Taxonomy & naming
Aphanotorulus rubrocauda was formally described by Oliveira, Rapp Py-Daniel, and Zawadzki in 2017 in Neotropical Ichthyology (volume 15, issue 1, DOI: 10.1590/1982-0224-20160028). The Catalog of Fishes (Eschmeyer, CAS) recognises this as a valid species within the genus Aphanotorulus, subfamily Hypostominae, family Loricariidae.
The genus Aphanotorulus accommodates a small group of Hypostominae armoured catfishes known from clear to dark Amazonian rivers. Generic diagnoses rely on a combination of colour pattern, bony-plate counts, and the morphology of the interopercular odontodes. Approximately eight valid species are currently placed in Aphanotorulus according to FishBase. No L-number has been assigned to this species, reflecting its absence from the ornamental fish trade.
Morphology
Adults reach a maximum recorded standard length of 9.5 in SL, placing A. rubrocauda among the mid-sized members of its subfamily. The body bears the characteristic loricariid armour of interlocking bony scutes running from the head to the caudal peduncle. The fin formula includes 2 dorsal spines, 7 dorsal soft rays, 1 anal spine, and 4 anal soft rays — counts consistent with the broader Hypostominae.
The defining diagnostic character is the reddish coloration of the caudal fin, which gives the species its name. This pigmentation distinguishes A. rubrocauda from all known congeners and, combined with specific plate counts and odontode morphology, forms the basis of its formal diagnosis. Ground body coloration follows the typical dark-brown to grey tones of the genus; precise pattern details beyond the caudal fin are not extensively documented in the published literature.
No data on sexual dimorphism have been published for this species.
Habitat
Aphanotorulus rubrocauda is known from the Rio Aripuanã system, a southern tributary of the lower Madeira River within the greater Amazon basin, spanning parts of Mato Grosso and Amazonas states, Brazil. The Rio Aripuanã drains a region of ancient Precambrian shield and delivers dark-coloured, tannin-stained water to the Madeira.
FishBase notes that the species inhabits rivers characterised by dark-coloured water, large boulders, and rocky substrates — the benthic, hard-substrate microhabitat typical of many Hypostominae. Beyond these broad habitat notes drawn from the original description, detailed field data on water temperature, pH, conductivity, current velocity, or depth ranges have not been published.
Feeding
No specific feeding data have been published for Aphanotorulus rubrocauda. By analogy with other Hypostominae occupying similar rocky, fast-water habitats, the species likely forages primarily on periphyton — the films of algae, diatoms, fungi, and associated organic matter that coat exposed rock surfaces. The ventrally positioned mouth and the broad, flat underside typical of the genus are well suited to scraping biofilm from hard substrates.
Data on diet composition in the wild remain unpublished for this taxon.
Mating
No observations of mating behaviour have been recorded for Aphanotorulus rubrocauda in the wild or in captivity. The species has not been maintained in aquaria, and no hobbyist or laboratory breeding reports exist. General reproductive strategies inferred from related Hypostominae are not sufficient to make species-specific claims here; data are sparse.
Breeding
No breeding data are available for Aphanotorulus rubrocauda. The species has not been bred in captivity, and no field observations of nest sites, clutch parameters, or parental behaviour have been published. Data sparse.
In the aquarium
Aphanotorulus rubrocauda is not known to be kept in the ornamental aquarium hobby. No husbandry records, care guides, or aquarium observations have been published. Given that the species inhabits rocky, fast-flowing Amazonian tributaries and attains approximately 9.5 in SL, a well-oxygenated aquarium with smooth-stone substrate and strong circulation would seem directionally appropriate based on habitat data alone — but no tested parameters are available.
This species should be considered unknown territory from a husbandry standpoint.
Conservation
Aphanotorulus rubrocauda has not been evaluated by the IUCN Red List. Its known range is confined to the Rio Aripuanã system of the Brazilian Amazon — a river subject to ongoing pressure from deforestation, agricultural encroachment, and planned or active hydroelectric infrastructure in the broader Madeira watershed. Rocky-rapid specialists with restricted distributions are generally considered more vulnerable to large-scale hydrological changes than habitat generalists, though no formal threat assessment has been conducted for this species.