilm feeder, rasping periphyton from smooth rock surfaces, submerged wood, and the undersides and upper surfaces of broad-leaved plants. In the wild, the diet consists principally of unicellular algae, diatoms, and the associated bacterial and detrital biofilm layer that accumulates on hard substrates in clear, swift streams.
In the aquarium, the species accepts flake food, algae wafers, spirulina-based tablets, and insect-based prepared foods. As with other hypoptopomins, a tank with established algae growth on surfaces provides the most natural feeding environment. The species grazes continuously during active periods and should not be kept in a sterile, newly set-up tank lacking a developed biofilm.
Mating
Detailed field observations of courtship in this species are sparse in the published literature. Based on aquarium observations, the species follows the general hypoptopomins pattern of pair formation in well-conditioned adults. Females become visibly plumper when gravid. Males show prolonged fin display behaviour near broad-leaved plant surfaces or smooth rock faces that appear to serve as preferred spawning sites.
Breeding
Hisonotus aky has been bred in captivity. Breeding was documented by aquarist Paul Dixon at Bolton Museum, England, with fry photographed at three days old. The spawning mode is consistent with other small hypoptopomins: eggs are deposited on hard surfaces — plant leaves, smooth stones, or aquarium glass — in a scatter or cluster, with no extended parental care beyond the spawning act.
Conditioning on a varied diet and the provision of clean, well-oxygenated, slightly cooler water (mimicking the cool southern Uruguayan streams of origin) appears to encourage reproductive behaviour. Fry are tiny and require infusoria-scale foods or dense biofilm for initial feeding.
In the aquarium
The extreme small size of H. aky — under 1.5 in — makes it a genuine nano-aquarium species, suitable for tanks as small as 10 US gal if water quality is maintained. Recommended parameters reflect the type-locality data: temperature 64–75 °F, pH 6.5–7.2. The upper end of that temperature range should be the norm rather than the ceiling; the fish originates from cool southern streams and does not tolerate the higher temperatures comfortable for many tropical loricariids.
Current is important. A small powerhead or spray-bar return providing a gentle but consistent flow is recommended, as the species is adapted to well-oxygenated running water. Décor should include smooth rocks, bogwood, and broad-leaved plants (Echinodorus species are ideal, replicating the natural habitat closely). Sandy substrate completes the environment.
The species is rarely available in the trade, and most fish encountered are wild-caught. It is genuinely peaceful and suitable for nano community tanks with small, non-aggressive tankmates. Groups are preferable to single specimens, as the fish are more active and less shy in the company of conspecifics. Given its rarity in commerce, captive-bred fish are to be sought when possible.
Conservation
The IUCN Red List assessed Hisonotus aky as Least Concern in 2020, reflecting a distribution across multiple river systems within the upper Río Uruguay basin and no evidence of significant range-wide decline at the time of assessment. The upper Río Uruguay region of Misiones and the adjacent Brazilian states remains relatively less impacted than many lowland South American drainages, though small-stream specialists such as H. aky can be sensitive to localised changes in water quality, catchment land use, and flow regime. Collection pressure for the ornamental trade is believed to be low.