Taxonomy & naming
Hypostomus careopinnatus was formally described by Martins, Marinho, Langeani and Serra in 2012 in Copeia (volume 2012, number 3, pages 494–500), based on specimens collected from the Rio Taquari drainage of the upper Rio Paraguay basin in Mato Grosso State, Brazil. The Catalog of Fishes (Eschmeyer, CAS) recognises the valid combination Hypostomus careopinnatus with authority attributed to the four authors and the year 2012.
Hypostomus Lacépède, 1803 is one of the most species-rich genera in Loricariidae, sitting within the subfamily Hypostominae. At the time of the species description the genus contained more than 130 valid species; subsequent revisions have expanded that figure considerably. Hypostomus careopinnatus keys out within the genus primarily by the absence of an adipose fin — a character otherwise unknown in Hypostomus except in H. levis. Within that two-species pairing it is distinguished by tooth morphology: the teeth are bifid, bearing a large mesial cusp and a small lateral cusp, compared with the arrangement in H. levis. The species carries 23–42 premaxillary teeth and 25–42 dentary teeth, and the body is peppered with small black spots.
No L-number or hobbyist trade code has been assigned, reflecting the species' rarity in the aquarium trade.
Morphology
Adults reach approximately 2.5 in standard length (SL), making H. careopinnatus one of the smallest species in a genus more commonly associated with fish two to three times that size. The body follows the typical loricariid plan: a dorsoventrally flattened profile, a suckermouth positioned well under the snout, and an armour of interlocking bony scutes covering the flanks and dorsal surfaces.
The most immediately diagnostic feature is the complete absence of the adipose fin — the small, un-rayed fin typically present between the dorsal fin and the caudal fin in Hypostomus and most other loricariids. In careopinnatus the space between the dorsal fin base and the caudal peduncle is bare. This character makes the species easy to identify if the specimen is complete.
Colouration is dark on the dorsal surface with a covering of small black spots on the body and fins. Teeth are bifid (two-cusped), with a dominant mesial cusp and a smaller lateral cusp — a form suited to scraping biofilm from hard substrates. Dimorphism has not been formally described for this species; secondary sexual characters typical of the genus (elongated odontodes on males) have not been documented in published sources for careopinnatus specifically.
Habitat
Hypostomus careopinnatus is known from the Rio Taquari drainage within the upper Rio Paraguay basin, Mato Grosso State, Brazil. The type locality — Alto Araguaia, a tributary of the Rio Ariranha within this drainage — lies at approximately 17°18′37″S, 53°32′22″W. PlanetCatfish records the species from the broader La Plata, Paraná, Paraguay, upper Paraguay and Taquari drainages, suggesting a distribution spanning the headwater tributaries of the Pantanal system.
The upper Paraguay tributaries in Mato Grosso are warm, seasonally flooded, and generally clear to slightly turbid during the dry season. Recorded water temperatures for the species span 77–86 °F. Specific pH and hardness data have not been published for this species; the wider Paraguay headwater system typically presents near-neutral to slightly alkaline conditions with moderate hardness, though these figures have not been confirmed for collection sites of H. careopinnatus.
As with most Hypostomus, a demersal, bottom-associated lifestyle is expected, with the fish rasping algae and biofilm from rock and wood surfaces in shallow to moderate-depth riverine habitat.
Feeding
No dietary data specific to Hypostomus careopinnatus have been published. The broader genus is characterised as algae scrapers and detritivores, using the ventrally positioned suckermouth and hard, peg-like or bifid teeth to rasp biofilm, periphyton, and fine organic matter from hard substrates — rock faces, submerged wood, and occasionally compacted sediment.
In the aquarium, standard loricariid husbandry for small biofilm-grazing species applies by analogy: algae wafers, spirulina-based foods, and blanched vegetable matter (courgette, cucumber, spinach) form a reasonable dietary base. Occasional offerings of small invertebrate or protein-rich foods are likely tolerated but not required. Given the species' small adult size, appropriately sized food particles matter more than with large Hypostomus.
Mating
No mating observations specific to Hypostomus careopinnatus have been published. In Hypostomus broadly, males are typically territorial around cave sites or crevices during the breeding season and compete for access to females. Interopercular odontodes and dorsal odontodes become more pronounced in reproductive males across many species in the genus, serving as visual and tactile signals during male–male competition and courtship.
Given careopinnatus's small adult size, any tank intended for spawning would need appropriately scaled cave or crevice structures. Data on breeding triggers, seasonal cues, or water-chemistry thresholds for this species are not available from published sources.
Breeding
No aquarium breeding reports for Hypostomus careopinnatus have been documented in the hobbyist literature or on PlanetCatfish. The species description (Martins et al., 2012) does not include reproductive data.
By analogy with other small Hypostomus, cave-spawning with paternal brood care is the expected reproductive mode: males guard and fan a clutch of adhesive eggs deposited in a confined cavity, with the female playing no further role after spawning. These inferences are based on the wider Hypostomus literature and should not be taken as confirmed for careopinnatus until direct observations are published.
In the aquarium
Hypostomus careopinnatus is essentially unknown in the aquarium trade. No hobbyist sightings or import records appear on PlanetCatfish, and it carries no L-number or commercial code. This is consistent with its restricted distribution in the upper Paraguay headwaters and its small, inconspicuous appearance — it lacks the dramatic size or bold pattern that drives ornamental collection of many Hypostomus.
For the rare keeper who obtains a specimen, the small adult size of approximately 2.5 in SL is a practical advantage. A well-filtered, oxygenated tank in the 15–20 US gal range with smooth river stones, a few pieces of driftwood, and shaded caves would suit the species. Water temperature in the range of 77–86 °F is supported by field data; other chemistry parameters should be kept moderate and stable, with clean, well-oxygenated water as the primary concern. The absence of the adipose fin makes specimens easy to identify but carries no known husbandry implication.
Conservation
The IUCN Red List assessed Hypostomus careopinnatus as Least Concern, with an assessment completed on 7 November 2018 by the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio). No population size estimate or specific threat assessment narrative was accessible from the published assessment page at the time of writing.
The upper Rio Paraguay basin, including the Pantanal wetland system downstream, faces pressures from agricultural expansion, deforestation in headwater catchments, and altered hydrology from farming operations in Mato Grosso. As a small, restricted-range species with no known trade pressure and no record of ornamental collection, H. careopinnatus is unlikely to be subject to harvest-related threat. Its Least Concern status reflects current knowledge, though the species remains research-sparse and its population dynamics have not been studied.