Taxonomy & naming
Ancistrus reisi was formally described by Fisch-Muller, Cardoso, da Silva, and Bertaco in 2005, in Revue Suisse de Zoologie (volume 112, number 2, pages 559–572). The holotype — a male measuring 2.5 in standard length — was collected on 15 December 2002 from córrego das Dores, a tributary of the rio Vermelho, in the município of Mambaí, Goiás state, Brazil (type locality: 14°29'S, 46°6'W). That stream drains into the upper rio Tocantins basin. The species is recorded in the Catalog of Fishes (Eschmeyer, CAS) under the combination Ancistrus reisi, which is the valid name.
Within Ancistrus — a notoriously species-rich and taxonomically complex genus of subfamily Hypostominae — A. reisi belongs to a small subset of species united by the complete absence of an adipose fin. In its place the fish bears a series of small unpaired platelets forming a low median ridge. Two other described Ancistrus share this character: A. tombador and A. jataiensis, both also from central Brazilian drainages. A. reisi is distinguished from A. jataiensis by a shorter predorsal length (43.8–46.4% SL vs. 47.5–49.3%) and a longer caudal peduncle (27.7–30.9% SL vs. 24.6–27.1%). From A. tombador the original authors cite additional meristic and proportional differences detailed in the 2005 description.
The species has not been assigned a DATZ L-number or any other aquarium trade code, reflecting its absence from commercial imports.
Morphology
Adults of Ancistrus reisi reach a maximum recorded size of 2.5 in standard length — a genuinely small Ancistrus. The body is covered in the overlapping bony scutes typical of loricariids, and the ventral surface is unplated and soft, as in all Ancistrus. The dorsal fin carries eight soft rays and two spines; the anal fin carries five soft rays. Vertebral count is 28, within the range typical for the genus.
The most distinctive external feature is the absence of an adipose fin. Where most loricariids display a single mid-dorsal adipose spine between the dorsal and caudal fins, A. reisi instead bears a row of small, unpaired dermal platelets forming a low, elongated ridge — a condition unique within Ancistrus and shared with only two congeners known at the time of description.
As in all Ancistrus, mature males develop branched, fleshy tentacles (odontodes) on the snout and along the interopercular region — the "bristlenose" characteristic from which the genus takes its common name. Females either lack these structures entirely or bear only rudimentary ones confined to the snout margin. The precise extent of tentacle development in A. reisi males has not been documented in detail in available literature, but the pattern follows the genus-wide rule.
The species is described as benthopelagic in freshwater and is a facultative air-breather, capable of supplementing gill respiration by gulping atmospheric air — an adaptation shared with many loricariids inhabiting warm, seasonally low-oxygen streams.
Habitat
Ancistrus reisi is known from the upper rio Tocantins drainage in the Brazilian state of Goiás, specifically from small tributary streams in the cerrado-dominated headwater region around Mambaí. The type locality, córrego das Dores, is a minor stream feeding the rio Vermelho, which in turn drains into the Tocantins system.
The cerrado is Brazil's vast tropical savanna, and its streams are characterised by seasonal flow regimes: high water during the wet season (October–March) and reduced, sometimes isolated pools or trickles in the dry season (April–September). Water temperatures in cerrado streams vary considerably with season and shade cover; dissolved oxygen can fall during the dry season as flow decreases and organic matter accumulates — conditions that likely underlie the species' documented capacity for facultative air-breathing.
No specific field water-chemistry data (pH, hardness, temperature at collection) have been recorded in the available literature for A. reisi. Given its cerrado headwater origin, soft-to-moderately mineralised, mildly acidic to neutral water is consistent with the broader regional pattern, but these parameters cannot be stated with certainty from current sources.
Feeding
No specific dietary data exist for Ancistrus reisi in the wild. Based on its loricariid morphology — a ventral, sucking mouth with fine comb-like teeth adapted for rasping — and the behaviour documented across the genus, A. reisi almost certainly feeds primarily on periphyton, the community of algae, diatoms, bacteria, and organic detritus that coats submerged rock and wood surfaces in flowing water. Aufwuchs grazing is the baseline feeding mode for virtually all small-bodied Ancistrus.
In the cerrado streams it inhabits, submerged rock, root, and leaf-litter surfaces are likely the primary foraging substrates. Small invertebrates associated with biofilm — midge larvae, ostracods, and similar meiofauna — may form an opportunistic protein supplement, as observed in other loricariid species of similar body size.
For any captive specimens (a rare situation given the species' absence from the ornamental trade), the dietary approach appropriate for small Ancistrus generally would apply: algae wafers, spirulina-based foods, blanched vegetables, and biofilm encouraged by providing rough-surfaced stones and driftwood.
Mating
No field or aquarium observations of mating behaviour in Ancistrus reisi have been documented in available literature. The genus Ancistrus follows a well-characterised reproductive strategy across its many species: males are territorial and defend suitable spawning cavities — crevices in rock, hollow logs, or undercut riverbank substrate — against rival males, displaying their tentacle arrays and engaging in physical contest if necessary.
Male territoriality in Ancistrus appears strongly tied to the quality and defensibility of the spawning site rather than to open-water courtship. In small headwater streams like those inhabited by A. reisi, suitable cavities may be limited by substrate type, which could intensify male-male competition during the breeding season.
Given A. reisi's small adult size (under 2.5 in SL), breeding behaviour consistent with other small Ancistrus species — compact cave defence, tentacle display, and female solicitation — is the most parsimonious expectation, but it has not been confirmed specifically for this taxon.
Breeding
Ancistrus reisi has not been reported in captive breeding in any source reviewed. The species has not entered the ornamental trade, and wild populations are known only from the original type series and a limited number of scientific specimens.
Across the genus, Ancistrus are cavity-spawners with paternal brood care. The male selects a tight cavity, entices a female to deposit adhesive eggs on the cavity ceiling or walls, and then remains with the clutch — fanning continuously with his pectoral fins and tail to aerate the eggs and removing any that fungus. Incubation in related species typically spans five to seven days at tropical temperatures. Fry emerge already bearing juvenile scute patterning and begin grazing on biofilm rapidly.
These patterns are presumed to apply to A. reisi based on genus-level biology, but no specific clutch-size, incubation duration, or fry-development data exist for this species in available sources.
In the aquarium
Ancistrus reisi is not established in the ornamental fish trade and is not currently available to hobbyists. No aquarium-keeping records have been located in any source reviewed for this article.
Should specimens become available through scientific collection or future trade development, the general husbandry framework for small Ancistrus would be the logical starting point. A tank of 15–20 US gal with gentle to moderate water flow, well-oxygenated water, abundant biofilm surfaces (smooth stones, small driftwood), and a range of hiding caves would suit a species from clear, seasonally variable cerrado streams. Water parameters consistent with the broader upper Tocantins region — soft to moderately hard, mildly acidic to neutral — would be a reasonable baseline, though no species-specific tolerance data are available.
Its small adult size (under 2.5 in SL) would make A. reisi a genuinely community-compatible pleco if it were to enter the hobby, and its unusual adipose-fin absence would make it immediately identifiable among aquarium Ancistrus.
Conservation
The IUCN Red List assessed Ancistrus reisi as Data Deficient (DD) — a status indicating that insufficient information exists to make a direct or indirect assessment of extinction risk. The assessment reflects the species' restricted known range (upper rio Tocantins headwaters in Goiás), the limited number of collection records, and the absence of systematic population surveys.
Data Deficient does not imply that the species is not at risk — it means the data required to evaluate risk are lacking. The upper Tocantins headwaters in the cerrado biome face ongoing pressures from agricultural expansion, particularly soy and cattle production, which drive stream siltation, riparian deforestation, and dry-season water extraction. Cerrado streams are among the most threatened freshwater habitats in South America by proportional area loss.
A. reisi's apparent endemism to a small geographic area, its association with clear headwater tributaries vulnerable to siltation, and its absence from the trade (meaning no captive backup population exists) make the Data Deficient designation a genuine conservation concern rather than a comfortable holding position.