Taxonomy & naming
Corydoras ourastigma was described by the Dutch ichthyologist Han Nijssen in 1972, on material from the Rio Purus basin in Amazonas, Brazil, and placed in Corydoras Lacépède, 1803 (family Callichthyidae). The epithet combines the Greek oura ('tail') and stigma ('mark' or 'spot'), for the dark blotch in front of the caudal peduncle that gives the fish both its scientific and common names.
When Dias, Tencatt and colleagues revised the Corydoradinae in 2024 and split the old omnibus Corydoras into several genera (Brochis, Hoplisoma, Gastrodermus, Osteogaster and others, alongside Aspidoras and Scleromystax), the name Corydoras was restricted to the long-snouted, 'saddle-snouted' lineage that contains the genus type species, Corydoras geoffroy. C. ourastigma — a long-snouted species — falls within that restricted Corydoras, so unlike the many spotted, blunt-headed corys that changed genus, it keeps the combination Corydoras ourastigma. It is one of a cluster of superficially similar smudge-tailed species and is most reliably distinguished by its conspicuously long snout, which the rounder-headed lookalikes (such as Gastrodermus guapore and Hoplisoma simile) lack.
Morphology
This is a medium-small cory reaching about 2.5 in in standard length. The build is the classic Corydoras form — a short, deep, flat-bellied body and an arched back — but the head is drawn out into a distinctly long, tapering snout, the hallmark of the restricted genus, with three pairs of barbels at the downturned mouth. Two rows of overlapping bony scutes armour each flank and meet along the back; the head is a hard helmet of bone.
The ground colour is pale, and the defining feature is a single dark, grey-to-black blotch with a soft, 'smudged' rather than sharply outlined edge, sitting just ahead of the caudal peduncle. The body otherwise carries faint mottling, and the fins are largely clear. As in all Corydoras, the dorsal and pectoral fins each begin in a stiff, mildly venomous spine. Females grow larger and noticeably broader than males, a difference best seen from above.
Habitat
Corydoras ourastigma is known from the Rio Purus basin, a major whitewater tributary system of the Amazon in western Brazil. Detailed field data are scarce, but like the great majority of corys it is a fish of quieter, shallow water — the sandy margins of streams and rivers, backwaters and flooded edges, over soft sand and mud with leaf litter and submerged wood.
These are warm Amazonian lowland waters, typically around 75–79 °F, soft and acidic to roughly neutral (about pH 6.0–7.2) and soft to moderately soft. As a callichthyid it is a facultative air-breather, rising to the surface to gulp atmospheric air and absorbing oxygen through a vascularised hindgut — the adaptation that lets corys cope with the warm, oxygen-poor shallows they frequent.
Feeding
The long-nosed smudge spot cory is a benthic omnivore and detritivore. Its long snout is a foraging tool: it probes deeper into sand and litter than the blunt-headed species, tasting out small invertebrates, insect larvae, worms, micro-crustaceans and organic detritus, and venting cleaned substrate through the gills. In the aquarium it accepts most sinking dried foods — tablets, granules, larger sinking flake — along with frozen and small live foods such as bloodworm, brine shrimp and chopped earthworm; a varied diet keeps it in good condition.
Mating
As in other Corydoras, this is an egg-laying shoaler with no pair bond and the genus's characteristic 'T-position' courtship. A receptive female lets a male caress her with his barbels, then the pair lock into the T-position, the male grasping her barbels between his pectoral fin and body as he releases milt; the sperm is thought to pass through the female and direct to her pelvic fins, which she cups into a basket to hold the one to few eggs she then carries to a chosen surface — often the aquarium glass in a high-flow spot — and presses into place. The cycle repeats, a few eggs at a time, with multiple males often pursuing the female between depositions.
Breeding
No specific captive-spawning account is documented for Corydoras ourastigma, but it can be expected to breed along standard Corydoras lines. A group conditioned on varied live, frozen and dried foods and skewed toward more males than females is brought into spawning by a large (50–70%) cool water change with increased oxygenation and flow, mimicking the rainy season; this is repeated until the fish spawn. The adhesive eggs, laid in small batches and stuck to glass, wood or plants, receive no parental care and will be eaten by the adults, so the breeder moves either the eggs or the parents. Eggs hatch in about three to four days; once the yolk sac is absorbed the fry take microworm and baby brine shrimp, and do best over a thin layer of sand.
In the aquarium
This is a peaceful, rewarding community catfish that needs the standard cory care. It is a shoaling species and should be kept in a group of at least six, in which it is far more confident and active; a footprint giving plenty of open bottom suits it better than height.
Substrate is critical: use fine, soft sand or very smooth rounded gravel, never sharp or chipped gravel, which abrades and infects the barbels — a long-snouted, dedicated sifter like this is especially prone to barbel damage on poor substrate. Keep the bottom scrupulously clean, the water warm (about 75–79 °F), well-filtered and well-oxygenated with some surface turbulence and flow, and avoid undergravel filtration. It makes an ideal companion for small characins, rasboras, pencilfish, dwarf cichlids and other peaceful catfish, and should not be kept with large or aggressive tankmates. As with all corys, handle gently — the fin spines snag in nets, and a stressed cory can release a skin toxin dangerous in the confined volume of a transport bag.
Conservation
Corydoras ourastigma has not been assessed by the IUCN Red List, so it carries no formal conservation status. That reflects a lack of data rather than a known problem: it is a little-studied species with a limited published range in the Rio Purus basin, and its population trends are unknown. The broad pressures on western Amazonian freshwaters — deforestation, agricultural run-off, mining and the prospect of damming — affect the shallow habitats it favours, and a genuinely narrow-ranged species could be more vulnerable to local disturbance than the widespread, hardy corys of the trade. A formal assessment would be needed to judge its status with any confidence.