Taxonomy & naming
Pseudolithoxus anthrax was formally described by Armbruster and Provenzano in 2000 in Ichthyological Exploration of Freshwaters (volume 11, number 3, pp. 241–254), originally under the combination Lasiancistrus anthrax. The type specimens were collected from a backwater of the Río Orinoco near a sand beach approximately half an hour upstream from Isla Temblador (3°04'N, 66°28'W), Estado Amazonas, Venezuela. The Catalog of Fishes (Eschmeyer, CAS) recognises the valid combination Pseudolithoxus anthrax; the earlier combination Lasiancistrus anthrax is now treated as a synonym.
The genus Pseudolithoxus was erected to accommodate a small, monophyletic group of Orinoco loricariids — including P. anthrax, P. dumus, P. tigris, P. nicoi, and P. stearleyi — that had initially been placed in Lasiancistrus but proved not to belong there on morphological grounds. Armbruster's work confirmed that these flat-bodied, boulder-clinging species form a clade distinct from Lasiancistrus proper. Within the Loricariidae, Pseudolithoxus sits in the tribe Ancistrini of subfamily Hypostominae (Armbruster, 2004). Among hobbyists the species is universally known by the DATZ code L235, under which it entered the trade long before formal description.
Morphology
Adults reach 5 in standard length (SL), making P. anthrax the largest species in the genus. The most immediately striking feature is the body profile: strongly depressed from dorsum to venter, with a broad, flattened head and a wide interorbital space — an adaptation for clinging to smooth, fast-water rock faces where a narrow or round-bodied fish would be carried away. The dorsal surface is charcoal grey to near-black, densely covered with small, rounded white spots; there is no pale band along the trailing edge of the caudal fin, which immediately separates it from the superficially similar P. nicoi.
Fin counts are diagnostic: two dorsal spines and seven to eight branched dorsal rays; one anal spine and four to five branched anal rays. Plate counts: lateral-line plates 24–26; dorsal plates 7–8; interdorsal plates 6–7; adipose-caudal plates 7–8; post-anal plates 12–14. The pectoral fins are large and held widely spread when the fish is resting on substrate — part of the suction-pad postural complex that anchors it in flow.
Sexual dimorphism is subtle. Breeding males develop longer, more numerous odontodes on the leading edge of the pectoral-fin spine compared to females; gravid females are noticeably broader when viewed from above. Interopercular odontode development in nuptial males is modest compared to genera like Ancistrus.
Habitat
The known range of P. anthrax spans the upper and lower Orinoco basin of Venezuela, with museum records from the Río Caura and Río Aro as well as the main Orinoco channel near Isla Temblador. All collection localities are in the Venezuelan Orinoco drainage; there are no confirmed records from outside Venezuela. The Caura is a large blackwater/clearwater tributary originating in the Guiana Highlands, renowned for its dramatic boulder-strewn rapids, deeply shaded gorges, and warm, very soft, acidic water.
Microhabitat is firmly associated with high-velocity, well-oxygenated zones: the species is found pressed against exposed rock faces, boulder undersides, and the upstream faces of large submerged rocks in rapids and cascades. The flat body and wide, splayed paired fins allow it to maintain position in flows that most loricariids avoid. Water temperatures in the Caura and lower Orinoco reach 75–81 °F during the dry season. PlanetCatfish records a pH range of 6.2–7.0, consistent with the softwater Orinoco tributaries that form its natural range. Dissolved oxygen is presumed high; the species is noted as tricky to acclimatise in captivity, which experienced keepers attribute directly to sensitivity to poor oxygenation.
Feeding
The feeding ecology of P. anthrax in the wild has not been studied in detail, but inference from habitat, gut morphology, and closest relatives points toward biofilm grazing as the primary mode. In fast-water rocky environments, exposed rock surfaces support dense epilithic biofilms — thin mats of algae, diatoms, cyanobacteria, fungi, and invertebrate larvae — that flat-bodied loricariids systematically rasp. The closely related genera Lithoxus and Exastilithoxus are reported to feed exclusively on insect larvae associated with these surfaces; P. anthrax probably takes a broader diet that includes algae and detritus in addition to invertebrate material.
In the aquarium, keepers report acceptance of algae wafers, spirulina tablets, blanched courgette, cucumber, and quality sinking pellets. The fish shows interest in soft spots on driftwood but is not a dedicated xylophage in the manner of Panaque or Panaqolus. A varied diet with substantial green and algae-based components, supplemented by occasional protein-rich foods (frozen bloodworm, daphnia), appears appropriate. Food should be delivered at night; the species is strongly nocturnal.
Mating
Mating behaviour in P. anthrax has not been documented in the scientific literature, and there are no registered breeding reports on PlanetCatfish as of 2025. Inference from the genus and from the wider Ancistrini suggests that males are territorial around preferred shelter sites — rock crevices or overhangs in high-current areas — and display elongated pectoral-spine odontodes to rivals and prospective mates. The species' rarity and exacting water-quality requirements have likely impeded aquarium breeding attempts, though a handful of European specialist keepers are thought to have made progress.
Females are judged by size (broader when gravid) and reduced pectoral odontode development relative to breeding males. Like many Ancistrini, males of most Pseudolithoxus species do not develop the elaborate interopercular bristle fans of Ancistrus, so the primary external sex indicator is pectoral-spine odontode length and, in ripe females, visible girth.
Breeding
No captive breeding has been officially reported in the published or hobbyist literature as of 2025. This puts P. anthrax in the category of loricariids that, while kept successfully by specialists, have not yet been reliably induced to reproduce ex situ. The most plausible reason is the species' sensitivity to dissolved oxygen and water quality: the trigger conditions present in Orinoco rapids — high flow, cool-end temperatures, significant daily fluctuation between dry-season low flows and wet-season flooding — are difficult to recreate in a standard aquarium.
By analogy with closely related Pseudolithoxus species and with other Ancistrini, P. anthrax is expected to be a cave or crevice spawner. Males would guard the eggs in a narrow rock crevice or cavity, fanning with the tail to maintain oxygen supply. Clutch sizes in medium-sized Ancistrini tend to run 20–60 eggs. Conditioning adult fish on a varied, high-quality diet and providing strong water movement, very soft water (conductivity below 100 µS/cm), and slight temperature drops to simulate seasonal change would be the logical first steps for any serious breeding attempt.
In the aquarium
Pseudolithoxus anthrax is regularly described as 'more delicate' than most pleco species, and experienced keepers are consistent in attributing this to water-quality demands rather than any intrinsic fragility. The non-negotiable requirements are excellent oxygenation, clean water low in nitrates, and a soft, slightly acidic chemistry: pH 6.2–7.0, temperatures 75–81 °F, and conductivity ideally below 150 µS/cm. A powerful external canister filter or a powerhead creating significant surface turbulence is essential; still or stagnant tank zones cause stress and immune compromise.
Tank size should be at least 30 US gal for a single specimen, with a generous footprint for territory and unimpeded flow. Décor should replicate a rocky riverbed: flat and rounded stones, slate pieces, and smooth boulders provide natural shelter and resting surfaces. Large pieces of smooth driftwood are also appreciated. The fish is primarily nocturnal and hides during daylight; subdued or red-spectrum lighting encourages more daytime visibility.
Conspecific males can be aggressive, so house only one male per tank unless the aquarium is large and thoroughly divided with rockwork. P. anthrax is generally peaceful toward non-competing species; it can be kept with small tetras, pencilfish, and other fast-water South Americans without issue. Avoid keeping with other large plecos in the same tank. The fish is rarely seen in the trade and commands a premium when available; wild-caught imports are seasonal.
Conservation
The IUCN Red List assessed Pseudolithoxus anthrax as Near Threatened (NT) in 2019, under criterion B2b(iii) — restricted area of occupancy with ongoing habitat quality decline. The species has a limited known range centred on Venezuelan Orinoco tributaries, particularly the Caura and Aro river systems. The Caura basin has historically been relatively well protected and is partially covered by Canaima National Park and surrounding conservation areas, but gold mining pressure, deforestation in the upper watershed, and illegal collection for the ornamental trade all contribute to habitat degradation.
The Near Threatened status reflects genuine concern: unlike Least Concern loricariids with broad distributions, P. anthrax is tied to clean, fast-moving, rocky-substrate rivers of a specific chemistry profile. Sedimentation from upstream deforestation smothers the epilithic biofilm that forms its food base; any increase in fine particulate load or nutrient enrichment that fuels algal blooms of the wrong type shifts the habitat away from what the species requires. Captive breeding would reduce collection pressure, but the lack of established captive populations remains a gap.