Taxonomy & naming
Ancistrus megacanthus was described by Widholzer, Borsoi, Reis, and Lehmann A. in 2024, with the original description published in Neotropical Ichthyology (volume 22, number 1, article e230078). The Catalog of Fishes (Eschmeyer, CAS) records the valid combination as Ancistrus megacanthus Widholzer, Borsoi, Reis & Lehmann A., 2024 — an unparenthesized authority, indicating the species was described directly in Ancistrus.
The type locality is the rio Vacacaí, on road RS-630, a tributary of the rio Jacuí within the Laguna dos Patos system, at São Gabriel, Rio Grande do Sul, Brazil (30°27'18"S, 54°22'26"W). The distribution encompasses the rio Vacacaí drainage and associated waters of the rio Jacuí and Guaíba system, all draining ultimately to the Laguna dos Patos on the Atlantic coast of Rio Grande do Sul.
The species belongs within the tribe Ancistrini of subfamily Hypostominae. No L-number has been assigned. As a 2024 description, A. megacanthus has not yet been assessed by the IUCN Red List.
Morphology
Ancistrus megacanthus reaches approximately 5 in standard length (SL) — a notably large size for Ancistrus, well above the 2.5–3.5 in typical of the most commonly kept species in the genus. The body is covered in the standard loricariid armour of bony scutes, with the ventral surface unplated.
The most distinctive morphological character — and the source of the species name — is the well-developed pectoral-fin spine. In males, this spine exceeds half the length of the leading ray of the pelvic fin, an unusual proportion that stands out within the genus. Dorsal fin formula comprises 2 spines plus 7 soft rays; 4 branched anal-fin rays are present.
Coloration is dark brown to black on the dorsal surface, overlaid with small white or yellow dots. Dorsal and caudal fins are plain dark brown to black, lacking the lighter spots or banding that appear in some congeners — a diagnostic character that aids visual identification. The ventral surface is paler, as is typical of the family.
Sexual dimorphism in A. megacanthus follows the Ancistrus pattern: mature males develop soft, fleshy tentacles on the snout and leading edge of the head, which are absent or greatly reduced in females. Females may develop a few small tentacles along the snout rim. The well-developed pectoral-fin spine described above is explicitly noted in males.
Habitat
Ancistrus megacanthus is known from the rio Vacacaí, a right-bank tributary of the rio Jacuí, itself part of the Laguna dos Patos drainage in the state of Rio Grande do Sul, Brazil. The type locality at São Gabriel sits at approximately 328 ft elevation, in the Pampa biome — the subtropical grassland-savanna landscape that covers most of southern Rio Grande do Sul and extends into Uruguay and Argentina. The Pampa is characterised by gently rolling terrain, campo grassland, and gallery forest along watercourses, with soils ranging from clay to gravel.
The collection habitat is described as swift streams with rocky and gravel substrate, consistent with the strong-current, hard-substrate microhabitats favoured by many loricariids. This is a cooler, temperate-subtropical environment compared with Amazonian Ancistrus habitats: Rio Grande do Sul has significant seasonal temperature variation, and river temperatures in this region can drop to below 59 °F in winter while reaching 79 °F or higher in summer. Specific water chemistry measurements for collection sites of A. megacanthus are not recorded in accessible sources, but the geology of the region suggests moderately hard, circumneutral to slightly acidic water (estimated pH 6.5–7.5).
The southernmost confirmed occurrence of Ancistrus in Brazil, the Pampa distribution of A. megacanthus is biogeographically significant: it demonstrates that the genus has successfully colonised temperate-subtropical drainages well beyond the Neotropical lowlands typically associated with the Ancistrini.
Feeding
No direct dietary data for Ancistrus megacanthus have been published. Diet is inferred from genus biology. Ancistrus species are algae grazers and biofilm scrapers, using the inferior, sucker-like mouth to rasp organic material from hard surfaces — rock, gravel, submerged wood, and clay banks. The swift, rocky streams of the Pampa biome would support diatom and periphyton communities on exposed stone surfaces, consistent with this feeding mode.
In aquarium practice, Ancistrus species broadly accept algae wafers, spirulina sinking foods, blanched courgette (zucchini), cucumber, and similar vegetable matter. Occasional protein supplementation is tolerated. These generalisations apply to the genus and are not specific to A. megacanthus, for which no published or hobbyist dietary observations exist.
Mating
Mating behaviour in Ancistrus megacanthus has not been observed or documented. General Ancistrus reproductive biology involves males becoming territorial around defended cavities — rock crevices, woody debris, or similar enclosed structures in swift water — and using these sites as spawning locations. The prominent snout tentacles of mature males are a species-recognition and possibly an intrasexual signalling character.
Given the swift-stream, rocky-substrate habitat of A. megacanthus, natural spawning sites would likely be crevices between stones or beneath large rocks in areas of moderate to strong current. No hobbyist sightings or courtship observations for this species are recorded on PlanetCatfish or in other accessible sources.
Breeding
No breeding reports for Ancistrus megacanthus exist in the accessible literature or in hobbyist databases. The species was described in 2024 and has not yet entered the aquarium trade to any documented degree, so captive breeding data are entirely absent.
Across the genus, Ancistrus is a well-characterised cave spawner: the male selects and defends a cavity, coaxes the female in for spawning, and then guards and fans the adhesive egg clutch until the fry are mobile and beginning to feed independently. Whether A. megacanthus conforms to this pattern cannot be confirmed without direct observation, but it is the most parsimonious expectation given its generic placement.
In the aquarium
Ancistrus megacanthus is not known to be present in the aquarium trade. No import records, hobbyist sightings, or care accounts have been located. Wild-habitat photographs are on record at PlanetCatfish, but no captive specimens have been documented.
The species' subtropical origin is an important consideration for any future aquarium interest. Unlike the majority of trade Ancistrus, which originate in lowland tropical Amazonian systems and require stable temperatures of 75–82 °F, A. megacanthus comes from a seasonally variable environment where temperatures range broadly across the year. This suggests the species may tolerate or even benefit from cooler conditions (possibly 61–75 °F) and might be a candidate for temperate-room aquaria without heating, at least seasonally — but this is inference from biogeography, not documented practice.
Its larger adult size (5 in SL) relative to common trade Ancistrus should be factored into any tank sizing considerations. The swift-stream origin suggests well-oxygenated water with moderate flow is likely preferred.
Conservation
Ancistrus megacanthus was described in 2024 and has not been evaluated by the IUCN Red List. Its status is therefore Not Evaluated (NE).
The species' known range is restricted to the rio Vacacaí drainage within the Laguna dos Patos system of Rio Grande do Sul — a geographically limited distribution within a biome, the Pampa, that is one of South America's most threatened. The Pampa biome has experienced extensive conversion to agriculture (soy, rice, livestock), urban expansion, and eucalyptus plantation forestry, with only a small fraction of native grassland and gallery forest remaining intact. Freshwater habitats in Rio Grande do Sul face additional pressure from agrochemical runoff, water extraction for irrigation, and periodic flooding amplified by land-use change.
Whether A. megacanthus is genuinely range-restricted or whether additional populations exist in the broader Jacuí and Laguna dos Patos systems is not yet known. Formal IUCN assessment will require survey data on distribution, abundance, and habitat condition. As the southernmost Ancistrus in Brazil, the species also represents a biogeographically irreplaceable lineage that warrants attention in regional freshwater biodiversity planning.