Taxonomy & naming
Corydoras petracinii was described in 2010 by Felipe Alonso and Adrián Calviño (Calviño & Alonso), in a paper that named two new corys from northwestern Argentina and redescribed C. micracanthus. The holotype (MACN 9233, 1.5 in SL) came from a small nameless stream near Salta city. The epithet honours Roberto Petracini (1941–2016), an Argentine aquarist recognised for his contributions to fishkeeping in South and Central America.
The species was placed in Corydoras Lacépède, 1803 (family Callichthyidae). The 2024 revision by Dias, Tencatt and colleagues split that overloaded genus into several lineages, and under that scheme C. petracinii falls into the resurrected genus Hoplisoma, so the current valid combination is Hoplisoma petracinii. The hobby and most literature still use 'Corydoras petracinii,' which we keep as the heading while noting the new placement; FishBase already lists the species under Hoplisoma.
Morphology
This is a small, moderately elongate cory of around 1–1.5 in in standard length. It carries 23 dorsolateral and 21 ventrolateral body plates, has notably small eyes (about 13–17% of head length), a short dorsal spine and short pectoral spines, and a slightly forked-to-emarginate caudal fin — a slimmer, shorter-finned build than the deep-bodied bronze cory.
The diagnostic colour mark is a row of five to seven squarish, dark differentiated blotches along the middle of the flank, set where the upper and lower body plates meet, over a pale ground; the caudal fin is hyaline with dark marks forming three or four irregular vertical bars. Three pairs of barbels fringe the mouth and the dorsal, pectoral and adipose fins each bear a stiff leading spine.
Habitat
Corydoras petracinii is known only from its type locality: a small stream at Finca Las Costas, near Salta city in Salta Province, northwestern Argentina, about 656 ft above its junction with the San Lorenzo river at roughly 1,728 ft elevation. That drainage feeds the Juramento river and ultimately the Paraná at Santa Fe — placing this fish firmly in the Río de la Plata basin, far south of the Amazon.
This is a high, cool, seasonal habitat unlike most cory streams. The stream has a rocky substrate, runs almost dry for much of the year, and flows swiftly during the summer wet season. Water there is cool — broadly in the 50–68 °F range across the year — and hard and alkaline, with a pH around 7.0–8.0. The species is a facultative air-breather, gulping surface air and absorbing oxygen through a vascularised hindgut, an adaptation well suited to the warm, low-oxygen pools that remain as the stream shrinks in the dry season.
Feeding
No dedicated diet study exists, but C. petracinii is a benthic omnivore like its relatives. It works the stream bottom, sifting sand and fine gravel with its snout and barbels to take small invertebrates, insect larvae such as chironomid (bloodworm) larvae, worms and organic detritus. In an aquarium it would be expected to take sinking dried foods alongside small live and frozen items such as bloodworm and Tubifex.
Mating
The spawning behaviour of C. petracinii has not been documented. Corys are egg-laying shoalers with a conserved courtship: a ready female cleans a site, a male takes the 'T-position' at right angles to her head, and a few adhesive eggs are clasped in a basket of her pelvic fins and pressed onto the chosen surface, the sequence repeating until the clutch is complete. This species is expected to follow the same pattern, though its seasonal, cool-water origins may mean spawning is keyed to the onset of the summer rains and a warming of the shrunken streams.
Breeding
There are no captive-breeding records for Corydoras petracinii, which is essentially absent from the hobby. If it spawns like its relatives, the trigger would be a seasonal change — here likely the arrival of the wet season and rising flow — the adhesive eggs would receive no parental care, and they would hatch in roughly three to five days, with fry taking microworm and baby brine shrimp. Its cool-water, strongly seasonal biology suggests it would need a pronounced temperature and flow cycle, rather than constant tropical warmth, to come into condition.
In the aquarium
Corydoras petracinii is not in the aquarium trade and is unlikely to enter it, so there is no established husbandry for it. What sets it apart from typical cory advice is temperature: this is a cool, subtropical, high-altitude fish that would want water well below the usual tropical range — roughly 64–72 °F rather than 75–79 °F — with hard, alkaline chemistry and a strong seasonal swing. Like all corys it is a shoaler needing groups of six or more over soft sand (which protects the barbels), strong filtration and good oxygenation. It is peaceful and would suit only equally calm, cool-water tankmates. The usual cautions apply: fin spines snag in nets, and a stressed cory can release skin toxins dangerous in the confined volume of a bag.
Conservation
Corydoras petracinii has not been evaluated by the IUCN. That is a real concern rather than a clean bill of health: the species is known only from a single small stream on the edge of Salta, one of the larger cities of northwestern Argentina, and a fish with so tiny a documented range is inherently vulnerable to local pressures — urban expansion, water abstraction from a stream that already runs nearly dry, pollution and habitat alteration. Until its true distribution is better understood, the safe reading is that this is a narrow-range endemic deserving of a formal assessment.