Taxonomy & naming
Hisonotus bockmanni was described by Carvalho and Datovo in 2012 in Copeia (volume 2012, number 2, pages 266–275, figures 1–9). Type specimens were collected from a sandbank in the Rio Cururu, a tributary to the Rio Teles Pires, which in turn drains into the Rio Tapajós, at coordinates 8°53'42.2''S, 57°14'27.8''W, within Jacareacanga Municipality, Pará State, Brazil. The holotype is a female of 1 in SL; a paratype female measured 1 in SL.
The Catalog of Fishes (Eschmeyer, CAS) and FishBase currently recognise Hisonotus bockmanni as the accepted name. However, Roxo et al. (2020) erected the genus Rhinotocinclus and transferred this and related species into it; PlanetCatfish has adopted the revised classification and lists the species as Rhinotocinclus bockmanni (Carvalho & Datovo, 2012), treating Hisonotus bockmanni as a synonym. The diagnosis of Rhinotocinclus rests in part on the canal cheek plate on the ventral surface of the head being posteriorly elongated and contacting the cleithrum — a character said to distinguish the genus from all other Hypoptopomatinae. This site follows the CoF-valid name Hisonotus bockmanni pending broader acceptance of the Roxo et al. (2020) revision.
The genus Hisonotus takes its name from the Greek hisos (equal, even) and notos (back), a reference to the smooth or evenly armoured dorsal surface characteristic of the genus. The species epithet bockmanni is a patronym honouring the Brazilian ichthyologist Flávio A. Bockmann, recognised for his contributions to the study of Neotropical catfishes.
Morphology
Adults of Hisonotus bockmanni are very small, with the holotype female measuring 1 in SL and a paratype female at 1 in SL; FishBase records a maximum of 1 in SL (female) and 1 in SL (male/unsexed). The species thus barely exceeds 1 in in most individuals, placing it among the genuinely diminutive Hypoptopomatinae.
A distinctive and diagnostic feature noted in the original description is the presence of a wide, obliquely transverse dark bar on the dorsal region — a pigmentation character that helps distinguish H. bockmanni from congeners. The species shares a V-shaped spinelet character with three other members of the genus, a feature relevant to phylogenetic placement within Hisonotus.
The body form follows the hypoptopomatine pattern: dorsoventrally flattened, with bony odontode-bearing scutes covering the flanks, a ventral sucking mouth with fine teeth for biofilm grazing, and the typical loricariid external armour. No adipose fin data are specifically noted for this species in the sources consulted; its placement in Hisonotus rather than Parotocinclus reflects osteological and meristic distinctions rather than the presence or absence of that fin.
Sexual dimorphism is documented for the related Rhinotocinclus species concept: males bear a urogenital papilla posterior to the anus, a prominent skin fold on the first pelvic-fin ray, a markedly enlarged nostril that elevates the snout profile in front of the eyes (reducing the internarial distance as seen from above), and pelvic fins that are proportionally longer, often reaching or nearly reaching the anal-fin origin. These characters are expected to apply to H. bockmanni under the synonymised concept, though direct confirmation from the type series — which consists of females — has not been published separately.
Habitat
The type locality is a sandbank on the Rio Cururu at 8°53'42.2''S, 57°14'27.8''W, Jacareacanga Municipality, Pará State, Brazil. The Rio Cururu is a tributary of the Rio Teles Pires, which flows into the Rio Tapajós — a major clearwater tributary of the Amazon in the state of Pará. The upper Tapajós basin in this region is characterised by clearwater rivers descending from the Brazilian Central Plateau, typically with relatively low conductivity, warm temperatures, and modest seasonal fluctuation in chemistry compared to Andean whitewater systems.
The sandbank microhabitat of the type locality suggests the species occupies shallow, productive zones with relatively stable substrate and sufficient light penetration to support biofilm and periphyton growth. PlanetCatfish's distribution record for this species (as Rhinotocinclus bockmanni) covers Amazon, Lower Amazon, Tapajós, Upper Tapajós, and Cururu river systems, indicating a range centred on the Tapajós drainage.
FishBase characterises the genus as having a capacity for facultative air-breathing, an adaptation common among Hypoptopomatinae and useful in shallow, seasonally variable environments.
Feeding
FishBase records a trophic level of 2.7 (± 0.1 SE) for Hisonotus bockmanni, classifying it as a herbivore or low-trophic omnivore consistent with a biofilm and periphyton grazing diet. The fine teeth and ventral mouth position of small Hypoptopomatinae are characteristic adaptations for scraping the aufwuchs community — microalgae, diatoms, cyanobacteria, and associated fine detritus — from rock, sand, and submerged wood surfaces.
No published dietary analysis or gut content data specific to H. bockmanni have been found in the sources consulted. The sandbank type locality may support a diet that includes both epilithic biofilm and fine organic material deposited on the sand surface between coarser substrate particles.
Mating
No mating or courtship observations are available for Hisonotus bockmanni. The species has no registered keepers or spotters in the aquarium hobby, and the original description was based on two female type specimens, leaving male-specific courtship behaviour undescribed.
In the revised Rhinotocinclus framework, the sexual dimorphism of males — including enlarged nostrils, elongated pelvic fins, and a pronounced skin fold on the first pelvic-fin ray — is consistent with competitive and display functions typical of small loricariids, where males may interact actively around preferred microhabitat sites and display to females using fin posturing. Specific mating behaviour for this species awaits observation.
Breeding
No breeding observations have been recorded for Hisonotus bockmanni in captivity or in the field. The species is absent from the ornamental trade and has generated no aquarium breeding accounts.
Based on the general reproductive biology of small Hypoptopomatinae and the closely related genus Otocinclus — where adhesive egg deposition on smooth hard surfaces is the documented mode — a similar strategy is probable for H. bockmanni: adhesive eggs placed on substrate such as glass, leaf surfaces, or smooth rock, with no parental care following spawning. This remains unconfirmed and should not be treated as established fact for this species.
In the aquarium
Hisonotus bockmanni is not established in the aquarium hobby. No husbandry parameters, feeding reports, or breeding accounts have been published. The species is very small — under 1 in SL — and likely has requirements similar to those of Otocinclus species: stable, mature aquaria with established periphyton on décor surfaces, excellent water quality (zero ammonia and nitrite), and supplemental algae-based feeding.
Its origin in the clearwater upper Tapajós basin suggests soft, slightly acidic to near-neutral water at typical tropical temperatures, with good oxygenation. The sandbank type locality hints at tolerance for moderate current and relatively open, lit environments rather than heavily shaded forest streams. Should this species enter the hobby, keeping a small shoal in a well-established aquarium with smooth-surfaced décor and algae growth would best reflect its natural ecology. Beyond these general principles, specific husbandry data for H. bockmanni are not available.
Conservation
Hisonotus bockmanni was assessed as Least Concern on the IUCN Red List, with the evaluation completed on 7 November 2018. The species is known from the Rio Cururu in the upper Tapajós basin, Pará State, Brazil.
The upper Tapajós basin in Pará has faced increasing development pressure, including road expansion, agricultural encroachment, and large-scale hydroelectric projects on the Teles Pires and Tapajós rivers. The Teles Pires hydroelectric complex in particular has raised concern among researchers about the fragmentation of fish communities in tributaries such as the Rio Cururu. For a species with a documented range limited to the upper Cururu, habitat alteration associated with infrastructure and agricultural development in this region could be locally significant even if the species is not globally threatened at present.
The concurrent taxonomic uncertainty — whether the species is valid as Hisonotus bockmanni or subsumed into Rhinotocinclus bockmanni — does not affect conservation status but does underscore the importance of continued systematic work to resolve the identity and range limits of small Hypoptopomatinae in the Tapajós drainage.