Taxonomy & naming
Hypostomus longiradiatus was formally described by Holly in 1929, in Anzeiger der Akademie der Wissenschaften in Wien, Mathematisch-Naturwissenschaftliche Klasse (volume 66, number 13, page 118), based on a single holotype. The original combination was Plecostomus longiradiatus; it is currently recognised under the valid combination Hypostomus longiradiatus (Holly, 1929) per the Catalog of Fishes (Eschmeyer, CAS).
The type locality presents an ongoing problem. Holly recorded the origin as the 'Madeira basin', but subsequent critical review — reflected in the FishBase species account — concludes that 'Rio Guana' in the original text almost certainly refers to the Rio Guamá, a river in the state of Pará, northeastern Brazil, that drains into the lower Amazon estuary near Belém. A paratype locality note mentions 'Ilha das Onças', which sits at the mouth of the Guamá near Belém and is consistent with this reinterpretation. The Madeira basin attribution is therefore considered doubtful, and the species is treated as endemic to the Guamá drainage.
The genus Hypostomus Lacepède, 1803 is among the largest and most taxonomically complex genera in Loricariidae, encompassing well over 100 valid species and a long history of synonymised names. Assigning H. longiradiatus to any infrageneric grouping or species complex is not possible from a single specimen without a modern morphological or molecular analysis, and no such work has been published.
Morphology
The single measurement on record is a maximum length of 15.5 in, cited by Burgess (1989). FishBase lists this figure as total length (TL) for a male or unsexed specimen, while PlanetCatfish records the same value as 15.5 in standard length (SL). The distinction is not trivial: SL excludes the caudal fin and is the standard measure in loricariid taxonomy, whereas TL includes it. Without access to the original specimen or a published re-examination, it is unclear which unit Holly intended and whether the 15.5 in figure is SL or TL. If SL, this would place H. longiradiatus among the larger members of the genus; if TL, total length would be consistent with a mid-sized Hypostomus. This uncertainty is noted and not resolved here.
Beyond the single size datum, no morphological description beyond the original Latin diagnosis is available in the secondary literature consulted. Members of Hypostomus share the family's characteristic body armour of keeled bony scutes, a ventrally placed suckermouth, a high dorsal fin, and — unique to this family — the capacity for facultative air-breathing via a highly vascularised stomach. Whether H. longiradiatus shows any distinctive coloration, spine form, or plate-row count relative to its congeners cannot be stated from available data.
Habitat
If the corrected type locality is accepted, H. longiradiatus originates from the Rio Guamá basin of Pará, Brazil — a blackwater-influenced lowland river draining the northeastern margin of the Amazon basin toward the tidal estuary near Belém. The lower Guamá is a broad, slow river running through seasonally flooded várzea forest and cerradão transition zones, subject to tidal influence near its mouth.
No habitat data specific to H. longiradiatus are recorded. The IUCN assessment notes that no ecological information is available for this species. General attributes expected of a lowland Amazonian Hypostomus — benthic lifestyle on submerged wood and rock substrates, tolerance of warm equatorial temperatures, soft and acidic to neutral water chemistry — are reasonable extrapolations from the broader genus but should not be attributed specifically to this species on current evidence.
Feeding
No feeding data exist for H. longiradiatus. Members of Hypostomus are broadly omnivorous scrapers: they rasp aufwuchs (periphyton, diatoms, cyanobacteria, and detritus) from hard substrates using the fine, multi-cusped or bicuspid teeth that characterise the genus. Some species show a degree of xylophagous (wood-eating) tendency, though this is far more pronounced in the Panaque and Panaqolus lineages. Beyond what can be inferred from close relatives, the diet of this species is unknown.
Mating
No behavioural observations exist for H. longiradiatus. In the genus Hypostomus broadly, males are typically the more territorial sex and develop more elaborate odontodes (spine-like dermal structures on the cheeks and pectoral spines) than females, particularly during reproductive condition. These features are used in male–male competition and appear to function in mate assessment. Whether H. longiradiatus conforms to this pattern cannot be determined.
Breeding
No breeding records exist for H. longiradiatus in the wild or in captivity. No registered keepers or spotters are on record at PlanetCatfish, and the species has not appeared in aquarium literature. Hypostomus species in general are cave-spawners: the male guards a burrow or crevice, the female deposits adhesive eggs on the interior wall, and the male fans and guards the clutch through incubation. Clutch sizes vary considerably across the genus. This pattern is cited as a genus-level expectation, not a documented attribute of this species.
In the aquarium
Hypostomus longiradiatus has not been recorded in the ornamental fish trade. No images, keeper reports, or husbandry observations exist for this species. With its uncertain size (potentially up to 15.5 in in one dimension), any aquarium maintenance would require a large system with appropriate filtration and submerged wood or rock substrate typical of Hypostomus care. Nothing further can be stated without fabricating detail that the data do not support.
Conservation
The IUCN Red List assessed Hypostomus longiradiatus as Data Deficient in 2018 (assessed 7 November 2018; published 2022; assessed by ICMBio, Instituto Chico Mendes de Conservação da Biodiversidade; reviewer: Carapeto, A.). The justification is direct: the species is known only from its holotype, the type locality carries unresolved uncertainty, and neither population size nor distribution area can be estimated. No habitat data are available and no trend information exists.
Given that the single known specimen was collected nearly a century ago and no subsequent records have been published, the possibility that H. longiradiatus represents a genuinely rare taxon — or that taxonomic confusion with related species has obscured its identity — cannot be excluded. A revision of the relevant Hypostomus species complex in northeastern Amazonia, incorporating the holotype in a morphological or molecular framework, would be necessary before a more informed assessment could be made.