Taxonomy & naming
Corydoras colossus was formally described in 2023 by Luiz F. C. Tencatt, Tim Grant and David Bentley in Neotropical Ichthyology, in a paper titled for its 'color-changing' subject. The holotype, a fish of about 2 in SL, was collected in 2022 from the rio Jutaí near its confluence with the rio Solimões, in the Amazonas State of Brazil. Long before that, the fish had circulated in the hobby under the collection code CW045 and the trade names 'Corydoras sp. Lessex' and 'false armatus,' for its resemblance to the larger long-snouted Corydoras armatus complex. The epithet colossus — Latin for a giant or colossal statue — marks it as one of the larger members of its group.
It is a short-snouted, deep-bodied species. When Dias, Tencatt and colleagues split the old mega-genus Corydoras in their 2024 phylogenetic revision (DOI 10.1093/zoolinnean/zlae053), resurrecting Hoplisoma Blyth, 1860 for a large clade of short-snouted forms, C. colossus fell into that genus, giving the current combination Hoplisoma colossus. The specialist site Corydoras World places it in the Hoplisoma lineage, consistent with the Catalog of Fishes. Because the description and the hobby still use 'Corydoras colossus,' we keep that as the heading while recording the Hoplisoma combination.
Morphology
True to its name, this is one of the larger, more massively built corys — a tall, deep-bodied, robust fish reaching roughly 3.5–4 in in total length, well above the size of a typical community cory. It has the standard generic armour of two rows of overlapping bony scutes along each flank, a hard helmeted head and three pairs of barbels, but on a notably high-backed, heavy frame.
The coloration is its other distinction. The body bears a fine reticulated or peppered pattern, broadly recalling the armatus group but without the heavy spotting, and the head carries a characteristic orange blotch. The species is genuinely colour-changing: individuals shift their depth of pattern and tone with mood, lighting and condition — the central observation of its description. The fins carry the family's stiff locking spines, and as in other corys females grow larger and rounder than males.
Habitat
Corydoras colossus is, on present knowledge, restricted to the rio Jutaí, a clear-to-blackwater southern tributary of the rio Solimões (upper Amazon) in Amazonas State, Brazil, reportedly near the Jutaí's confluence with the main river. It is likely to occur more widely in the streams draining into that river and further upstream, but the documented range is narrow.
The habitat is lowland Amazonian: warm, soft, often acidic water over sand, with leaf litter and submerged wood, the kind of slow-to-moderate forest stream and river margin that suits a large sand-sifting cory. In aquarium terms the species is reported to do well in warm water of roughly 73–81 °F, soft to moderately hard, around pH 6.0–7.5, with good oxygenation and some current — conditions that mirror its clearwater-tributary origins. As a Corydoras it is a facultative air-breather, gulping atmospheric air at the surface when oxygen runs low.
Feeding
Like all corys, C. colossus is a benthic omnivore that forages over the bottom, sifting sand and leaf litter with its barbels and downturned mouth for small invertebrates, insect larvae, microcrustaceans and organic detritus. Its larger size simply means a larger appetite; the niche is the same sand-sifting one as its smaller relatives. In the aquarium it readily takes sinking dried foods together with small live and frozen items such as bloodworm and brine shrimp, and as a big, active forager it benefits from a varied, generous diet rather than scavenged leftovers.
Mating
Detailed spawning has not been published for Corydoras colossus, but the genus and its segregate Hoplisoma are highly uniform in reproduction. These are egg-laying shoalers with no pair bond and the characteristic 'T-position' courtship: the male presents himself crosswise to the female's head, she takes up his milt, and a few adhesive eggs are clasped in a basket of her pelvic fins and pressed onto a cleaned leaf, stone or pane. The sequence repeats a few eggs at a time. C. colossus is expected to follow this pattern, scaled to its larger body and correspondingly larger eggs.
Breeding
There is no well-established aquarium breeding record for Corydoras colossus, and no published clutch sizes or fry data for the species. By analogy with other large Hoplisoma/Corydoras, the expectation is a clutch of adhesive eggs laid in small batches on hard surfaces, typically triggered by a cool, fresh water change mimicking the rains, with no parental care — the adults will eat the eggs, so a breeder removes either the eggs or the parents. As a larger species it would likely produce relatively large eggs and robust fry, which would take baby brine shrimp and microfoods after absorbing the yolk sac, but the specifics remain to be documented.
In the aquarium
The colossus cory is a desirable but uncommon aquarium fish, and its main difference from a standard cory is scale: it is a large, deep-bodied species that needs room. Keep it as a group of five or six or more — corys are shoaling fish stressed when kept alone — and give them a tank with a generous footprint rather than a nano set-up; an aquarium on the order of 30 US gal or larger suits a small group.
Provide a soft fine-sand substrate to protect the barbels, with shade among driftwood, rocks and planting. Water should be warm (about 73–81 °F), clean, well-filtered and well-oxygenated with some current and regular changes; soft to moderately hard, around pH 6.0–7.5, suits it. Despite its bulk it is a peaceful, calm fish, good with other medium-sized peaceful community species. The usual cory cautions apply: the stiff fin spines tangle in nets, and a badly stressed cory can release a skin toxin that can harm a whole group in the confined volume of a bag, so transport with care.
Conservation
Corydoras colossus has not been assessed by the IUCN — it was described only in 2023 and carries no Red List status, so its conservation situation is formally unknown rather than confirmed secure. On current evidence it is known only from the rio Jutaí drainage, a narrow documented range, which is the kind of distribution that warrants assessment in its own right. The wider upper Solimões/Amazon region faces deforestation, expanding agriculture and, in places, mining pressures, all of which can degrade the clearwater tributary habitats this species occupies. It is also a fish with a hobby-trade history under its old CW045 code, so any wild collection should be monitored. A future Red List evaluation will need basic distribution and population data that do not yet exist.