Taxonomy & naming
Corydoras undulatus was described by the British ichthyologist Charles Tate Regan in 1912, as part of his revision of the South American catfishes of the genus Corydoras. The original description was notably brief, based only on external features, and for over a century the species remained poorly defined — a confusion that entangled it with Corydoras latus Pearson, 1924. A full redescription by Bono and colleagues, published in PLOS ONE in 2019, finally clarified its identity and fixed its diagnosis.
The species was placed in Corydoras Lacépède, 1803 (family Callichthyidae). That genus proved non-monophyletic, and the 2024 revision by Dias, Tencatt and colleagues (DOI 10.1093/zoolinnean/zlae053) split it into several genera. Under that scheme C. undulatus belongs to the resurrected genus Gastrodermus Cope, 1878 — a move foreshadowed in the 2019 redescription, which noted that the name Gastrodermus might be resurrected for this group — and FishBase now lists it as Gastrodermus undulatus, the current valid combination. The hobby still uses 'Corydoras,' so we keep that heading while noting the change. The Catalog of Fishes treats the name as valid.
Morphology
This is a small cory, reaching only about 1.5 in standard length, of compact armoured build: two rows of overlapping bony plates down each flank meeting in a dorsal ridge, a hard helmet of a head, and short barbels around the down-turned mouth.
The colour pattern is the source of the name. On a pale ground, dark purplish spots tend to run together and coalesce into undulating, wavy longitudinal bands along the body — 'undulatus' meaning 'wavy.' The effect is of broken, rippling lines rather than the discrete blotches of the spotted corys. The dorsal, pectoral and adipose fins each carry the usual stiff defensive spine.
Habitat
Corydoras undulatus is a fish of the temperate south of the continent, recorded from the lower Paraná and Paraguay river basins, the Uruguay river basin and the coastal rivers and Laguna dos Patos system of southern Brazil, across Argentina, Uruguay and southern Brazil. It is found in clear, well-vegetated water over gravel and sand; collectors have noted it in densely planted reaches with limpid water and a gravel bottom.
What sets this species apart from its tropical relatives is the climate of its range: these subtropical-to-temperate waters cool markedly in winter, sometimes to single digits Celsius, so the fish tolerates cooler conditions than most corys. FishBase gives an indicative aquarium band of about 72–79 °F, but the species naturally experiences a wider, cooler swing. Like all corydoradines it is a facultative air-breather, surfacing to gulp air and absorb oxygen through a vascularised hindgut.
Feeding
Corydoras undulatus is a benthic omnivore and detritivore, sifting the gravel and sand of its well-vegetated streams for small invertebrates, insect larvae, worms, micro-crustaceans and organic detritus, expelling cleaned substrate through the gill openings. It is an opportunist, taking whatever the bottom offers. In captivity it accepts the usual small sinking foods — pellets, frozen bloodworm and brine shrimp — worked from a clean substrate, and should not be expected to live on tankmates' leftovers alone.
Mating
No detailed spawning study specific to Corydoras undulatus has been published, and the account here is the standard corydoradine pattern, given as the expected behaviour. These are egg-laying shoalers with no pair bond: a ready female cleans a site, a male takes the classic 'T-position' at right angles to her head, and she clasps a few eggs in a basket formed by her pelvic fins before pressing them onto a cleaned leaf, stone or plant. The cycle repeats a few eggs at a time until a clutch of sticky eggs is scattered across the chosen surfaces.
Breeding
There are no well-documented aquarium breeding reports for Corydoras undulatus, which is an uncommon fish in the trade. By analogy with the genus, spawning would be expected to follow the familiar pattern — adhesive eggs laid in small batches on firm surfaces, no parental care, and a cool, large water change as the trigger. For a temperate-edge species like this, the natural seasonal cooling of its range suggests a winter chill followed by warming and a water change might be the most effective cue. Until it is bred and documented, this remains informed expectation rather than established husbandry.
In the aquarium
Corydoras undulatus is an occasional import rather than a staple, and its chief peculiarity for the keeper is temperature: coming from the cooler south, it does well in an unheated or low-heat tank and does not want persistently warm tropical water. Keep it as a shoal of six or more — it is a sociable fish that frets alone — over a soft, fine substrate, with the clean, well-oxygenated, planted conditions it knows from the wild. Gentle current, good filtration and regular water changes suit it, and it is a peaceful companion for other temperate-tolerant community fish. As with all corys, avoid sharp gravel that damages the barbels, handle it with a cup rather than a net (the fin spines snag), and transport it carefully, since a stressed cory can release a skin toxin lethal in the small volume of a bag.
Conservation
Corydoras undulatus is assessed by the IUCN as Least Concern, in an evaluation dating to 2020. It is reasonably widespread across the lower Paraná, Uruguay and southern Brazilian drainages and is not subject to heavy targeted collection. Like all fishes of the heavily settled temperate Plata basin it faces pressures from agriculture, water management, pollution and habitat modification, but as a species it is not presently considered at risk.