Taxonomy & naming
The grey cory was described by the Austrian zoologist Maximilian Holly in 1940 as Corydoras griseus, and the specific name is simply the Latin griseus, 'grey,' for its drab colouration. It entered the long roster of Corydoras Lacépède, 1803, the enormous genus of South American armoured catfishes in the family Callichthyidae, and for the better part of a century it was traded, bred and written about as Corydoras griseus.
That genus, however, had become a grab-bag of well over 170 species that molecular work showed was not a single natural lineage. In 2024 Dias, Tencatt and colleagues published a large phylogeny-based revision (Zoological Journal of the Linnean Society, zlae053) that split Corydoras into several genera — among them Brochis, Hoplisoma, Gastrodermus and Osteogaster — and under that scheme the grey cory becomes Hoplisoma griseum. FishBase and the Catalog of Fishes now carry the species under Hoplisoma. The change is recent and the aquarium trade still overwhelmingly says 'Corydoras griseus,' so we keep that hobby-familiar heading while noting the current combination. The species sits in the Hoplisoma lineage of small, short-snouted three-barbel corys.
Morphology
This is a small, short-snouted cory reaching only about 1.5 in in standard length. Like all members of the subfamily it is armoured rather than scaled: two overlapping rows of bony plates (scutes) run the length of each flank, meeting along the back, and the blunt head is a hard helmet. Three pairs of barbels fringe the downturned mouth, the tools it uses to taste and sift the substrate.
The colouration is understated, which is exactly where the name comes from: a pale to smoky grey or greyish-brown ground, often with a faint darker shading over the back and a slightly iridescent sheen, and largely clear fins. There are no bold blotches, bars or mask of the kind that make many corys instantly recognisable; it is a plain, almost cryptic little fish. As in other Corydoras the dorsal, pectoral and adipose fins each bear a stiff, mildly venomous spine that can deliver a sharp jab and snag in a net. Sexual differences follow the family pattern — adult females grow a little larger and noticeably rounder-bodied than the slimmer males, most obvious when females are carrying eggs.
Habitat
Corydoras griseus is recorded from the Amazon River basin of Brazil, where it lives in small forest streams and tributaries rather than the open main river. These are warm, shaded, slow-moving waters over soft sand and mud, strewn with leaf litter and submerged wood — the typical haunt of a small benthic cory.
FishBase gives an environmental envelope of roughly 72–79 °F, a pH spanning the acidic to slightly alkaline range (about 6.0–8.0) and soft to moderately hard water (dH around 2–25), figures that reflect the broad tolerances reported for small Amazonian corys generally rather than fine-grained field measurement for this species. Like its relatives it is a facultative air-breather, periodically darting to the surface to gulp air and absorb oxygen through a modified, vascularised hindgut — an adaptation that lets corys persist in warm, oxygen-poor pools where many fish could not. Its apparently restricted range within the basin is central to its conservation status.
Feeding
Like other small Corydoras, the grey cory is a benthic omnivore and detritivore. It works steadily over the bottom, burying its snout and barbels in soft sand to taste out small invertebrates, insect larvae, worms, micro-crustaceans and the organic film of detritus, expelling cleaned sand through the gill openings as it goes. It is an opportunist rather than a specialist feeder. In the aquarium it readily takes sinking pellets and wafers, frozen and live foods such as bloodworm and brine shrimp, and whatever edible particles settle to the substrate, though it should never be left to scavenge leftovers alone. Detailed gut-content studies specific to this species are lacking, and the feeding picture is inferred from close relatives.
Mating
Corydoras are egg-laying shoalers with no pair bond and the distinctive courtship that defines the group. When a female is ready she cleans a site — a leaf, a stone, the aquarium glass — and the group grows active; a male takes up the classic 'T-position' at right angles to her head, and she appears to take his milt at the mouth. Sperm is thought to pass rapidly through her gut to fertilise the small clutch of eggs she clasps in a basket formed by her pelvic fins. FishBase, drawing on observations of the species, records the female holding two to four eggs between her pelvic fins while the male fertilises them for roughly thirty seconds before she swims off to press the very sticky eggs onto a chosen surface, repeating the sequence until on the order of a hundred eggs have been laid.
Breeding
Reproduction follows the standard Corydoras pattern and, where it has been spawned, the grey cory behaves much like its better-known relatives. Spawning is typically prompted by a cool, large water change that imitates the onset of the rainy season. The adhesive eggs are laid in small batches and scattered across cleaned surfaces; on the order of a hundred may be deposited over a session. The adults give no parental care and will eat the eggs, so a breeder removes either the eggs or the parents. Eggs hatch in a few days depending on temperature, and the fry, once their yolk sacs are absorbed, take microworm, baby brine shrimp and fine powdered foods. Dedicated breeding accounts for this particular species are sparse — it is uncommon in the trade — so much of the husbandry is extrapolated from the wider genus.
In the aquarium
The grey cory is a peaceful, undemanding community fish for those who can find it, but it has the same non-negotiable needs as any Corydoras. It is a shoaling species and is genuinely stressed when kept singly or in pairs — keep six or more, and they reward the keeper with constant, confident foraging. A group is comfortable in a modest tank with plenty of floor area; footprint matters more than height.
Substrate is the critical detail. Provide soft sand or very smooth, rounded fine gravel: corys feed by sifting, and sharp or coarse gravel abrades and infects their barbels, leaving the stubby, eroded faces that are a hallmark of poor husbandry. Keep the bottom clean and the water warm (about 72–79 °F), well-filtered and well-oxygenated, with gentle flow and regular changes; the species tolerates a broad pH and hardness range. It makes an ideal companion for tetras, pencilfish, rasboras and dwarf cichlids, and should never be housed with anything large enough to bully it. As with all corys, handle with a cup rather than a net to avoid snagging the fin spines, and transport with care, since a distressed cory can release a skin toxin that, in the confined volume of a bag, can kill the whole group. Given its threatened wild status, captive-bred stock should be preferred wherever available.
Conservation
Corydoras griseus is assessed by the IUCN Red List as Endangered (assessed 2020, criterion B1ab(iii)), a notably serious listing for a cory. The assessment reflects a restricted distribution within the Brazilian Amazon and a continuing decline in the quality of its habitat — the small forest streams it depends on are vulnerable to deforestation, agricultural run-off, siltation and other pressures that degrade headwater systems. The species is of no commercial fishery interest and appears only sporadically in the aquarium trade, so collection pressure is not the headline threat; habitat loss is. Its plight underscores a wider point about the cory radiation: many of these little catfishes are narrowly distributed specialists of particular drainages, and a drab, easily overlooked species can be far more imperilled than the familiar, mass-bred community corys.