Taxonomy & naming
Hisonotus notatus was described by Carl H. Eigenmann and Rosa Smith Eigenmann in 1889, with the type locality given as rio Guandu at the Fazenda Santa Cruz — Emperor Pedro II's estate at Santa Cruz, Rio de Janeiro state, Brazil (approximately 22°54'40"S, 43°41'7"W). Eigenmann & Eigenmann simultaneously erected the monotypic genus Hisonotus for this species, making H. notatus the type species of the genus by original designation. The Catalog of Fishes (Eschmeyer, CAS) records two synonyms: Otocinclus notatus (C.H. Eigenmann & R.S. Eigenmann, 1889) and Microlepidogaster notatus (C.H. Eigenmann & R.S. Eigenmann, 1889), both referring to the same 1889 description under earlier generic assignments.
For much of the twentieth century, populations now assigned to Hisonotus thayeri were treated as conspecific with H. notatus. A 2016 revision by Martins & Langeani re-examined the taxonomy, redescribed H. notatus sensu stricto, and separated the nome H. thayeri for populations previously conflated with it. The current valid combination Hisonotus notatus Eigenmann & Eigenmann, 1889 is recognised by CoF; the authority does not appear in parentheses because the species was originally described in Hisonotus itself.
The genus Hisonotus derives from Greek isos (equal) and noton (back), a reference to the even, flat dorsal profile of the genus that lacks the pronounced ridges of some loricariid relatives. The genus belongs to subfamily Hypoptopomatinae (Schaefer in Reis et al. 2003; CoF), which includes Otocinclus, Parotocinclus, and allied small loricariids. Hisonotus itself is a diverse genus of over 25 species distributed across southern and southeastern Brazil and adjacent parts of Uruguay and Argentina.
Morphology
Hisonotus notatus is a small loricariid, reaching a maximum of 1.5 in SL (FishBase, Ref. 95507). The body form is typical of small Hypoptopomatinae: moderately depressed, broad and slightly flattened in the head region, with a ventrally positioned oral disc and overall compact proportions. Fin-ray counts (from the description and FishBase): dorsal soft rays 7, anal soft rays 7. The genus Hisonotus is characterised by the absence of an abdominal groove and by the relatively uniform, flat dorsal profile that gives the genus its name.
Colouration is brown to olive-brown on the dorsum, with a pattern of dark and pale spots or mottling across the body and fins. The species can be distinguished from close relatives in part by the overall spotted-to-reticulate pattern. The 2016 Martins & Langeani revision provides formal morphometric and meristic characters that distinguish H. notatus sensu stricto from H. thayeri, primarily based on body proportions and the disposition of cephalic sensory pores.
Sexual dimorphism in Hisonotus is generally subtle. In many congeners, gravid females become noticeably broader when viewed from above, and males may develop small odontodes on the pectoral spine and ventral surface. The degree to which these apply specifically to H. notatus has not been documented in detail in the revised species description, but the pattern is consistent with the genus.
Habitat
Hisonotus notatus is endemic to Brazil, occurring in a small cluster of coastal drainages in Rio de Janeiro state. The primary records are from the São João River basin and adjacent rivers draining into Guanabara Bay and Sepetiba Bay — a group of lowland to mid-altitude coastal systems that are the most densely urbanised and industrialised freshwater zone in South America. The type locality (rio Guandu at Santa Cruz) lies in a Sepetiba Bay tributary that today flows through suburban western Rio de Janeiro.
FishBase records the environmental parameters as pH 6.5–7.5 and hardness to 20°dH, with a temperature range of 72–75 °F — reflecting the subtropical/tropical Atlantic coastal climate of this region, which lacks extreme seasonality in temperature but experiences a marked wet season (October–March) when rivers swell and water chemistry changes. Microhabitat is among aquatic vegetation, submerged roots, and rocks with algal growth in slower to moderate current sections; like other small Hypoptopomatinae, the species is most often associated with submerged surfaces on which periphyton grows. The genus is noted as facultatively air-breathing (FishBase, Ref. 126274), which aids survival in stagnant or low-oxygen conditions that may develop in overgrown lowland streams during dry seasons.
Feeding
Hisonotus notatus is a periphyton and biofilm grazer, consistent with the feeding ecology of all small Hypoptopomatinae. The oral disc and fine teeth are adapted for rasping algal films, diatoms, and associated microbial communities from hard surfaces: plant leaves, rocks, submerged wood, and artificial surfaces in captivity. The diet in the wild is predominantly periphyton supplemented by fine detrital particles and the microinvertebrates (rotifers, nematodes, small crustaceans) incidentally ingested with the biofilm scrape.
In aquaria, suitable foods mirror those used for Otocinclus species: algae wafers and spirulina discs form the staple; blanched vegetables (zucchini, cucumber, spinach) on a clip or the substrate are accepted; and a tank with an established algal film on glass or décor provides the natural grazing activity that keeps the fish occupied and healthy. The species is not a wood-eater and does not require driftwood for dietary reasons, though wood with biofilm is grazed readily. Heavy protein foods are unnecessary and should not form the core diet.
Mating
Mating behaviour in Hisonotus notatus has not been documented in detail in the published literature. The genus Hisonotus shows variation in reproductive strategy: the congener H. leucofrenatus (which has an article elsewhere on this site) is documented as a non-guarder that scatters eggs on plant leaves and substrate surfaces in a manner reminiscent of Corydoras — a striking contrast to the cave-guarding strategy of most loricariids. Whether H. notatus shares this scatter-spawning mode or instead uses a more conventional loricariid cave or crevice approach is not established in the literature.
The limited number of field specimens and the absence of established captive colonies means that courtship has not been described for this species. Males in the genus may show slight dimorphism (pectoral odontodes) during the breeding season. The subtropical Atlantic Forest seasonal cycle, with summer rains driving river flooding and temperature peaks in December–March, presumably influences reproductive timing in wild populations.
Breeding
No captive breeding records have been published specifically for Hisonotus notatus. The species is not commonly established in the ornamental trade; most Hisonotus offered in the hobby are undetermined or misidentified species, and H. notatus itself (sensu stricto, post the 2016 Martins & Langeani revision) is rarely if ever imported as a named trade species.
By comparison with the well-documented H. leucofrenatus, some Hisonotus species spawn in open water or on plant leaves without a specific cave territory, producing non-adhesive or weakly adhesive eggs that receive no parental care. If H. notatus shares this pattern, aquarium breeding would be more accessible than for cave-guarding species: a healthy group in a stable, algae-rich, well-maintained tank might be sufficient. The water parameters documented for H. notatus (temperature 72–75 °F, pH 6.5–7.5, hardness up to 20°dH) suggest standard tropical water-quality maintenance is appropriate, with no need for extreme soft-water conditions. However, without confirmed breeding observations, these are extrapolations from congeners rather than documented fact.
In the aquarium
Hisonotus notatus is rarely available in the ornamental trade under its correct name, and confirmed specimens are uncommon outside specialist circles or research institutions. Aquarists interested in keeping the species should be aware that much material sold as 'Hisonotus sp.' in Brazilian domestic trade may be H. notatus or a close relative, but exact species identification requires morphometric data from the Martins & Langeani (2016) revision.
For hobbyists, the care requirements are similar to those of Otocinclus: a well-established, algae-colonised tank is essential; the fish are sensitive to poor water quality and the lack of appropriate food. Temperature 72–77 °F is suitable; pH 6.5–7.5 and moderate hardness (up to approximately 15°dH) match the natural range. The species is small, peaceful, and suited to community tanks with non-aggressive tankmates. Like all small Hypoptopomatinae, H. notatus should not be kept with large, boisterous, or territorial fish that may outcompete it for food or stress it. Groups of at least five to six individuals are preferable to reduce conspecific competition and provide social context.
The coastal Rio de Janeiro water systems where H. notatus occurs are extremely urbanised; in the wild this fish persists in moderately degraded lowland streams as well as cleaner headwaters, suggesting some tolerance of moderate turbidity. Nevertheless, clean, well-filtered aquarium water with zero ammonia and nitrite is the minimum standard for long-term health.
Conservation
The IUCN Red List assessed Hisonotus notatus as Least Concern (LC) in November 2018, reflecting a distribution within a small but so far unextirpated set of coastal drainages in Rio de Janeiro state. The species has been recorded from the São João River, Guanabara Bay tributaries, and Sepetiba Bay drainages — an area that encompasses some of the most heavily impacted freshwater habitats in Brazil, including the Baía de Guanabara watershed, which has been the focus of environmental remediation efforts for decades.
The fact that H. notatus survives in this highly modified landscape implies a degree of environmental resilience, but the small total range remains a vulnerability. The coastal Atlantic Forest drainages of Rio de Janeiro have experienced large-scale deforestation, agricultural conversion, sewage pollution, and urban expansion for centuries. The separation of H. notatus from the previously synonymised H. thayeri (Martins & Langeani 2016) means the precise geographic limits of the valid species need to be resurveyed; some historical records labelled as H. notatus sensu lato may represent the other species, making accurate population trend data difficult to establish. Ongoing monitoring, especially in the context of continuing urbanisation of the Rio de Janeiro metropolitan region, would be warranted even given the current LC status.