Taxonomy & naming
Hopliancistrus xikrin was formally described by de Oliveira, Zuanon, Rapp Py-Daniel, Birindelli, and Sousa in 2021 in PLoS ONE (volume 16, number 1, e0244894) as part of a full taxonomic revision of the genus Hopliancistrus. The holotype (INPA 37903, 5 in SL) was collected on 15 October 2012 from Rio Bacajá, approximately 1 mi upstream from its confluence with Rio Xingu, Altamira, Pará, Brazil (3°31'10"S, 51°42'35"W). The Catalog of Fishes (Eschmeyer, CAS) recognises Hopliancistrus xikrin de Oliveira, Zuanon, Rapp Py-Daniel, Birindelli & Sousa, 2021 as the valid combination.
The species was known in the aquarium trade for decades before formal description under two codes: L171, first documented by Stawikowski in DATZ magazine as a new Xingu/Iriri import, and LDA015. Both codes are now formally synonymised with H. xikrin. The revision by de Oliveira et al. (2021) elevated the species count in Hopliancistrus from one (H. tricornis, described 1989) to six, recognising four entirely new species — H. xikrin among them — and redescribing the type species.
Hopliancistrus belongs to tribe Ancistrini of subfamily Hypostominae (Armbruster, 2004). The genus is characterised within Ancistrini by having a small number of very strong, recurved interopercular odontodes on the cheek region, and a patch of strong, stiff odontodes on the pectoral-fin spine, a diagnostic combination shared by no other Ancistrini genus in quite the same form.
Morphology
Adults reach approximately 5.5 in SL at maximum (FishBase; de Oliveira et al. 2021 cite the largest examined specimen at 5.5 in SL), making H. xikrin a medium-small loricariid. The body is typical of the genus: depressed, widest at the cleithrum, and gradually narrowing to the caudal peduncle. Five lateral plate series cover the caudal peduncle. The head is large and wide with a roughly square anterior profile in dorsal view; the nuchal plate is exposed and covered by odontodes — distinguishing H. xikrin from H. xavante, in which the nuchal plate is concealed beneath thick skin.
The diagnostic coloration in life is a ground colour of light to dark brown on the head, dorsum, and flanks, with inconspicuous whitish spots on the trunk and flanks that are generally smaller than the pupil — a useful difference from H. tricornis and H. munduruku (in which spots are always larger than the pupil) and from H. wolverine (whose body is near-black with vivid greenish-yellow dots). The dorsal fin carries irregular dark spots on the rays; the caudal and anal fins bear small whitish spots. Juveniles may show a pale yellow blotch on the distal tips of both caudal-fin lobes, absent in adults.
The pectoral-fin spine is notably long relative to body length (32.1–35.7% of SL), shared with H. wolverine and distinguishing both from the other congeners. Cheek odontodes include three large, strong, and curved structures reaching the posterior margin of the branchial opening, with a fleshy sheath covering roughly two-thirds of their length. The dorsal fin formula is II,7; the anal fin i,5 with five branched rays — the latter separating H. xikrin from H. tricornis (four branched anal-fin rays). Vertebral count 27–28.
Habitat
Hopliancistrus xikrin is known exclusively from medium- and small-sized tributaries of the middle portion of the Rio Xingu basin in Pará state, Brazil. The confirmed drainage core is the Rio Bacajá and its tributary the Rio Bacajaí, within the Volta Grande area — a stretch of the Xingu characterised by extensive rapids, rocky outcrops, and substrate of exposed bedrock and boulders. Additional records exist from Igarapé Triunfo and the Rio Jauri system (sub-drainage of Rio Fresco), extending the known range somewhat into southern Pará. The type locality sits at approximately 3°31'S, 51°42'W.
This is a classical Xingu rheophilic microhabitat: fast, well-oxygenated, warm, and oligotrophic. The Rio Bacajá flows across Pre-Cambrian shield geology with extremely low nutrient content, very soft and mildly acidic water, and high transparency. The rock surfaces and crevices that characterise such rapids are the preferred microhabitat for most Hopliancistrus. The species does not penetrate the main channel of the Xingu itself to the degree that some other loricariids do, being concentrated in the mid-sized tributaries that drain into the main river along this segment.
Water temperature in the Xingu system at this latitude averages around 82–86 °F in the dry season, with pH values typically 6.0–7.0 and very low conductivity, reflecting the ancient shield bedrock. Seasonal hydrological variation is pronounced, with high-water periods inundating marginal rock faces and expanding the available microhabitat.
Feeding
Like other Hopliancistrus species, H. xikrin is presumed to be primarily a grazer of periphyton and biofilm attached to rock surfaces. The structure of the mouth and teeth — short, thin, bicuspid teeth with a slightly larger mesial cusp, disposed on a moderately sized oral disc covered with small papillae — is consistent with scraping algae and biofilm from smooth rock faces rather than excavating wood.
The Bacajá tributaries that H. xikrin inhabits are warm, oligotrophic, and rock-dominated, and under these conditions the dominant food source for Ancistrini is the mat of diatoms, cyanobacteria, and associated organic material (aufwuchs) that coats submerged boulders. Field studies on related Ancistrine plecos from similar habitats in the Xingu indicate that gut contents are dominated by algal material, filamentous cyanobacteria, and fine detritus, with occasional invertebrate matter.
In the aquarium, this species should be provided with regular aufwuchs-bearing surface (i.e., mature bogwood, smooth rocks, and roots) and supplemented with algae wafers, spirulina-based foods, and blanched vegetables such as courgette and sweet potato. Animal protein can be offered in small quantities — thawed bloodworm, mysis, or high-quality sinking pellets — to replicate the opportunistic invertebrate feeding likely occurring in the wild. The long pectoral spine and active swimming posture suggest a fish that forages actively across current-exposed surfaces.
Mating
Specific observations on courtship in Hopliancistrus xikrin are not documented in the published literature or in aquarium-keeping records to date. The genus as a whole belongs to the Ancistrini, a tribe in which cave-spawning with male brood-care is universal, and there is no reason to suppose H. xikrin departs from this pattern.
Breeding males of Hopliancistrus develop notably hypertrophied odontodes on the interopercular cheek plates — the genus-defining recurved spines from which the genus takes its name (Greek hoplon = weapon, agkistron = hook). These cheek spines appear to function in both male–male competition and in courtship signalling; the degree of development is greatest in reproductively active males. Males also develop odontodes on the pectoral-fin spines. Sexual dimorphism based on cheek odontode development is the most reliable external indicator of sex in the hobby, though genital papilla differences visible in fully mature fish provide a secondary indicator.
Territorial behaviour around preferred crevice or cave sites is expected in captivity, and male–male aggression in confined spaces can be significant given the armature these animals carry. Providing multiple large crevices and line-of-sight breaks in the aquarium is important when keeping more than one individual.
Breeding
No captive breeding of Hopliancistrus xikrin has been reported on PlanetCatfish or in the aquarium literature at time of writing. Given the rarity of the species in the trade and the restricted availability of wild-caught specimens following changes to Brazilian export regulations, documented captive reproduction remains absent.
Based on well-characterised breeding biology in the closely related H. tricornis and by analogy with other Ancistrini, the species is expected to be a cave-spawner with paternal brood care. Eggs are likely large, few in number, adhesive, and deposited in a narrow crevice, hollow, or tube selected and defended by the male. The male fans and guards the clutch through incubation; based on temperatures typical of the Xingu system, incubation would likely run 5–8 days at 81–86 °F.
Conditioning for breeding would require high water quality, regular partial water changes, and elevated flow and oxygenation to simulate the Bacajá river regime. A gradual lowering of temperature by a few degrees followed by a rise, mimicking seasonal inflow, is a common trigger in Xingu loricariids. Provision of appropriate cave sites — narrow terracotta tubes, tight rock crevices, or hollowed driftwood — is essential. This species remains a target for dedicated Loricariidae breeders.
In the aquarium
Hopliancistrus xikrin is uncommon in the hobby, appearing sporadically as wild-caught imports from the Pará region, typically mislabelled or sold simply as 'L171'. It is not established as a captive-bred species. Given the restricted Brazilian export environment and the species' Xingu identity, expect this fish to remain a niche item sought by Loricariidae specialists rather than a mainstream retail pleco.
The demands of the species are those of a Xingu rheophile. Strong water movement is important — the fish in the wild inhabits rocky rapids, and the aquarium must provide sustained, high-volume current with excellent oxygenation. Temperatures of 79–86 °F match the warm clearwater conditions of the Bacajá drainage. pH should be kept in the soft-acid to neutral range (6.0–7.2), and hardness low. The fish does not tolerate stagnant or poorly oxygenated conditions well.
A tank of at least 47–59 in in length is appropriate for one or two specimens. Rockwork providing multiple crevices and caves is strongly recommended. Smooth-surfaced stones on which biofilm can develop make ideal feeding substrate. Cohabitation with other Hopliancistrus is risky in all but the most spacious and well-structured tanks given the males' formidable cheek odontodes. Other rheophilic plecos, mid-water fish adapted to warm, fast-flowing water (Altum angelfish, Geophagus, or characids from the same river system), and robust bottom-dwellers are generally compatible if territory is well-defined. H. xikrin is peaceful toward non-pleco tankmates.
Conservation
Hopliancistrus xikrin has not been formally assessed by the IUCN Red List. The species was described in 2021, and no assessment had been published at time of writing (FishBase v.2025-2 records it as Not Evaluated). Conservation concern for this species is nonetheless real and immediate.
The Rio Xingu basin in Pará has been profoundly affected by the Belo Monte hydroelectric complex, the main dam of which altered the hydrology of the Volta Grande stretch — the core range of H. xikrin. The Bacajá drainage specifically sits within and adjacent to areas affected by water diversion during Belo Monte's construction and operation; changes to flow regime, sediment dynamics, and water temperature in this reach have been documented by environmental impact studies. For a species restricted to rocky-rapid microhabitats in a small geographic area, hydrological disruption is the principal threat. Illegal gold mining (garimpagem) in smaller tributaries adds pressure via increased turbidity and habitat destruction. The proximity of portions of the range to Xikrin indigenous territory, which affords some degree of protection, is a positive factor. Formal IUCN evaluation and monitoring are needed.