Taxonomy & naming
The American palaeontologist and naturalist Edward Drinker Cope described this fish in 1872 as Corydoras amphibelus, from material collected in tributaries of the Río Ambyiacu (Ampiyacu) near Pebas, at roughly 3°10'S, 71°50'W, in the Peruvian upper Amazon. The holotype — a single, unique specimen, ANSP 8290 — remains the only name-bearing type. It belongs to the family Callichthyidae, subfamily Corydoradinae, the South American armoured catfishes whose flanks are sheathed in two rows of bony plates.
The name was carried as Corydoras amphibelus for a century and a half. The genus Corydoras, however, had become an unwieldy assemblage of more than 170 species that molecular phylogenetics showed to be non-monophyletic, and in 2024 Dias, Tencatt and colleagues published a major revision (DOI 10.1093/zoolinnean/zlae053) that broke it into several lineages — among them the resurrected Brochis and Hoplisoma. Under that scheme amphibelus was placed in Hoplisoma, and the Catalog of Fishes now lists the valid combination as Hoplisoma amphibelum (Cope 1872). The change is recent, and because the aquarium literature and trade still overwhelmingly use 'Corydoras', we keep that as the heading while recording the current name. The epithet itself derives from Greek roots referring to a circle or surrounding form, an allusion to the plating; it is best treated as descriptive.
Morphology
Reliable, detailed morphology for this species is thin, because almost everything known traces back to the single nineteenth-century type and the original description. In general form it is a typical small Corydoradinae catfish: a stocky, plated body with the two interlocking rows of lateral scutes that define the subfamily, a hard bony head, a downturned mouth flanked by three pairs of barbels, and stiff, mildly venomous leading spines in the dorsal, pectoral and adipose fins. Its precise maximum size, definitive colour pattern in life, and the features that cleanly separate it from its many congeners are not well established in the modern literature — which is precisely why hobbyists and ichthyologists alike call it the 'mystery cory'. We deliberately avoid stating a confident maximum length or colour description that the sources do not support.
Habitat
Corydoras amphibelus is known from the upper Amazon River basin, the type locality being a tributary system of the Río Ambyiacu near Pebas in Loreto, Peru; the species is generally cited as ranging across the upper Amazon of Peru and into Ecuador. Beyond the type locality, almost nothing specific is documented about the streams it occupies or its microhabitat, and there are no well-attested modern field collections to flesh out the picture.
Reported aquarium-reference water parameters for the species follow the broad envelope typical of upper-Amazon corys — temperatures of roughly 72–79 °F, a pH from acidic to slightly alkaline (about 6.0–8.0) and soft to moderately hard water — but these are general values rather than measurements from its actual habitat. As with other members of the subfamily it is a facultative air-breather, able to gulp atmospheric air at the surface and absorb oxygen through a vascularised hindgut, an adaptation suited to warm, oxygen-poor pools.
Feeding
There is no diet study specific to Corydoras amphibelus. On the basis of its membership in the Corydoradinae it can be expected to be a benthic omnivore and detritivore, working soft substrate with its barbels to taste out small invertebrates, insect larvae, worms and organic detritus and expelling sifted sand through the gills. In captivity — for the few specimens that have ever been kept — it would be expected to take the standard cory diet of sinking pellets and wafers, frozen bloodworm and brine shrimp, and other small foods that reach the bottom.
Mating
No courtship or spawning behaviour has been described for this species. The genus as a whole shares a distinctive and well-studied pattern: corys are egg-laying shoalers with no pair bond, in which a receptive female cleans a site and a male adopts the 'T-position' at right angles to her head, she apparently taking milt into her mouth to fertilise the small clutch of eggs cradled in a pelvic-fin basket before pressing them onto the cleaned surface. C. amphibelus can reasonably be assumed to follow this template, but in the absence of any direct observation that remains an inference rather than a record.
Breeding
Corydoras amphibelus has, as far as the published and hobby record shows, never been bred in captivity — Planet Catfish notes no breeding report — and there is no documented account of its reproduction in the wild. Anything written about how it might spawn is extrapolation from better-known relatives: a likely response to cool, rain-simulating water changes, adhesive eggs deposited on firm surfaces with no parental care, and a few days to hatching. Given how rarely the fish enters the trade, even those generalities are untested. This is one of the species for which honesty about the gaps is more useful than a confident but invented breeding protocol.
In the aquarium
For practical purposes Corydoras amphibelus is not an aquarium fish: it is essentially never collected for the trade, and most people who believe they have it are far more likely to have a similar, better-known species. There is consequently no body of husbandry experience specific to it. Were genuine specimens to appear, they should be treated as any sensitive wild cory: kept as a group of six or more, over soft sand to protect the barbels, in clean, well-oxygenated, gently flowing water of moderate temperature, with shade from wood and plants and an open foraging area, alongside peaceful small tankmates. The standard cautions hold — fin spines snag in nets, and a stressed cory can release a skin toxin lethal in the small volume of a transport bag — but in reality the species' importance is taxonomic and historical rather than practical.
Conservation
Corydoras amphibelus was assessed by the IUCN in 2014 and listed as Least Concern, on the reasoning that it is presumed to be reasonably widespread within the vast upper Amazon and faces no identified species-specific threat. That status sits oddly with how little is actually known: the assessment rests on inference about its range rather than on population data, because the fish is so seldom encountered. The general pressures on western Amazonian headwaters — deforestation, agriculture, road-building and oil and mining activity — apply here as everywhere, but for a species this poorly documented the more honest statement is that its true distribution, abundance and trends remain largely unknown.