Taxonomy & naming
The rust cory was described by Francesca La Monte in 1941 as Corydoras rabauti, from a specimen collected on a tributary of the Amazon roughly a week's river travel above Manaus, in Amazonas state, Brazil. The epithet honours Auguste Rabaut (1894–1985), the French-born explorer and natural-history collector who gathered fishes across South America in the 1930s and 1940s for institutions including the American Museum of Natural History and the Smithsonian. It was placed in Corydoras Lacépède, 1803 (family Callichthyidae), the huge genus of South American armoured catfishes.
For a long time rabauti was confused with the very similar Corydoras zygatus, and many authors treated the two as one species. That was settled by breeding work — notably by Joe Fuller in the 1980s — which showed that, although the adults look alike, the two have distinctly different colour patterns in their larval and juvenile stages, confirming them as separate species. The difference at the tail end is the field mark: in rabauti the dark dorsal band continues down onto the lower part of the caudal peduncle, whereas in zygatus it does not.
The genus was long overgrown — over 170 species and not a single natural lineage — and in 2024 Dias, Tencatt and colleagues published a large phylogenomic revision (Zoological Journal of the Linnean Society 203(3); DOI 10.1093/zoolinnean/zlae053) that split it into several genera, resurrecting Osteogaster, Brochis, Hoplisoma and Gastrodermus among them. Under that scheme the rust cory becomes Osteogaster rabauti, and the Eschmeyer Catalog of Fishes and FishBase now list it under Osteogaster — the same genus to which the revision moved the bronze cory. The combination is recent and not yet universal, and the trade still says 'Corydoras rabauti,' so we keep that heading while recording the new placement.
Morphology
This is a small, fairly deep-bodied cory reaching about 2–2.5 in in standard length, with the family's standard build: two rows of overlapping bony scutes along each flank, a hard helmeted head, three pairs of barbels around a downturned sifting mouth, and a stiff, mildly venomous spine in the dorsal, pectoral and adipose fins. Females grow larger and noticeably broader than males, most obviously when gravid and viewed from above.
The adult is the draw: a warm rusty orange-brown body — the source of both 'rust cory' and its trade reputation — overlaid in good condition with iridescent violet, orange and metallic-green sheen, and crossed by a single dark band that runs diagonally from the nape and upper back down to the lower part of the caudal peduncle. The juvenile, however, looks markedly different, lacking the uniform rust ground and instead showing a contrasting dark patterning that gradually resolves into the adult coloration over the first few months of growth. That ontogenetic colour change is the species' signature and was the very character that separated it from C. zygatus. An ornamental albino form also circulates in the trade.
Habitat
Corydoras rabauti is an upper-Amazon fish, recorded from the Solimões and Rio Negro drainages and adjacent upper-Amazon tributaries, with the type material from the rio Javarí region on the Brazil–Peru border; it occurs in Brazil and Peru and probably Colombia, though the exact limits of its range remain uncertain. It inhabits a range of quiet-water settings — floodplain lakes, oxbows and the margins of slow river reaches — typically over soft sand and leaf litter.
These are warm, soft, often acidic to neutral waters. Reported tolerances run to roughly 72–81 °F, a pH band from acidic to neutral (about 5.5–7.2, with FishBase citing 6.0–8.0 at the broad end) and soft to moderately hard water. Like other corys it is a facultative air-breather, surfacing to gulp air and absorb oxygen through a vascularised hindgut — an adaptation for the warm, low-oxygen still waters and seasonal pools it favours.
Feeding
The rust cory is a benthic foraging omnivore. In the wild it works soft substrate continuously, sinking its snout and barbels to taste out small invertebrates, insect larvae (including chironomid larvae), worms, micro-crustaceans, seeds and the organic film of detritus, expelling cleaned sand through the gills. It is an opportunist rather than a specialist. In the aquarium it readily takes sinking dried foods and small live and frozen fare — bloodworm, Tubifex, mosquito larvae and the like — and should be fed deliberately rather than left to scavenge tankmates' scraps, which slowly starves a group.
Mating
Like all corys, rabauti is an egg-laying shoaler with no pair bond and the genus's characteristic courtship. A ready female cleans a spawning site — a leaf, a stone, the glass — and the group becomes active; a male takes up the right-angled 'T-position' against her head, and she appears to take his milt into her mouth. The sperm is thought to pass quickly through her gut to fertilise the handful of eggs held in a basket of her clasped pelvic fins, which she carries to the cleaned surface and presses into place. The sequence repeats batch by batch until the clutch is laid.
Breeding
The rust cory is bred in the hobby, set up in a dedicated tank with soft substrate, broad-leaved plants, gentle aeration and mature, soft, slightly acidic water. Spawning is triggered by a large (50–70%) water change with cooler, often rain-soft water and increased oxygenation, after conditioning the group on live foods; breeders commonly run a surplus of males, around two or more per female. A female may lay up to roughly a hundred adhesive eggs, scattered in small batches over leaves, plants and glass. The adults give no parental care and will eat the eggs, so they are removed once spawning ends. Eggs hatch in about two to four days depending on temperature; the fry absorb the yolk sac and become free-swimming a couple of days later, then take infusoria and rotifers followed by microworm and baby brine shrimp. They demand excellent water quality, and over the following months they undergo the species' juvenile-to-adult colour change.
In the aquarium
The rust cory is an excellent, beginner-friendly community catfish given the usual cory essentials. It is a shoaling species and is genuinely stressed kept alone or in twos — keep six or more and they become confident and outgoing; a group is comfortable in a tank of around 23.5 in / 20 US gal or more, where bottom area counts for more than height.
Substrate is functional rather than decorative: provide soft sand or very smooth, rounded fine gravel kept clean, because corys feed by sifting and sharp or dirty substrate erodes and infects their barbels, leaving the blunted faces that signal poor husbandry. The rust cory prefers soft, slightly acidic, well-filtered and well-oxygenated water with gentle current and regular changes, and a planted, wood-strewn, lightly shaded layout shows off its colours and suits its origins. It is wholly peaceful and pairs well with small to medium cyprinids, tetras, pencilfish, dwarf gouramis and dwarf cichlids, never with anything large enough to bully it. Two cory-wide cautions apply: the fin spines snag in nets (use a cup), and a badly stressed fish can release a skin toxin that, in the small volume of a bag or bucket, can wipe out the whole group — so transport them carefully.
Conservation
Corydoras rabauti has not been formally assessed on the IUCN Red List (Not Evaluated). It is a widely distributed upper-Amazon species of no interest to food fisheries, and a growing share of the aquarium supply is captive-bred, so collection pressure on wild stocks is limited — but no Red List determination of its status exists. The broad pressures on western Amazonian freshwaters — deforestation, agriculture, mining and pollution — degrade the quiet floodplain and stream habitats it favours. Because its range is still imperfectly mapped and it was long conflated with C. zygatus, even basic distribution data would benefit from confirmation before any meaningful assessment.