Taxonomy & naming
Parotocinclus spilurus was originally described by Fowler in 1941 as Plecostomus spilurus, based on the holotype ANSP 69403 collected from the rio Salgado at Icó Municipality, Ceará state, Brazil, in 1937. The Catalog of Fishes (Eschmeyer, CAS) recognises the current valid combination Parotocinclus spilurus, with the original basionym placed in synonymy under Parotocinclus.
Parotocinclus is a species-rich genus of small hypoptopomatine loricariids distributed across South American drainages from Venezuela to Argentina. Within the family Loricariidae, Hypoptopomatinae is a predominantly South American subfamily of small, dorso-ventrally flattened catfishes closely associated with hard, rocky substrates in flowing water. The genus Parotocinclus is distinguished from the superficially similar Otocinclus (with which hobbyists sometimes confuse it) by the presence of an adipose fin and certain cranial features. A formal redescription of P. spilurus, published in Neotropical Ichthyology, refined the species' morphological diagnosis and documented the four localities currently known for the species.
The specific epithet spilurus derives from the Greek spilos (spot or mark) and oura (tail), referring to dark spots or marks on the caudal fin that are diagnostic for the species.
Morphology
Adults reach a maximum known standard length of 2 in (approximately 2 in SL); the holotype measured 1 in SL. By loricariid standards P. spilurus is a very small fish — comparable in size to an Otocinclus — with the low, flattened body plan typical of rheophilic Hypoptopomatinae. The body is covered in bony scutes and the head is broad and depressed.
The caudal fin bears the dark spots or marks for which the species is named; this patterning can be used to separate P. spilurus from other Parotocinclus species within the Jaguaribe system where ranges or museum lots might overlap. An adipose fin is present, distinguishing the genus from Otocinclus.
Sexual dimorphism is present and has been documented. Males bear a urogenital papilla, a thickened first pelvic-fin ray with a posterodorsal dermal flap, and clusters of odontodes on the caudal peduncle. Females lack these characters. This pattern of male secondary ornamentation is broadly consistent across rheophilic loricariids where males compete for or guard benthic territories.
Habitat
Parotocinclus spilurus is endemic to the upper rio Jaguaribe drainage in Ceará state, north-eastern Brazil — a semi-arid region dominated by the Caatinga biome. The four confirmed localities span the rio Salgado at Icó, the rio Lima Campos near Lima Campos municipality, riacho Machado near Lavras da Mangabeira, and the rio Jaguaribe near Saboeiro municipality. The total area of occupancy is estimated at approximately 10 mi².
The species occupies clear-water streams with moderate current over rocky bottoms and gravel. The redescription records the species as currently restricted to rocky areas immediately below artificial dams — a distribution almost certainly shaped in part by the altered hydrology of the upper Jaguaribe system, where impoundment has affected flow regimes, sediment transport, and water quality across much of the drainage.
The Caatinga is a seasonally dry biome subject to pronounced wet and dry seasons. Stream flows in the upper Jaguaribe can vary dramatically between seasons; during the dry season many minor tributaries cease flowing entirely. P. spilurus appears to persist in the perennial reaches maintained by dam releases and subsurface flows, but detailed physico-chemical data for its occupied localities have not been published.
Feeding
Like other hypoptopomatine loricariids, Parotocinclus spilurus is a benthic grazer equipped with a ventral suctorial mouth for rasping aufwuchs — the mixed biofilm of algae, diatoms, fungal hyphae, bacteria, and fine organic matter that colonises submerged rock surfaces in flowing water. Rocky substrates in clear, well-lit streams with moderate current are typically the most productive aufwuchs habitat, which aligns closely with the microhabitats where P. spilurus has been collected.
Specific dietary studies for this species have not been published. By analogy with congeners and other small hypoptopomatines, the diet in nature is presumed to consist primarily of periphyton and epilithic algae supplemented by fine detritus and invertebrate matter incidentally ingested with biofilm. Current is important for maintaining the algal and biofilm communities on which such grazers depend.
Mating
Reproductive behaviour in Parotocinclus spilurus has not been documented in the literature. The pronounced secondary sexual characters in males — including the caudal peduncle odontodes and modified pelvic fin — are consistent with male-male competition or mate assessment behaviours seen in other rheophilic loricariids. Odontodes on the caudal peduncle in particular are often associated with male combat or with the clasping behaviours some loricariids use during spawning.
In related Parotocinclus species and other small hypoptopomatines, pair formation appears to be transient, with spawning occurring over or among rocks in areas of adequate current. Whether P. spilurus shows any territorial defence of spawning sites by males is unknown.
Breeding
No published accounts of the breeding biology of Parotocinclus spilurus are available, whether from field observations or captive reproduction. Given the species' Endangered status and extreme range restriction, it has not become established in the aquarium hobby and there are no hobby breeding reports.
By analogy with small hypoptopomatines more generally, eggs are likely deposited in sheltered positions on or among rocks, possibly with some adhesion to the substrate. Parental care investment by one or both parents is unknown. Given the limited wild population, deliberate captive-breeding programmes would have conservation relevance, but none are known to be underway.
In the aquarium
Parotocinclus spilurus is not present in the commercial aquarium trade and should not be expected to become available. Its Endangered status, extreme endemism, and very restricted wild population make collection for export inappropriate. No hobbyist breeding populations are known.
Any future husbandry — relevant only to accredited institutions conducting conservation programmes — would need to replicate clear, well-oxygenated, moderately fast-moving water over rocky substrates, with abundant aufwuchs or algae-covered surfaces to graze. Temperature guidance for the Caatinga is difficult to specify without published field data for the species' localities; semi-arid north-eastern Brazilian streams can be warm, though perennial reaches maintained by dam outflows may be cooler and more stable than seasonally variable natural channels. Water chemistry data for the known sites have not been published.
Conservation
Parotocinclus spilurus is assessed as Endangered on the IUCN Red List (B2ab(iii); assessed 7 November 2018), qualifying under Criterion B2 for a very small area of occupancy (approximately 10 mi²), habitat that is highly fragmented or known from very few locations, and a continuing decline in the area, extent, and quality of its habitat. The species is also listed on Brazil's official national list of endangered species.
The upper rio Jaguaribe basin in Ceará sits within one of the most heavily degraded and water-stressed river systems in north-eastern Brazil. The semi-arid Caatinga is subject to chronic water scarcity exacerbated by climate variability, and anthropogenic pressures are severe: large-scale water extraction for agriculture and municipal supply, construction of dams and reservoirs that alter flow regimes and trap sediment, and agricultural runoff carrying nutrients and pesticides into streams. The species appears to have been pushed into the narrow zones of permanently flowing, relatively clean water below dam structures — a precarious refuge that is itself subject to management decisions by dam operators.
With only four known localities and a total area of occupancy of roughly 10 mi², any further degradation of even one or two sites could be catastrophic. Formal conservation actions — protected status for key stream reaches, restoration of riparian vegetation to reduce runoff and maintain stream flow, and monitoring of existing populations — are urgently needed.