Taxonomy & naming
Pseudacanthicus major was formally described by Carine C. Chamon and Thiago Costa e Silva in 2018, published in Zootaxa 4387(3): 499–510 (DOI: 10.11646/zootaxa.4387.3.5). The species is placed within the tribe Ancistrini of subfamily Hypostominae, the same tribe that contains the genus's better-known spotted relatives. The Catalog of Fishes (Eschmeyer, CAS) recognises the valid combination Pseudacanthicus major; GBIF likewise accepts it. FishBase's public-facing database had not yet incorporated this species as of mid-2026, underscoring the value of cross-checking with CAS for recently described loricariids.
In the ornamental trade, this fish circulated under the code L186 for several years before its formal description — a common pattern in a family where new Loricariidae species reach European aquarium markets long before taxonomic publication catches up. The L-number remains its most widely used identifier in the hobby.
Morphology
Pseudacanthicus major is among the largest loricariids known, with a recorded maximum of 23.5 in total length (TL). Like all members of Pseudacanthicus — a genus informally called cactus plecos — the body is covered in densely arranged, sharp odontodes (tooth-like projections) on the scutes and fin spines that give the skin a rough, bristling texture.
The single most diagnostic character is the plain dark-brown ground colour devoid of spots or blotches. This immediately separates P. major from spotted congeners such as P. leopardus, P. pirarara, P. nyktos, and P. histrix, which all carry conspicuous pale spotting against a dark field. The dorsal and caudal fins display transverse pale bands, adding a subtle pattern contrast against the otherwise uniform body. The anterior pterotic process does not contact the orbit margin, and the parieto-supraoccipital posterior edge lacks a conspicuous crest — characters cited in the original description as additional distinguishing features.
Dimorphism has not been formally documented for this species. By analogy with congeners, sexually active males likely develop denser and longer odontodes along the body flanks and opercular regions, but this has not been confirmed in published literature.
Habitat
Pseudacanthicus major is known exclusively from the rio Tocantins basin in central Brazil. The type locality is recorded as the Rio Areias near its confluence with the Rio Tocantins, Porto Nacional, Tocantins state (approximately 10°50′30″S, 48°23′35″W). No collection records from outside this drainage have been reported.
The rio Tocantins is one of Brazil's major river systems, draining a vast stretch of the Brazilian Highlands before joining the Pará River near the Atlantic coast. Water conditions at the type locality are characterised as moderately acidic to near-neutral (pH 6.0–7.5) and warm (77–84 °F), consistent with the oxbow lakes, rapids, and rocky substrates typical of Tocantins tributaries. Like other large Pseudacanthicus, this species is presumed to favour rocky substrates and boulder-strewn rapids where its armoured body and powerful pectoral spines provide secure anchorage against current.
Feeding
Pseudacanthicus belongs to a predominantly carnivorous lineage within Loricariidae, and P. major is no exception. In captivity, specimens accept frozen foods — Artemia, mosquito larvae, mysis shrimp, and krill — as dietary staples, supplemented with sinking pellets and tablets formulated for carnivorous catfish.
In the wild, large Pseudacanthicus are believed to prey on molluscs, crustaceans, invertebrates embedded in rocky surfaces, and possibly carrion. The powerful jaws and stout body of P. major are consistent with this carnivorous ecology. Unlike wood-eating loricariids such as Panaque or Panaqolus, Pseudacanthicus species do not require driftwood for nutritional purposes, though shelter structures such as caves and crevices remain important for behavioural wellbeing.
Mating
No mating behaviour has been documented for Pseudacanthicus major in the literature or in reliably reported captive observations. Given the species' extreme size — adults at 23.5 in TL require extraordinary tank volumes — captive maintenance at the scale necessary for breeding is rarely achieved by hobbyists.
By comparison with smaller, better-studied congeners, male Pseudacanthicus are expected to be territorial around crevice or cave sites, with odontode development intensifying in sexually active individuals. The species' size and the territorial disposition typical of large carnivorous plecos suggest that multiple adults in confined quarters would be incompatible without very large volumes and abundant refuge space.
Breeding
No captive breeding of Pseudacanthicus major has been documented in the reviewed sources, and none is expected to have occurred given the practical constraints imposed by the fish's adult size. A species reaching 23.5 in TL would require a tank of several thousand litres before conditions approximating those necessary for conditioning and spawning could be provided.
Smaller Pseudacanthicus species are cave-spawners with paternal brood care — a reproductive strategy likely shared by P. major, given its position within the genus. In those species, the male guards and fans the egg clutch within a rock crevice or hollow until the fry become free-swimming. Whether P. major has ever completed this cycle in captivity is unknown.
In the aquarium
Pseudacanthicus major is not a fish for the typical community aquarium, or indeed for most large-fish aquaria. At 23.5 in TL it is among the largest loricariids in cultivation, and juveniles grow toward that ceiling steadily if well fed. A single adult would require a tank of at minimum 1,0.5 US gal — and arguably considerably more — with powerful filtration capable of handling the substantial bioload produced by a large carnivore.
Water parameters should reflect the Tocantins origin: temperature 77–84 °F, pH 6.0–7.5, moderate flow and high dissolved oxygen. Large smooth rocks and substantial cave structures are important both for shelter and for the fish's behavioural needs. The dense odontodes covering the body mean handling is hazardous without thick gloves, and the powerful pectoral spines can lock in a net and injure both fish and handler.
P. major is likely to be strongly territorial toward conspecifics and potentially toward other large bottom-dwellers. Tankmates, if kept at all, should be robust and occupy different water layers. The species' rarity in the trade — a direct consequence of its restricted Tocantins range and the low numbers exported — means captive specimens represent a taxonomically significant responsibility.
Conservation
Pseudacanthicus major has not been formally assessed by the IUCN Red List. Its known distribution is restricted to the rio Tocantins basin of Brazil — a river system that has experienced significant anthropogenic pressure, most consequentially from the Tucuruí Dam, one of the world's largest hydroelectric impoundments, which flooded a vast stretch of the lower Tocantins and altered flow regimes, sediment transport, and habitat structure throughout the basin.
The type locality lies in the upper Tocantins above the reservoir, in a region that has not been directly inundated but is subject to ongoing deforestation, agricultural runoff, and river-basin modification. A species with such a narrow known range and large body size — which typically correlates with slower maturation and lower reproductive turnover — merits careful monitoring, even in the absence of a formal Red List assessment.