Taxonomy & naming
Hisonotus depressicauda was originally described by Miranda Ribeiro in 1918 in the Revista do Museu Paulista (volume 10, page 635). It has passed through several genus assignments in the course of loricariid systematics and has been listed under the synonyms Microlepidogaster depressicauda and Otocinclus depressicauda, reflecting older placements in genera that have since been revised or synonymised.
The current valid combination, Hisonotus depressicauda (Miranda Ribeiro, 1918), is recognised by the Catalog of Fishes (Eschmeyer, CAS). The species belongs to the subfamily Hypoptopomatinae — the dwarf suckermouth group within Loricariidae — alongside Otocinclus and several other small genera of algae-grazing catfishes found across tropical and subtropical South America.
Morphology
Hisonotus depressicauda reaches approximately 2 in standard length (SL). The body form is characteristic of Hypoptopomatinae: slender and moderately depressed, with the typical loricariid covering of bony scutes and a ventrally placed sucker mouth equipped with fine rasping teeth. The genus name derives from the Greek isos (equal) and noton (back), likely alluding to an even or flattened dorsal profile.
No photographs of Hisonotus depressicauda have been published on FishBase or PlanetCatfish, and the original description is the primary morphological record for the species. Detailed colour pattern and any diagnostic external characters beyond size are not available in the accessible literature. Sexual dimorphism has not been documented for this species specifically; in related hypoptopomatine genera, females are typically broader-bodied than males when gravid.
Habitat
The type locality of Hisonotus depressicauda is Sorocaba, in São Paulo State, Brazil, placing the species in the upper Paraná drainage via the Tietê River basin. The Tietê is one of the major rivers of São Paulo State, draining the central Paulista plateau before joining the Paraná near the state's western border.
The upper Tietê and its tributaries have been heavily modified by urbanisation, industrialisation, and reservoir construction — the greater São Paulo metropolitan area occupies much of the headwater region. Downstream reaches retain more natural character. Specific habitat data for this species (substrate, current preference, vegetation association, water chemistry) are not documented in the published record.
Feeding
As a member of Hypoptopomatinae, Hisonotus depressicauda is presumed to be an aufwuchs micro-grazer — feeding on soft algae, diatoms, biofilm, and fine organic matter adhering to hard surfaces. This is the dietary mode shared by Otocinclus and other hypoptopomatines, and the sucker mouth and fine teeth of the genus are consistent with this specialisation.
No specific feeding observations have been published for H. depressicauda, and the species is not represented in the aquarium literature.
Mating
No observations of courtship or mating behaviour have been published for Hisonotus depressicauda. No records of the species in captivity are known from the aquarium literature or the PlanetCatfish database.
Breeding
No breeding records for Hisonotus depressicauda exist in the aquarium or scientific literature. Based on the general biology of related hypoptopomatines, the species would be expected to scatter adhesive eggs on hard surfaces without providing parental care, but this is extrapolation and should not be taken as documented fact.
In the aquarium
Hisonotus depressicauda has no established presence in the aquarium hobby. No keepers, sightings, or breeding reports are registered on PlanetCatfish, and no photographs have been submitted. The species is not known to be commercially collected or traded.
Any hobbyist who does encounter this species should approach its care as they would other small hypoptopomatines: a stable, mature aquarium with well-established biofilm and algae growth, soft to moderately hard water reflecting its subtropical São Paulo origin, good filtration, and a group rather than solitary keeping. Small hypoptopomatines are sensitive to poor water quality and to the stress of transport and acclimation. Beyond these general principles, no species-specific husbandry data are available.
Conservation
The IUCN Red List assessed Hisonotus depressicauda as Least Concern (LC), last evaluated in November 2018. The assessment places the species in the Tietê River basin, São Paulo State, Brazil, within the upper Paraná drainage.
However, this drainage has been subject to severe anthropogenic modification. The upper Tietê was for decades one of the most polluted rivers in South America, heavily affected by industrial and domestic effluent from the São Paulo metropolitan area. Restoration efforts since the 1990s have improved water quality in some reaches, but large impoundments throughout the basin have fragmented the river and altered habitat character substantially. The degree to which H. depressicauda's specific habitat is intact or degraded is not addressed in available sources.