Taxonomy & naming
The sailfin cory was described by C. Tate Regan in 1913 as Corydoras macropterus. A second nominal species, Corydoras bertoni Eigenmann, 1942, was later shown to be the same fish and is treated as a synonym. As with the other bristle-cheeked corys, it was transferred to Scleromystax Günther, 1864 — the small south-eastern Brazilian genus whose mature males grow patches of erectile cheek odontodes and show pronounced sexual dimorphism in colour and finnage.
The 2024 revision of the Corydoradinae by Dias, Tencatt and colleagues, which dismantled the old super-genus Corydoras into several lineages (Brochis, Hoplisoma, Osteogaster, Gastrodermus and more), kept Scleromystax intact as a valid, deeply divergent group. The Catalog of Fishes and FishBase therefore list this fish as Scleromystax macropterus, the combination we adopt. It is closely allied to — and easily confused with — the more southerly S. salmacis, from which it differs in pattern and fin-ray counts and in its much stronger sexual dimorphism.
Morphology
This is a medium-sized Scleromystax, reaching roughly 3–3.5 in in standard length. The body is the deep, armoured, flat-bellied form typical of the corys, but the finnage is anything but typical: in adult males the dorsal fin is drawn out into a tall sail and the pectoral spines are likewise elongated, giving the species both its English and its scientific name (Greek makros, 'long', and pteron, 'fin').
Sexual dimorphism is marked. Males are larger-finned and more intensely patterned, with bold dark marbling and reticulation over a pale ground; females are smaller-finned, plainer and rounder-bodied. Breeding males also carry the genus's hallmark cheek bristles — patches of stiff odontodes across the side of the head. Three pairs of barbels fringe the mouth, and the dorsal, pectoral and adipose fins each bear a hardened spine.
Habitat
Scleromystax macropterus occurs in the coastal river basins of south-eastern and southern Brazil — across the states of São Paulo, Paraná and Santa Catarina — and in some headwater tributaries of the upper rio Paraná system, with records extending to adjacent Paraguay. These are clear, well-oxygenated streams of the Atlantic Forest and adjacent uplands, running over sand, gravel and leaf litter.
Crucially, this is a subtropical, cool-water fish. Reported temperatures run from about 57–68 °F — markedly colder than most tropical corys tolerate — over soft, acidic to neutral water, with a pH around 5.0–7.0 and low to moderate hardness. That preference for cool, clear, flowing water is the single most important fact for anyone keeping it, and it tracks the species' restriction to the cooler coastal and upland streams of Brazil's south-east rather than the warm lowlands of the Amazon.
Feeding
Like all Scleromystax, the sailfin cory is a foraging benthic omnivore. In its streams it sifts sand and combs through leaf litter for insect larvae, small crustaceans, worms and organic detritus, tasting the substrate with its barbels and venting cleaned grit through the gills. In the aquarium it readily accepts sinking dried foods and takes live and frozen fare with enthusiasm — bloodworm (chironomid larvae) and Tubifex are particular favourites — and a varied diet keeps it in peak condition and breeding readiness.
Mating
Spawning follows the standard corydoradine sequence: an egg-scattering shoaler with no pair bond and the characteristic 'T-position' embrace. A receptive female cleans a surface, a male aligns at right angles to her head, and a small number of eggs are fertilised and carried in the basket of her pelvic fins to be pressed onto the chosen site. The display is repeated, a few eggs at a time. Males' extravagant fins and bristled cheeks are thought to function in this courtship, advertising condition to females and in contests with rival males.
Breeding
The sailfin cory has been spawned in the aquarium, broadly as for other corys but with the proviso that it wants cool, clean water throughout. Breeders typically condition the fish hard, then trigger spawning with a large (50–70%) water change using cooler water and stepped-up aeration to mimic rain-swollen, oxygen-rich streams. One male per female is a common ratio. The adhesive eggs are laid on the glass or vegetation and receive no parental care; incubation runs about three to four days. The fry demand excellent water quality and small live foods, and are noticeably more delicate to raise than those of the common tropical corys.
In the aquarium
This is a specialist's fish, and its great non-negotiable is temperature: it must be kept cool. A summer-warm tropical tank will stress and eventually kill it; aim for the mid-to-high teens up to about 68 °F, which usually means a dedicated, unheated or chilled set-up rather than a mixed community. Keep a group of six or more over soft sand, with smooth wood, leaf litter and brisk, well-oxygenated flow to suit a stream fish. Water should be soft and acidic-to-neutral. It is peaceful and gregarious, but its cool-water needs limit suitable tankmates to other subtropical species; it does not belong with warm-water tropicals. Handle with a cup rather than a net, since the elongated fins and spines tangle readily. Given the right cool, clean conditions, the displaying males are among the most beautiful of all the corys.
Conservation
Scleromystax macropterus has not been formally assessed by the IUCN. It is, however, a fish of the coastal-stream habitats of the Atlantic Forest, one of the most heavily cleared and fragmented biomes in South America, and at least some of its restinga and coastal-stream populations are regarded by Brazilian researchers as threatened by habitat loss. Its dependence on cool, clear, well-oxygenated flowing water makes it sensitive to deforestation, water abstraction and warming, and its range, while spanning several states, is patchy. The absence of a Red List assessment reflects a data gap rather than safety, and a formal evaluation would be valuable.