Taxonomy & naming
Hisonotus thayeri was formally described by Martins and Langeani in 2016 in Neotropical Ichthyology (volume 14, number 2, pages 1–18), in a paper that simultaneously provided a redescription of the type species of the genus, H. notatus Eigenmann & Eigenmann, 1889. The Catalog of Fishes (Eschmeyer, CAS) recognises Hisonotus thayeri Martins & Langeani, 2016 as a valid species, assigned CoF spid 74367.
The species is placed in the subfamily Hypoptopomatinae — the 'otocinclus group' of loricariids — a clade of small to very small algae and biofilm grazers widespread across South American coastal and interior drainages. Hisonotus is one of the most speciose hypoptopomatine genera, with diagnoses often resting on fine plate-count and odontode-pattern characters.
The species epithet thayeri commemorates the Thayer Expedition of 1865–1866, a major scientific survey of Brazil's freshwater fauna led by the Swiss naturalist Louis Agassiz and funded by the American banker Nathaniel Thayer. The expedition made the original collections that eventually, a century and a half later, were linked to this new species — a striking illustration of the timelines involved in neotropical fish taxonomy.
H. thayeri is distinguished from its congeners by a combination of characters: the anterior portion of the snout is completely covered by odontodes (no odontode-free band), the anterior abdomen is covered by large plates arranged in three longitudinal series without naked spaces, and the spinelet is ovoid to rectangular (not V-shaped with anteriorly directed lateral projections). It differs from the closely related H. leucofrenatus by the configuration of the head stripe (ending near the vertical through the distal opercle tip, not bifurcating further), presence of a bifid neural spine on the 7th vertebra, and basipterygia contacting along the midline to the anterior margin. Vertebral count is 27–28.
Morphology
Hisonotus thayeri is a very small loricariid, reaching a maximum recorded standard length of 1.5 in SL. Body form is elongate and strongly depressed (dorsoventrally flattened) in the typical hypoptopomatine fashion, allowing the fish to press tightly against flat substrate surfaces while grazing.
The anterior snout is fully covered in odontodes — a diagnostic character separating H. thayeri from several congeners that bear an odontode-free band in that region. The abdomen in adults is entirely covered by large plates arranged in three distinct longitudinal series without naked gaps between them; this covered-abdomen condition distinguishes it from species with naked spaces or plates not arranged in longitudinal series.
The spinelet — the locking mechanism of the dorsal fin — is ovoid to rectangular in shape, placing H. thayeri in a different morphological group from congeners with V-shaped spinelets bearing anteriorly directed lateral projections (such as H. acuen, H. bockmanni, H. chromodontus, and H. vespuccii). Vertebral count is 27–28, distinguishing it from H. hungy (25–26 vertebrae) and H. armatus (29 vertebrae).
The mid-dorsal plate series consists of 15–18 plates (mode 17), forming a generally continuous series — a character shared with and helping to distinguish it from H. notatus within the redescription paper. The lateral line is present but not fully documented in the cache data. Coloration details beyond the broadly typical hypoptopomatine pattern of brownish body with lighter underparts are not available from the cached sources.
Habitat
Hisonotus thayeri is known from multiple coastal drainage basins of south-eastern Brazil: the Rio Paraíba do Sul, Lagoa Feia, Rio Macaé, Rio Itabapoana, Rio Itapemirim, Rio Novo, Rio Benevente, and Rio Doce. This is a notably broad distribution for a hypoptopomatine species, spanning coastal rivers across Rio de Janeiro and Espírito Santo states.
These south-eastern Brazilian coastal drainages are distinct from the large interior basins (Amazon, Paraná) and are characterised by relatively short, steep rivers draining the Serra do Mar coastal escarpment. Waters are typically clear to slightly tea-stained, soft to moderately hard, and well-oxygenated in the upper and middle reaches where substrate-grazing loricariids are most common.
Specific microhabitat data for H. thayeri are not available from the cached sources. By analogy with other Hisonotus species, the likely microhabitat involves rock surfaces, submerged wood, and coarse substrate in areas of moderate current, where biofilm is sufficiently developed to support grazing.
Feeding
Hisonotus thayeri, like all members of the subfamily Hypoptopomatinae, is a micro-grazer. These fish use their ventral suckermouth to scrape biofilm — the complex community of algae (principally diatoms and green algae), bacteria, fungi, detritus, and associated microinvertebrates — from hard substrate surfaces including rock, wood, and leaf litter.
This feeding mode is analogous to, and convergent with, the well-known otocinclus catfishes (Otocinclus spp.), which occupy the same functional niche and are far more commonly kept in aquaria. The trophic level and dietary breadth of H. thayeri specifically are not documented, but the functional feeding group is well-established for Hypoptopomatinae as a whole.
In an aquarium context, the care requirements for feeding would mirror those for similarly sized hypoptopomatines: mature aquaria with established biofilm growth, supplemented with sinking algae wafers, spirulina-based foods, and blanched vegetables. However, given the extreme rarity of this species in the trade, practical captive feeding records are not available.
Mating
No specific observations of courtship or mating behaviour in Hisonotus thayeri have been published or reported in the available literature. The species has not been recorded in the ornamental trade, and captive breeding accounts are absent.
Within Hypoptopomatinae broadly, reproductive behaviour has been studied in the more commonly kept genera (particularly Otocinclus). These typically spawn in open water or on plant leaves and vertical surfaces, with eggs scattered or attached adhesively, and no parental care. Whether Hisonotus follows this pattern has not been documented from captive observation of this particular species.
Sexual dimorphism, if any, has not been characterised for H. thayeri in the available literature.
Breeding
Breeding biology of Hisonotus thayeri is undocumented. The species was described only in 2016, has a very limited presence in the hobbyist literature, and does not appear in the ornamental trade. No captive spawning records are known from the cached sources.
Hypoptopomatinae in general tend toward non-guarding spawning strategies, depositing adhesive eggs on surfaces with no subsequent parental investment, but this has not been confirmed for H. thayeri specifically. Clutch size, incubation period, and fry development are unknown for this species.
In the aquarium
Hisonotus thayeri is essentially absent from the ornamental trade and is not a species that hobbyists are likely to encounter. Its description in 2016 from museum material and field collections means it has not entered commercial collection networks, and no husbandry records have been published.
By analogy with ecologically similar hypoptopomatines of comparable size (around 1.5 in SL), a suitable aquarium would be a mature, well-planted or biofilm-rich tank of modest volume — 10–20 US gal — with soft, slightly acidic to neutral water reflecting south-eastern Brazilian coastal drainage conditions. Fine sand or smooth gravel substrate, pieces of smooth rock, and driftwood providing surfaces for biofilm establishment would be appropriate.
The tiny adult size of H. thayeri (under 2 in SL) makes it suitable only for a quiet aquarium without large or aggressive tankmates. Peaceful nano fish — small tetras, rasboras, dwarf corydoras — would be the appropriate companions, with care taken to ensure food competition does not disadvantage the bottom-dwelling grazers.
Should specimens ever become available — through specialist biotope collectors or Brazilian hobbyist networks — detailed water parameter matching to the specific source drainage would be important given the lack of published captive care data.
Conservation
Hisonotus thayeri has not been evaluated by the IUCN Red List. The species was described in 2016, and newly described species typically require several years before formal assessment is possible. Its status is therefore Not Evaluated.
The south-eastern Brazilian coastal drainages that H. thayeri inhabits — the Rio Paraíba do Sul, Rio Doce, and associated coastal rivers — are among the most impacted river systems in Brazil. The Rio Doce in particular experienced a catastrophic tailings dam collapse in 2015 (the Samarco/Fundão disaster), which released iron-ore mining waste across the entire length of the river to the sea. Whether H. thayeri populations in the Rio Doce survived or have since recovered is unknown.
Broader threats to these coastal drainages include deforestation of the Atlantic Forest — the biome that historically covered the coastal escarpment and provided the organic input and shade that maintain the cool, clear-water conditions these loricariids depend upon — agricultural runoff, urbanisation, and water abstraction for irrigation and municipal supply. The Atlantic Forest is one of the world's most threatened biodiversity hotspots, and its aquatic fauna face compounding pressures.
Given the recent description date and the absence of population data, H. thayeri's conservation status in practice is uncertain. Its apparent occurrence across multiple coastal drainages (if correctly identified throughout the range) suggests it is not immediately restricted to a single threatened watershed, but the Rio Doce populations are of concern.