Taxonomy & naming
Pterygoplichthys scrophus was formally described by Edward Drinker Cope in 1874 in the Proceedings of the Academy of Natural Sciences of Philadelphia (vol. 26, p. 136), from type specimens collected at Nauta, Peru — a town at the confluence of the Marañón and Ucayali rivers in the upper Amazon basin. The original combination was not Pterygoplichthys; the species has passed through the genera Liposarcus (as Liposarcus scrophus) and Glyptoperichthys (as Glyptoperichthys scrophus) before its current placement was established by Page et al. (1996). Both former combinations are now treated as synonyms under the valid Pterygoplichthys scrophus. The type series could not be located (Page et al. 1996); the new combination was established from the original description. The Catalog of Fishes (Eschmeyer, CAS) recognises Pterygoplichthys scrophus (Cope, 1874) as valid.
Pterygoplichthys is a medium-to-large Hypostominae genus whose members are most readily separated from other large plecos by dorsal fin ray count: 10 or more dorsal rays is diagnostic of the genus, compared to 8 or fewer in most other large loricariids including Hypostomus. No L-number has ever been assigned to P. scrophus — the species was well-established in science and the hobby before the L-number coding system began, and pre-dating importations under a named species make a number unnecessary. Mitochondrial D-loop sequences from P. scrophus were used by Montoya-Burgos and Weber in a molecular phylogeny of Hypostomus and related genera (GenBank AJ318362).
Morphology
Adults reach a maximum standard length of 11 in SL, with total length typically in the range of 13–14 in. The body shape is elongated — consistent with the FishBase morphological record — and heavily armoured with rows of bony scutes bearing odontodes across the dorsal and lateral surfaces. Four prominent longitudinal keels run from the head towards the caudal fin, giving the fish its characteristic 'alligator ridge' appearance. These keels are sharper and more spiny anteriorly.
The dorsal fin is large and sail-like, typical of the genus, with 12 soft rays — a high count relative to most loricariids. The presence of 10 or more dorsal rays remains the most reliable field character for distinguishing Pterygoplichthys from other large armoured catfishes. Above and between the large nostrils, P. scrophus bears two prominent nasal flutes (sometimes described as 'horns') that rise conspicuously from the snout — the feature that gives rise to the 'rhino pleco' name. A single median crest of raised armour also runs along the top of the head, a feature otherwise found in Leporacanthicus among loricariids.
Coloration ranges from uniformly dark chocolate-brown to sandy brown with darker saddles across the back reaching toward the belly, sometimes with a subtle pale flash on the upper ray of the caudal fin. Large adults often appear uniform dark mocha. The ventral surface is paler and unarmoured. Sexing in mature fish relies on genital papilla comparison: in males the papilla is a small but thick, protruding stump; in females it is recessed or lies flat against the body.
Habitat
Pterygoplichthys scrophus is native to the upper Amazon basin of Peru, specifically the Marañón and Ucayali river drainages. The type locality — Nauta, Peru — sits at the junction of these two great rivers as they converge to form the Solimões–Amazon proper. The Marañón descends from the Andes through deep gorges before spreading into broad, turbid, white-water lowland channels; the Ucayali drains the eastern Andean slopes and is similarly a productive, turbid, nutrient-rich system.
Both are Andean whitewater systems: sediment-laden, moderately alkaline in the lowlands, warm (75–82 °F), and with relatively stable chemistry year-round compared to clearwater or blackwater rivers. In these environments, P. scrophus is a demersal species associated with the muddy and sandy riverbanks, submerged root systems, and soft-bottomed slower margins that characterise the lower reaches of these rivers. The species is a facultative air-breather — it can supplement gill respiration by gulping air from the surface, an adaptation particularly valuable in seasonally hypoxic floodplain environments. PlanetCatfish records the preferred pH range as 6.0–7.6 and temperature 72–82 °F.
Feeding
Pterygoplichthys scrophus is omnivorous with a strong tendency toward algae and aufwuchs grazing, particularly in juveniles. Younger fish are especially effective algae eaters on glass and hard surfaces. Adults expand their dietary range and will consume detritus, organic sediment, biofilm, plant matter, and opportunistically any food scraps — earning the 'pig pleco' description from PlanetCatfish's Catfish of the Month feature (Dignall, 2011). They are emphatically not fish that can be kept on algae alone.
In the aquarium, the diet should consist of a varied foundation of sinking algae wafers, spirulina discs, and high-quality sinking catfish pellets, supplemented with blanched vegetables (courgette, cucumber, sweet potato, peas), and occasional meaty foods such as bloodworm, Daphnia, or prawn. The species is an enthusiastic, constant feeder and tends to monopolise food sources if kept with slower or shyer species. Its high food intake translates directly into high waste production, which demands strong filtration and regular maintenance.
Mating
In the wild, Pterygoplichthys species are known to be burrow-nesters: the male excavates and defends a tunnel in the soft clay or sand of a riverbank, into which the female deposits eggs. This breeding biology is described by PlanetCatfish on the basis of field observations of the genus. The egg mass, consisting of several hundred to several thousand bright orange, adhesive eggs, is deposited on the walls of the burrow interior, and the male remains in the burrow throughout incubation, guarding and aerating the clutch.
Captive breeding of P. scrophus has not been reported on PlanetCatfish. In the aquarium, courtship observations would require a very large system with soft substrate deep enough for excavation or suitable artificial caves. Sexing adults is feasible through papilla examination — the male's papilla is a small, thick, protruding stump, while the female's is recessed. Males are likely territorial toward rival males, requiring ample space and hiding places to reduce aggression if multiple individuals are kept.
Breeding
Captive breeding of Pterygoplichthys scrophus has not been documented in the hobby literature or on PlanetCatfish. In the wild, breeding is timed to the Peruvian Amazonian seasonal cycle — the timing of the wet season flood pulse and associated temperature rises provide the environmental cues that trigger reproductive readiness in these river systems.
The natural nesting method — deep burrows in soft bank substrate — is difficult to replicate in a standard aquarium. The egg mass in the wider genus is notable for its sheer size: clutch counts in the thousands have been documented for related species such as Pterygoplichthys multiradiatus and P. pardalis in field studies. Eggs are bright orange and adhesive. Incubation takes approximately 6–10 days at tropical temperatures. Successful captive breeding would require either very deep sandy substrate for natural burrow excavation, or purpose-built artificial burrows of appropriate dimensions for adults of this size. Dedicated breeding projects with Pterygoplichthys have been achieved with commoner species (P. pardalis, P. multiradiatus), suggesting the group is not inherently difficult if space and conditions allow, but P. scrophus specifically has not been a focus of captive breeding efforts.
In the aquarium
Pterygoplichthys scrophus is one of the more rewarding large plecos in the hobby for aquarists with the space to house it properly. At up to 11 in SL — roughly 13 in total length — it is noticeably smaller than the common pleco (P. pardalis), which regularly exceeds 19.5 in, making it more manageable for large community or cichlid aquaria. A minimum of 90–105 US gal is appropriate for a single adult; the Catfish of the Month feature (PlanetCatfish, July 2011) suggests each adult pair requires a territory roughly 50 × 39.5 in in size.
Water parameters should reflect the productive Amazonian white-water origin: temperature 72–82 °F, pH 6.0–7.6, soft to moderately hard. The species is more tolerant of moderate fluctuations in water quality than many sensitive Loricariidae, making it a robust choice where water consistency is variable. Filtration must be proportionate to the species' high metabolic output.
Decor should include multiple large cave structures — PVC pipe of appropriate diameter, ceramic tubes, or large slate caves — since adults are strongly territorial around refuge sites. Keeping multiple individuals together is feasible in a large aquarium (118 in or more in length) if sufficient caves are provided. The species is an excellent companion for large robust cichlids (Oscars, Cichla species, large Geophagus), benefiting from the food waste such fish generate and weathering the associated water quality variability. It is not suited to planted tanks or tanks with small, delicate fish.
Conservation
The IUCN Red List assessed Pterygoplichthys scrophus as Least Concern in 2014, noting a wide distribution across the Marañón and Ucayali systems and no evidence of significant population decline. The species is exported in small numbers from Peru for the ornamental trade, though it is not among the most frequently traded Pterygoplichthys species in the international aquarium market.
The broader Peruvian Amazon basin faces increasing pressure from deforestation, agricultural expansion, artisanal gold mining (which directly affects river sediment loads and mercury levels), and infrastructure development. The Marañón drainage in particular has been the subject of controversy over proposed hydroelectric dam projects. While none of these threats currently approach the level needed to alter this species' Red List status, they represent genuine long-term concerns for the health of the riverine ecosystems that support P. scrophus and its community of associated species.