Taxonomy & naming
The bluespotted cory was described by the British ichthyologist Charles Tate Regan in 1912, in his revision of the South American siluroid fishes of the genus Corydoras, from material out of the Guianas. The specific name melanistius refers to its black dorsal fin — the 'black sail' — and the fish has been a familiar aquarium import for many decades.
Its history is muddied by two subspecies that were later untangled and removed. The form once called C. melanistius brevirostris is now recognised as the distinct species Corydoras brevirostris, and what was labelled C. melanistius longirostris turned out to be C. ambiacus; melanistius proper is the short-snouted nominal fish. The species is also easily confused with the closely similar C. delphax — they are best separated by fin-ray counts and pattern, melanistius having far more dorsal rays and lacking barring in the tail — and a long-snouted Suriname cory once traded under the melanistius umbrella was eventually described as the separate C. wotroi. So the honest position is that 'C. melanistius' has historically been a catch-all for several look-alikes, now mostly sorted out.
The over-large genus Corydoras was shown not to be a single natural lineage, and in 2024 Dias, Tencatt and colleagues split it into several genera. Under that revision melanistius falls into the resurrected genus Hoplisoma, making the current combination Hoplisoma melanistium. The change is recent and not yet universally used — the hobby still says 'Corydoras melanistius' — so we keep that heading while recording the new name.
Morphology
This is a fairly small, deep-bodied Corydoras reaching roughly 2–2.5 in in length, with the usual genus architecture: a hard, blunt helmeted head, a downturned barbelled mouth, and two overlapping rows of bony scutes down each flank in place of scales. Females, as is typical, grow larger and rounder than males.
Its colour pattern is what makes it worth keeping. The ground is a pale, pearly grey-white with a soft iridescent bluish cast, finely sprinkled with small dark spots over the body. Over this sit two strong markings: a black bar or mask through the eye, broadening across the head, and a separate black blotch at the base of the dorsal fin that extends up into the fin's leading rays — the 'black sail' of the common and scientific names. Some fish show an iridescent blue gleam around the eye and gill cover. The unbarred, mostly clear tail helps separate it from the similar C. delphax. As in all Corydoras, the dorsal, pectoral and adipose fins carry a stiff, mildly venomous spine.
Habitat
Corydoras melanistius is a fish of the Guiana Shield, recorded from the coastal river systems of Guyana, Suriname and French Guiana, with the Essequibo drainage in Guyana usually cited as its heartland and the basis of much of the exported stock. These are clear to tea-stained lowland rivers and their quieter margins, draining old, nutrient-poor crystalline terrain to the Atlantic.
Within them it keeps to the standard Corydoras niche: shallow, gently flowing or still stretches over soft sand and mud, among leaf litter, roots and submerged wood. The water is warm — broadly 72–79 °F — and tends to be soft and acidic to roughly neutral, often with a low mineral content reflecting the shield geology, though the fish tolerates a fair span of pH and hardness. As with its relatives, it can supplement its oxygen by gulping air at the surface and absorbing it through a vascularised hindgut, a useful trick in warm, sluggish, low-oxygen backwaters.
Feeding
The bluespotted cory is a benthic omnivore and is easy to feed. In the wild it forages over the bottom, sifting sand and leaf litter through its barbels and mouth to take small invertebrates, worms, benthic crustaceans, insect larvae and plant and detrital matter, expelling the cleaned substrate through its gills. It has no specialised requirements. In the aquarium it readily accepts sinking pellets and tablets, frozen and live foods such as bloodworm, daphnia and brine shrimp, and the organic odds and ends that reach the bottom — but it should be fed purposefully and not relied upon to clean up after tankmates.
Mating
Corydoras melanistius is an egg-laying group spawner with the courtship typical of the genus and no pair bond. A ripe female cleans a spawning site — a leaf, a stone, a stretch of glass — and the group becomes active; a male takes up the characteristic 'T-position' at right angles to her head, and she appears to take in his milt, which is thought to pass quickly through her gut to fertilise the few eggs held in the basket of her clamped pelvic fins. She carries each small batch to the cleaned surface and presses the adhesive eggs into place, repeating the sequence until the chosen surfaces are studded with eggs. Spawnings tend to be most successful with a surplus of males in the group.
Breeding
The bluespotted cory is bred in the hobby but is regarded as a little less casually prolific than the bronze cory, and it is more sensitive to water quality. Spawning is encouraged in the usual Corydoras way: a well-conditioned group is given a generous, cooler water change with soft, clean water to mimic the rainy season, often after a period of feeding on live and frozen foods. The adhesive eggs are deposited in small batches on plants, décor and glass and receive no parental care — the adults will eat them — so the eggs or the parents are separated. Eggs hatch in roughly three to five days depending on temperature, and the free-swimming fry take microworm, baby brine shrimp and fine powdered foods. The fry are reportedly somewhat more delicate than those of the hardier aeneus-group species and want clean, mature, stable water; the species is genuinely intolerant of ammonia and nitrite and should never be spawned or raised in an immature system.
In the aquarium
The bluespotted cory is a rewarding, peaceful community fish, a touch more demanding of water quality than the beginner-proof bronze cory but not difficult. It is strongly shoaling and should be kept in groups of six or more; in a proper group it is active and confident, whereas isolated fish become shy and stressed. A footprint of around 23.5 in / 20 US gal or more suits a group, bottom area mattering more than height.
As with every Corydoras, substrate is critical: use soft sand or very smooth, rounded fine gravel, because these fish feed by sifting and sharp or coarse gravel wears away and infects their barbels, leaving the eroded faces that mark poor husbandry. This species is notably sensitive to nitrogenous waste, so it wants a fully mature, well-filtered tank, gentle current, good oxygenation and reliable regular water changes; warm water (about 72–79 °F), soft to moderately hard and slightly acidic to neutral, suits it best. It mixes well with tetras, rasboras, dwarf cichlids and other calm community fish. The usual genus cautions apply: the fin spines snag in nets, so a cup is safer, and a badly stressed cory can release skin toxins that may prove lethal in the small volume of a transport bag, so it should be moved with care.
Conservation
Corydoras melanistius has been assessed by the IUCN as Least Concern. It is a reasonably widespread fish across the Guianas, occurring in river systems that remain comparatively lightly developed, and a large share of the aquarium supply is captive-bred, which limits pressure on wild populations. No species-wide threat is recognised. As with all the region's freshwater fishes it is not wholly insulated from the broad pressures of deforestation, gold-mining sedimentation and pollution that can degrade Guiana Shield rivers, and the lingering taxonomic confusion with its former subspecies and look-alikes is a reminder that range and status figures for 'melanistius' in older sources may have included fish now treated as separate species.