Taxonomy & naming
Hoplisoma noxium was described in 2025 by Luiz Tencatt and colleagues in the journal Neotropical Ichthyology, in a paper that simultaneously named a second new species, C. tenebrosum, from the same broad region of the rio Tapajós basin. The two were treated together because they are close relatives and because their description gave the authors a chance to discuss the morphology of the mesethmoid bone of the snout across the Corydoradinae — a structure whose shape changes conspicuously as a fish grows, which has repeatedly confused the identification of long-snouted corys.
The species belongs to the genus Corydoras Lacépède, 1803 as the hobby and most of the trade still use it, and that is the heading we keep. But it was described against the backdrop of the major 2024 revision by Dias, Tencatt and co-authors, which split the old super-genus Corydoras into several lineage genera. Under that scheme this is a member of the resurrected genus Hoplisoma, so the current combination is Hoplisoma noxium. We note the new combination here and follow the Catalog of Fishes for the name itself; the change is recent, and aquarists will mostly still encounter the fish as 'Hoplisoma noxium.'
This fish was long listed as Corydoras noxium; Eschmeyer's Catalog of Fishes places the species in Hoplisoma, and the current combination Hoplisoma noxium is followed here.
Morphology
This is a moderately sized, fairly elongate cory, with type material reaching roughly 2.28 in in standard length — larger than the dwarf species but still a small fish. It has the standard Corydoras build: a body sheathed not in scales but in two rows of overlapping bony plates (scutes) down each flank, a hard helmeted head, a downturned mouth fringed with three pairs of barbels, and stiff leading spines in the dorsal and pectoral fins.
The colour pattern is what marks it out. A dark mask-like blotch crosses the eye and runs back across the head toward the gill region. The flanks are not cleanly striped but broken up: small dark blotches and short, disrupted irregular lines rather than the solid longitudinal bands of some relatives, set against paler ground colour, with pale blotches on the forward part of the body. The underside of the trunk carries small to relatively large coalescent platelets that form a distinctive mosaic-like texture. Importantly, the snout shape shifts with age — becoming more produced as the fish grows — so juveniles and adults can look surprisingly different, a point the describing authors stressed.
Habitat
Hoplisoma noxium is known from small tributaries of the rio Tapajós basin in Pará State, Brazil, including the type stream, the igarapé do Buriti (locally the igarapé Sonrizal), and other forest creeks such as the igarapé Miuçuzinho and igarapé do Pinto. This is classic Amazonian terra-firme creek habitat.
The fish lives in clear, faintly tea-stained streams running over sand and fine gravel, with accumulations of leaf litter and submerged tree roots along the margins. The describing team observed it near the banks among submerged roots, active at night. These are warm, soft, slightly acidic forest waters typical of the southern Amazon clear- and blackwater tributaries; precise long-term temperature and chemistry data for the type localities are limited, so aquarium guidance below is drawn from the general envelope of similar Tapajós-basin corys rather than from published measurements of this species.
Feeding
Like all members of the genus, Hoplisoma noxium is a benthic forager that works the stream bed, sifting sand and leaf litter with its barbels to find small invertebrates, insect larvae, microcrustaceans, worms and the organic detritus that collects among roots and fallen leaves. Nothing in its description suggests a specialised diet; it is an opportunistic bottom omnivore like its relatives. No dedicated gut-content study of this newly described species has been published, so this account reflects the well-documented feeding ecology of the genus rather than species-specific data.
Mating
No spawning observations have been published for Hoplisoma noxium specifically. Corydoradine catfishes are egg-laying shoalers without a pair bond, and across the genus they share a distinctive courtship: a receptive female cleans a surface, a male takes up the classic 'T-position' at right angles to her head, and a small clutch of eggs is fertilised and pressed, a few at a time, into the cleaned site held in a basket of her pelvic fins. It is reasonable to expect this species follows the same pattern, but that should be treated as an inference from its relatives, not a recorded fact.
Breeding
There is no published captive-breeding record for Hoplisoma noxium, which is unsurprising for a fish formally named only in 2025 and barely present, if at all, in the aquarium trade. If it breeds like its many relatives, it would lay small batches of adhesive eggs on firm surfaces, give no parental care (adults of the genus readily eat their own eggs), and produce fry that hatch within a few days and grow on small live and powdered foods. Anyone obtaining wild or early captive stock would be charting this for the first time; the genus-typical approach — a cooler, larger water change to simulate the rains, then removing eggs or parents — is the sensible starting point.
In the aquarium
Hoplisoma noxium is, for now, essentially a scientific novelty rather than an established aquarium fish; it is not yet a fixture of the trade, and most keepers will never encounter it. Should it become available, it would want exactly what its better-known Tapajós relatives need: a shoal of six or more for confident behaviour, a soft sand bottom that will not abrade its barbels, gentle current, clean well-oxygenated water, and warm, soft, slightly acidic conditions with leaf litter and wood to mimic its forest-creek home.
Its name is also a husbandry warning. The epithet noxium refers to the potent toxin the fish releases from its skin under stress — a defence shared across the genus but evidently pronounced here — which in the small closed volume of a transport bag or bucket can poison and kill every fish travelling with it, the donor included. Corys must therefore be bagged in small numbers, with plenty of water and care, and never crowded or roughly handled. As with all corys, the fin spines can tangle in a net, so a cup is the safer way to catch them.
Conservation
Hoplisoma noxium has not been assessed by the IUCN; as a species described only in 2025 it currently has no Red List status. That makes any statement about its conservation provisional. It is known so far from only a handful of small tributary streams in the rio Tapajós basin, which on its face suggests a restricted range — but newly described corys are frequently found to be more widespread once collectors look, so a narrow known range is as likely to reflect limited sampling as genuine rarity. The general pressures on southern-Amazon clearwater tributaries — deforestation, agriculture, road-building and dam projects on the Tapajós system — are real and worth flagging, even though this particular fish has not yet been evaluated against them.