Taxonomy & naming
The sail cory was described by Albert Günther in 1868 as Callichthys armatus, from material collected in the upper Río Huallaga drainage of Peru ('Xeberos and Huallaga'). It was later moved into Corydoras Lacépède, 1803, the vast genus of South American armoured catfishes in the family Callichthyidae, and that is the name under which it has been listed and traded ever since. The epithet armatus — Latin for 'armed, armoured' — probably alludes either to the body plates or to the prominent dorsal-fin spine, though Günther did not say.
Corydoras had long been understood to be far too large and not a single natural lineage. The 2024 phylogenomic revision by Dias, Tencatt and colleagues (Zoological Journal of the Linnean Society) split it into several genera, including the resurrected Hoplisoma, and under that classification this species becomes Hoplisoma armatum — a member of the large lineage-9 Hoplisoma group. FishBase already uses Hoplisoma armatum, while the hobby still says 'Hoplisoma armatum,' so we keep the familiar heading and note the new combination. Catalog of Fishes treats armatus as a valid species. Fish traded from the upper Rio Madeira in Brazil and Bolivia under invented names such as 'dorsalis' and 'ogawae' (CW086) are officially considered conspecific, though the enormous distance between those populations and the Peruvian type locality suggests the matter deserves further study.
This fish was long listed as Corydoras armatus; Eschmeyer's Catalog of Fishes places the species in Hoplisoma, and the current combination Hoplisoma armatum is followed here.
Morphology
This is a medium-small, comparatively slender cory of the long-snouted type, reaching around 2–2.6 in in standard length; FishBase gives a maximum of about 2 in SL. Like all members of the group it is armoured rather than scaled, with two rows of overlapping bony plates down each flank and a hard head, and it carries the typical three pairs of barbels on a downturned, sediment-sifting mouth.
The ground colour is pale to greyish, peppered and reticulated with darker markings, and the most distinctive feature is the dorsal fin: tall, with a long, slightly curved leading spine and a conspicuous dark blotch in the membrane — the 'sail' of the common name. This longer, curved dorsal spine helps separate it from the superficially similar C. loretoensis, which has a shorter, straight spine and a shallower body. As in other corys the pectoral and adipose fins also bear stiff spines, and females grow larger and rounder than males, most obviously when gravid.
Habitat
Hoplisoma armatum is a fish of the upper Amazon basin. Its type locality lies in the Río Huallaga watershed of Peru — the Huallaga being a tributary of the Marañón, which joins the Ucayali to form the Amazon proper — and the species may be more or less endemic to that upper-Amazon region, though records also exist from the upper Rio Madeira system in Brazil and Bolivia.
These are warm, lowland-to-piedmont waters: streams, river margins and connected pools over sand and soft sediment with leaf litter and wood. Reported conditions span roughly 72–79 °F, a near-neutral to slightly alkaline pH (about 6.0–8.0) and soft to moderately hard water. Like its relatives it is a facultative air-breather, rising periodically to gulp atmospheric air that it absorbs through a modified, highly vascularised intestine — an adaptation that lets corys survive in warm, oxygen-poor water.
Feeding
The sail cory is a foraging benthic omnivore. In the wild it works over sand and soft substrate, using its barbels to taste out small invertebrates, insect larvae, microcrustaceans and organic detritus, and sifting mouthfuls of sand that it expels through the gills. In the aquarium it accepts most sinking dried foods — pellets, tablets and granules — alongside small frozen and live foods such as bloodworm, Tubifex and Daphnia. As with all corys a varied diet keeps the fish in condition, and it should never be relied upon to live on the leftovers of its tankmates or to 'clean' the tank.
Mating
Hoplisoma armatum follows the classic egg-laying courtship of the genus, with no pair bond. When a female is ready the group becomes active and she cleans a spawning site; a male positions himself at right angles to her head in the well-known 'T-position.' The female holds a small number of eggs — observations of this species report just two to four at a time — clasped between her pelvic fins, which the male fertilises over roughly thirty seconds before she swims to a chosen surface and presses the very sticky eggs into place. The pair repeats the process until on the order of a hundred eggs have been laid.
Breeding
Reproduction in H. armatum matches the familiar Corydoras template, and where it has been observed the female lays in the typical batched, T-position manner described above, accumulating around a hundred adhesive eggs over a spawning. Spawning is generally encouraged by a cool, generous water change that mimics rainy-season conditions. The eggs receive no parental care and the adults will eat them, so breeders remove either the eggs or the parents. Eggs hatch in a few days, and the fry take microworm, baby brine shrimp and powdered foods after absorbing the yolk sac. The species is not bred in large numbers, and most fish in the trade are wild-caught Peruvian imports.
In the aquarium
H. armatum is an attractive but only occasionally available cory that does well given the standard, non-negotiable cory requirements. It is a peaceful, gregarious, shoaling fish and should be kept in a group of at least six (four to six is the practical minimum); kept alone it becomes stressed and reclusive. A group needs bottom area more than height and is comfortable in a medium community tank.
Provide soft sand or very smooth, rounded fine gravel — corys feed by burying their snouts to sift, and sharp or chipped gravel erodes and infects their barbels. Keep the substrate clean and the water warm (about 72–79 °F), well-filtered and well-oxygenated, with gentle current, some shade from wood or planting, and regular water changes; the species tolerates a broad pH and hardness range. It is an excellent companion for tetras, rasboras, dwarf cichlids and other calm fish, and should not be housed with anything large or aggressive. The usual cory cautions apply: the stiff fin spines can pierce skin and tangle in nets (use a cup), and a badly distressed cory can release skin toxins that may harm the whole group in the confined volume of a transport bag.
Conservation
Hoplisoma armatum is assessed by the IUCN as Least Concern (assessed 2020). It is fairly widely distributed across the upper Amazon and faces no specific identified major threat, and the broad scale of the Amazon basin buffers it against rapid global decline. None of that exempts it from the general pressures on western-Amazonian freshwaters — deforestation, mining and associated pollution, and habitat alteration — which degrade the shallow habitats it uses. Because almost all trade fish are wild-collected in Peru, sensible, well-managed ornamental collection rather than captive breeding is the relevant conservation consideration, and the unresolved question of whether the distant Madeira populations are truly the same species is a taxonomic loose end worth bearing in mind.