Taxonomy & naming
Rhinotocinclus marginalis was described by Roberto E. Reis and Pablo Lehmann A. in 2022 in Neotropical Ichthyology (volume 20, number 2, article e220002), as part of the work that established the genus Rhinotocinclus. The Catalog of Fishes (Eschmeyer, CAS) recognises the valid combination Rhinotocinclus marginalis Reis & Lehmann, 2022; because the species was both named and placed in this genus in the same publication, the authority carries no parentheses.
The genus Rhinotocinclus sits within the subfamily Hypoptopomatinae, the group of small, biofilm-grazing armoured catfishes that includes Otocinclus, Parotocinclus and their relatives. The 2022 revision redistributed a number of species that had previously been parked in the catch-all genus Parotocinclus, and R. marginalis was among the taxa anchoring the new genus. FishBase records the species from tributary habitats in Brazil; beyond the type material and the original description, published natural-history data are sparse.
No earlier combinations or synonyms apply to this taxon, which is known only under its original name.
Morphology
Rhinotocinclus marginalis is a very small loricariid. The original description reports a maximum size of about 1 in standard length, with the largest measured specimen at 1 in SL — placing it firmly among the dwarf members of the family. As in other Hypoptopomatinae, the body is clad in the overlapping bony plates (scutes) typical of the suckermouth catfishes, and the ventrally positioned mouth forms a sucker-like oral disc adapted for grazing on flat surfaces.
Detailed external description, coloration and meristic counts for this species are given in the 2022 description; broader popular accounts are not available, and the species is not figured on FishBase. No data on sexual dimorphism have been published for R. marginalis.
Habitat
FishBase records Rhinotocinclus marginalis as a freshwater, demersal, tropical species found in tributaries in Brazil, within the broad Amazon drainage region. Like other dwarf Hypoptopomatinae, it would be expected to associate with submerged structure — leaf litter, woody debris, root tangles and aquatic vegetation — in shallow, well-oxygenated flowing water, where it grazes the biofilm coating these surfaces.
Specific water-chemistry measurements at the type locality have not been published in the sources consulted here, so precise temperature, pH and conductivity figures for this species are not available.
Feeding
No diet study specific to Rhinotocinclus marginalis has been published. FishBase assigns the species a trophic level of about 2.6, consistent with the herbivore-detritivore feeding ecology that characterises the dwarf Hypoptopomatinae. Members of this group typically rasp aufwuchs — the film of algae, diatoms, bacteria and fine detritus that coats submerged surfaces — using their suckermouth oral disc.
Unlike the wood-eating Panaqolus and Panaque of the Hypostominae, fishes in this lineage are not xylophages; they graze biofilm rather than consuming wood. Detailed feeding observations for R. marginalis are not available.
Mating
No courtship or mating behaviour has been documented for Rhinotocinclus marginalis. The species is known essentially from its original description, and reproductive biology was not reported.
Breeding
No breeding data have been published for Rhinotocinclus marginalis. The species has not, to the knowledge of the sources consulted here, been bred in captivity, and no information on spawning mode, clutch size, egg deposition or parental care is available. Many small Hypoptopomatinae are egg-layers that attach adhesive eggs to plant leaves or other surfaces, but this is a general expectation for the subfamily rather than an observation of this species.
In the aquarium
Rhinotocinclus marginalis is not an established aquarium species, and no husbandry guidance specific to it has been published. It does not appear to enter the ornamental trade in any documented way.
Given its tiny adult size and biofilm-grazing ecology, any care requirements would be expected to resemble those of related dwarf suckermouths such as Otocinclus — mature, well-oxygenated water with abundant biofilm-covered surfaces and stable, clean conditions — but in the absence of direct observation these remain inferences rather than recommendations.
Conservation
Rhinotocinclus marginalis has not been evaluated by the IUCN Red List. As a recently described species known from a limited body of material, its distribution, population size and trends are poorly documented, and no conservation assessment has been made.
The Brazilian tributary habitats occupied by small Hypoptopomatinae are broadly subject to pressures from deforestation, agricultural runoff and changes in flow regime, but no species-specific threats have been identified for R. marginalis.