Taxonomy & naming
Ancistrus marcapatae was originally described by Charles Tate Regan in 1904 as Chaetostoma marcapatae in the Transactions of the Zoological Society of London (volume 17, part 3, number 1, page 246, plate 14, figure 1), based on a single specimen collected from the Marcapata Valley in eastern Peru. The basionym Chaetostoma marcapatae and the older spelling Chaetostomus marcapatae are treated as synonyms of the current valid combination Ancistrus marcapatae (Regan, 1904), as recognised by the Catalog of Fishes (Eschmeyer, CAS) and by Lujan et al. (2015) in a molecular phylogenetic study of the Chaetostoma clade (Copeia 103(3): 664–701).
The transfer from Chaetostoma to Ancistrus reflects the reappraisal of generic boundaries within the tribe Ancistrini conducted by Lujan and colleagues, and places H. marcapatae within the largest and most familiar genus of bristlenose plecos. Ancistrus Kner, 1854 is distinguished by the branched tentacles (cirri) that erupt from the snout and interopercular region of adult males — a character not found in Chaetostoma sensu stricto.
FishBase lists the species under ID 50047. PlanetCatfish carries the species under ID 2018 with photographic records of holotype lateral, dorsal, and ventral views — the only images publicly available. The name Ancistrus bolivianus is treated as a synonym of this species in Eschmeyer's Catalog of Fishes. The name Ancistrus heterorhynchus is treated as a synonym of this species in Eschmeyer's Catalog of Fishes.
Morphology
The published maximum size is 5 in total length (TL) per FishBase; PlanetCatfish records 4 in SL for the type specimen. These figures are consistent and describe a medium-sized Ancistrus. All morphological data on record derive from the single holotype collected in 1904; no additional specimens are known to science.
As a member of Ancistrus, adult males would be expected to develop the genus's signature branched snout tentacles (cirri) — fleshy, branching projections that erupt from the rostrum and interopercular region and that serve in male-to-male competition and possibly mate assessment. Females may develop minor, unbranched tentacles at the snout margin. This is genus-level biology; whether the holotype (of unknown sex or maturity) displayed these features is not stated in the available record.
Beyond size, no verified pigmentation description, meristic count summary, or osteological character data have been retrieved for this account. The formal morphological diagnosis resides in Regan's 1904 paper, which was not directly accessed during research.
Habitat
The type locality is the Marcapata Valley, eastern Peru, in the upper Inambari River basin, which drains into the Madre de Dios and ultimately into the Madeira River drainage of the Amazon basin. The Inambari basin spans the Andean foothills (piedmont) and upper lowlands of the Puno and Cusco regions of Peru — elevational zones characterised by swift, highly oxygenated mountain streams and foothill rivers descending through cloud forest and transitional lowland forest.
No field-measured temperature, pH, dissolved oxygen, or conductivity data have been retrieved for Ancistrus marcapatae or the specific collecting site. The Andean foothill streams of the Inambari system are generally cool to warm (depending on elevation), well-oxygenated, and relatively low in dissolved solids — typical of montane Amazonian tributaries.
Critically, only the holotype is on record. Whether the species persists at the type locality, whether it was ever found elsewhere, or whether additional habitat in the Inambari basin was ever surveyed for this taxon is unknown. The Marcapata Valley sits at the junction of Andean foothills and the Amazon piedmont, an area of high fish diversity but also significant historical and ongoing land-use pressure.
Feeding
No diet data for Ancistrus marcapatae are available. The species is not known to have been kept in captivity, and no field observation of feeding behaviour has been recorded.
Ancistrus as a genus are primarily aufwuchs and algae grazers, rasping epilithic algae, biofilm, and diatoms from hard substrates with their specialised oral disc. Many Ancistrus also accept and process soft wood surfaces in captivity, and in the wild may ingest fine organic detritus and small invertebrates incidentally. The Andean foothill rivers of the Inambari system support dense algal growth on boulders and bedrock in areas of high light penetration, which would be consistent with suction-grazing ecology — but these are inference from genus biology, not observations of this species.
Mating
No information on courtship or mating behaviour in Ancistrus marcapatae exists in any source retrieved for this account. The species is known only from a museum holotype collected over 120 years ago.
At the genus level, Ancistrus males are cavity-territory holders that use their branched rostral cirri in male-to-male competition and pre-spawning displays. Males typically select or excavate cavities in wood, root masses, or rock crevices, and defend these sites vigorously against rival males while attempting to attract gravid females. Whether A. marcapatae follows this pattern cannot be confirmed.
Breeding
No breeding records for Ancistrus marcapatae exist. The species has never been documented in captivity, and its entire scientific record consists of the 1904 holotype.
All Ancistrus species that have been studied or bred in captivity are cave spawners with paternal brood care: the male guards adhesive eggs deposited inside a cavity, fans them with his pectoral and caudal fins, and continues to guard the fry for a period after hatching. This is a deeply conserved reproductive strategy across the genus and tribe. Clutch size, incubation period, and fry behaviour in A. marcapatae are entirely unknown.
In the aquarium
Ancistrus marcapatae has never been kept in the aquarium hobby. No specimens beyond the holotype are scientifically documented, and the species does not appear in the ornamental fish trade. PlanetCatfish lists no registered keepers.
Given its Endangered status, extremely restricted known range, and the fact that it is known from a single specimen, this species should be considered effectively unavailable to the hobby. Any speculative husbandry guidance would be fabrication, and none is provided here.
If the species were ever to be encountered alive by field researchers, the genus-level framework for Andean Ancistrus would be a reasonable starting reference: clean, well-oxygenated, moderately cool water with ample rocky and woody cover, and a diet centred on plant matter and biofilm.
Conservation
The IUCN Red List assessed Ancistrus marcapatae as Endangered (EN) on 23 April 2014. The Endangered listing reflects the species' extreme rarity in the scientific record — it is known only from a single holotype collected in 1904 from the Marcapata Valley, Puno/Cusco region, Peru — and its correspondingly tiny, point-source known range within the Inambari River basin of the Amazon's Madeira drainage.
The Andean foothill region of eastern Peru faces substantial and accelerating habitat pressure: expansion of agriculture into the Andean piedmont, road construction (notably the Interoceanic Highway corridor through Madre de Dios), artisanal and small-scale gold mining in the Inambari basin, and associated deforestation and sedimentation. Whether A. marcapatae remains extant at or near its type locality is unknown — no surveys are on record.
The near-complete absence of data for this species makes any population trend assessment impossible. The Endangered listing is an honest acknowledgement of extreme uncertainty compounded by severe range restriction. The holotype resides in a museum collection; the fish in the wild, if it still exists, has gone unrecorded for more than a century.