Taxonomy & naming
Pseudacanthicus nyktos was formally described by Lopes, Sousa, and Chamon in 2024 in Neotropical Ichthyology (volume 22, number 3, article e240033), from type specimens collected at Vitória do Xingu, Pará, Brazil. The Catalog of Fishes (Eschmeyer, CAS, species ID 77882) recognises the valid name Pseudacanthicus nyktos Lopes, Sousa & Chamon, 2024.
Because the formal description appeared only in 2024, no IUCN assessment has been conducted; the species is too recently named to have passed through the Red List evaluation cycle. This recency is worth noting: a species unknown to science until 2024 underscores how much of the Xingu's ichthyofauna remains undocumented, particularly under the pressure of ongoing hydroelectric development in the basin.
The species had long circulated in the hobby under the provisional code L185 — one of the DATZ L-number assignments given to undescribed loricariids entering the German aquarium trade before formal description. That code is now a synonym for P. nyktos. In Brazil it is known colloquially as assacu-preto (black assacu), and in Danish-speaking hobbyist communities as sortmønstret kaktussugemalle.
Pseudacanthicus is placed in subfamily Hypostominae and in the Acanthicus clade, a group of large, heavily spined plecos whose name references the genus Acanthicus. The clade is characterised by hypertrophied odontodes on the snout margin and longitudinal rows of keeled odontodes on the body flanks — features especially pronounced in P. nyktos. Within Pseudacanthicus, the new species is diagnosed from all congeners by a combination of its dark body with light gray vermicular spots, a reduced tooth count (three teeth in the premaxilla, five in the dentary), a crown-shaped anterior supraoccipital edge with three tips (one larger central, two smaller lateral), a thin and elongated nasal shape, the absence of a rectangular lateral ethmoid with lateral projection, and a short, straight anterior process of the metapterygoid.
Morphology
Pseudacanthicus nyktos is a large loricariid, reaching at least 15.5 in standard length (SL) — placing it among the bigger members of its genus and well above the 8.5 in of P. leopardus or the 10.5 in of P. spinosus. The body is elongated and dorsoventrally flattened in the typical loricariid fashion, clad in rows of bony scutes.
Ground colouration is dark brown to near-black, overlaid with light gray vermicular (worm-like) spots distributed across the dorsal and ventral surfaces and onto the fins. This pattern gives the fish its common name — the spots scattered against a dark background evoke the light patterns of a night sky. The pattern shows on both surfaces, which is unusual and forms part of the formal diagnosis.
The Acanthicus-clade hallmarks are prominent: hypertrophied odontodes line the snout margin, and longitudinal rows of keeled, spine-like odontodes run along the flanks. These structures are not merely ornamental — they function as defence and as intrasexual weaponry during territorial disputes. The dorsal fin has eight rays; the anal fin has five branched rays.
Sexual dimorphism has not been formally described for this species, but in keeping with other large Pseudacanthicus, males likely develop more elaborate odontodes during the reproductive season, particularly on the pectoral-fin spines and snout region. Males in the description paper are noted to confine females in burrows during egg-laying, implying the male is physically larger or at least strong enough to enforce this behaviour.
Habitat
The type locality is Vitória do Xingu, on the lower Xingu River, Pará, Brazil (02°49'13.6"S, 52°00'42.7"W). The Xingu is an Amazonian blackwater and clearwater river flowing from the Brazilian Planalto northward to the Amazon; in its lower reaches near Vitória do Xingu the river is deep, fast-moving, and rocky — a very different character from the slow, silty Amazon mainstream.
According to the description, P. nyktos is found at depths of up to 66 ft in the main river channel, sheltering near and beneath submerged logs. This deep main-channel habitat is consistent with other large Pseudacanthicus, which tend to occupy the deeper, faster, oxygen-rich zones of large Amazonian rivers rather than the marginal shallows. The Xingu at this location runs warm, typically in the range of 79–84 °F, and water transparency is high relative to whitewater Amazonian rivers.
The species is documented from the Amazon basin, specifically the lower Amazon and Xingu River drainage. No other populations or drainages are confirmed from the formal description.
Feeding
Pseudacanthicus nyktos is carnivore-leaning. In captivity the species feeds primarily on small aquatic invertebrates — consistent with the wider Pseudacanthicus pattern, in which the reduced, curved teeth and robust body build favour a diet of hard-shelled prey items and meaty foods rather than algae or wood.
In the aquarium, appropriate foods include prawns (fresh or frozen), chopped mussel, cockle, and other shellfish; sinking meaty pellets formulated for carnivorous catfish; and occasional bloodworm or similar invertebrate supplements. Some vegetable matter — algae wafers, blanched courgette — can be offered as supplementation, and most individuals will take it, but the diet should not be predominantly plant-based. The cactus-pleco lineage lacks the spatulate wood-rasping teeth of Panaque and the cellulose-digesting capacity of wood-eaters; a high-protein diet is appropriate.
FishBase notes a low population resilience for this species (minimum population doubling time 4.5–14 years), which, combined with its apparently restricted known range, suggests the wild population may not sustain heavy extraction pressure.
Mating
The primary description documents a distinctive and forceful mating behaviour: during the reproductive period, males actively confine females within burrows until egg-laying is complete. This is not passive cohabitation — the male physically restricts female egress from the nest site, presumably to ensure fertilisation and to prevent egg predation or disturbance during the vulnerable spawning window.
This behaviour has been corroborated by the small number of captive breeding records held by PlanetCatfish (three reports on record, including photographic documentation of eggs inside a cave). Territorial aggression is pronounced in this species; the description specifically notes intraspecific aggression toward sympatric populations, and captive experience reflects this. Males will defend cave sites vigorously against other males and may harm conspecifics if space is inadequate.
Given the large adult size, breeding-season territorial disputes can cause significant physical injury. A spacious tank with multiple well-separated cave options and visual barriers is essential if more than one individual is maintained.
Breeding
Pseudacanthicus nyktos is a cave-spawning loricariid with paternal brood care, following the pattern typical of the Pseudacanthicus genus. The male selects and defends a burrow or tight cavity — in captivity, a large ceramic cave, a section of hollow driftwood, or a PVC pipe of appropriate diameter — and confines the female within it for the duration of egg-laying.
Only three captive breeding events are on record as of the cache date, reflecting the species' rarity in the hobby and the challenges of conditioning large Amazonian carnivores for reproduction. Photographic evidence from one breeding includes an image of eggs inside a cave, consistent with the cave-spawning mode documented in the description paper.
Given the freshness of the formal description (2024), detailed captive breeding protocols have not yet been widely developed. General guidance for large Pseudacanthicus applies: a large tank (at minimum 80 US gal for a breeding pair), a high-protein diet with live or frozen meaty foods to condition the fish, stable warm water (79–84 °F), and a significant partial water change or simulated rainfall event to trigger spawning. The male fans and guards the eggs; the female should be removed after spawning if aggression is a concern.
In the aquarium
With a maximum known size of 15.5 in SL — equivalent to roughly 17.5–19.5 in total length — P. nyktos is a large predator suited only to large aquaria. A single adult requires a tank of at least 105 US gal; the long-term minimum is better set at 160 US gal or above, with a footprint large enough to allow the fish to turn freely. This is not a community pleco in the conventional sense.
Water temperature should be maintained in the 79–84 °F range matching Xingu conditions. The Xingu in the lower reaches near the type locality is clear and oxygen-rich; high flow and strong filtration are important — large carnivorous plecos produce substantial waste, and the species naturally inhabits current-exposed channel habitat.
Furnishings should include multiple substantial caves (large ceramic tubes, hollow logs, or custom-built plywood structures backed with slate or rock), with enough spatial separation that territorial individuals cannot dominate all retreat sites simultaneously. Driftwood and rockwork add structural complexity. Plants are generally not appropriate in a tank for large Pseudacanthicus.
Compatibility is a serious consideration. The species is territorial with conspecifics and may be aggressive toward any loricariid attempting to occupy shared space. Despite its formidable odontodes, it can be displaced or harassed by faster-moving large cichlids. Best kept as a species-focused display, or with large, robust tankmates that do not compete for bottom territory. PlanetCatfish records only eleven registered keepers and seven documented sightings in the hobby as of the cache date, emphasising its rarity in captivity.
Conservation
Pseudacanthicus nyktos has not been assessed by the IUCN Red List. Described only in 2024, it has not yet passed through the evaluation cycle; its conservation status is formally Not Evaluated (NE).
Several factors warrant attention even absent a formal assessment. The type and only known locality is the lower Xingu River near Vitória do Xingu — the precise stretch of river that was the epicentre of controversy surrounding the Belo Monte hydroelectric dam complex, one of the largest hydroelectric projects in the world. Construction and operation of Belo Monte significantly altered flow regimes, water temperature, sediment dynamics, and submerged wood habitat in the lower Xingu — the same deep-channel, log-associated habitat that the description identifies for P. nyktos.
FishBase records a low resilience score for the species (minimum population doubling time 4.5–14 years), consistent with a large-bodied, apparently site-faithful carnivore with slow recruitment. The ornamental trade collected L185 specimens from the Xingu before the formal description existed; collection pressure is an additional variable that has not been formally quantified. Until a range survey and population estimate are completed, the conservation standing of P. nyktos cannot be meaningfully characterised — but the habitat context makes precautionary concern appropriate.