Taxonomy & naming
Corydoras psamathos was described in 2022 by Tencatt, Ohara, Sousa and Britto, from the Rio Jamanxim and the small tributary igarapé Santa Júlia in the Tapajós basin, Pará State, Brazil — the type locality. It was placed in Corydoras Lacépède, 1803, the very large genus of South American armoured catfishes (family Callichthyidae). The species' name is a literary flourish: it honours Psamathos Psamathides, the sand-sorcerer of J. R. R. Tolkien's tale Roverandom, built on the Greek psammos, 'sand,' for the fish's habit of living in and on sandy bottoms.
Because Corydoras had grown to over 170 species and was shown not to be a single natural group, Dias, Tencatt and colleagues split it in a 2024 phylogenetic revision into several genera, including Brochis, Hoplisoma, Osteogaster and Gastrodermus. Corydoras psamathos falls in the lineage now recognised as Hoplisoma, so its current combination is Hoplisoma psamathos; FishBase already lists it that way. As the name is only a few years old and the hobby uses 'Corydoras psamathos,' we keep that heading while recording the Hoplisoma placement honestly.
Morphology
This is a true dwarf cory. Reported sizes are small — on the order of 1 in standard length for the type material, with aquarium accounts giving roughly 1.5 in for males and up to about 1.5 in for the larger females. The body is the compact Corydoras form: short and deep, with a blunt armoured head, a downturned mouth fringed by three pairs of barbels, and two rows of overlapping bony plates instead of scales running down each side.
Its colouration is pale and cryptic, suited to a life on bright sand. Wild fish show a light, greenish-yellow iridescent ground colour over the body; aquarium-raised individuals can look greyer-white. A short dark stripe runs from the front-lower edge of the eye toward the upper lip, and fine, scattered dark markings break up the pale flanks. The slim males and rounder, fuller-bodied females differ as in other corys, the difference clearest in conditioned fish.
Habitat
Corydoras psamathos is, so far, known only from Brazil — the main channel of the Rio Jamanxim and its tributary igarapé Santa Júlia, in the Novo Progresso area of Pará State, within the Tapajós drainage of southern Amazonia. It is a clearwater fish and, as its name promises, a sand specialist: it occupies very shallow margins, on the order of twenty centimetres deep, over clean sand and fine gravel with slow flow, where it lies half-buried and is largely active at dusk and into the night.
These are warm, soft, near-neutral clearwater habitats. Husbandry sources cite a comfortable range of roughly 73–81 °F and a pH around 6.0–7.2, consistent with the lowland forest streams of the Tapajós system. Like other Callichthyidae the species is a facultative air-breather, gulping surface air and taking up oxygen through a vascularised hindgut — useful in the warm, shallow, sometimes low-oxygen shoreline water it favours.
Feeding
As a sand-dwelling cory, Corydoras psamathos is a benthic micro-omnivore that works the substrate for its food. It sifts fine sand through its mouth and gills, using its barbels to taste out tiny invertebrates, insect larvae, micro-crustaceans, worms and organic detritus. Its small size and sand-specialist habits point to a diet of correspondingly small prey and biofilm. In the aquarium it readily takes a mixed menu of small sinking granules and tablets, frozen bloodworm and other small frozen and live foods, and good-quality flake taken from the bottom — best offered as a varied diet rather than relying on leftovers.
Mating
Courtship has not been documented in detail for Corydoras psamathos, but it is expected to follow the genus pattern. Corydoras are egg-laying shoalers without pair bonds. A ready female cleans a spawning surface and the group grows active; a male adopts the 'T-position' at right angles to her head, she appears to take milt directly, and the sperm is thought to pass quickly through her gut to fertilise the few eggs she holds in a basket made by her pelvic fins. She presses each small batch onto the chosen surface and the process repeats. Given the species' affinity for sand and shallow margins, suitable clean surfaces in those settings would be the likely natural spawning sites.
Breeding
There is no established, widely published captive-breeding account specific to Corydoras psamathos; expectations rest on its relatives. Spawning in the genus is generally cued by a cool, large water change imitating the rainy season. The adhesive eggs are laid in small batches on firm surfaces and given no parental care — the adults will eat them, so a breeder removes the eggs or the parents. Eggs commonly hatch in three to five days depending on temperature, and the fry take microworm, baby brine shrimp and powdered foods once the yolk sac is gone. As a small, recently described and not-yet-common species, it is only rarely bred in the hobby.
In the aquarium
Corydoras psamathos is an occasional, specialist import that rewards a setup built around its sand-dwelling nature. It is a shoaling fish and is stressed in ones and twos; keep six or more and they behave naturally, foraging and resting on the open bottom together.
The single most important provision is fine, soft sand — not a luxury but a necessity for a species that buries itself and sifts to feed; coarse or sharp gravel abrades and infects the barbels, producing the blunted faces that mark poor husbandry, and denies the fish its natural behaviour. Because it is tiny (under 1.5 in), a small, well-planted tank with open sand patches, leaf litter, gentle well-oxygenated flow and a clean bottom suits a group; bottom area matters more than height. Keep the water warm but not hot (about 73–81 °F), soft and slightly acidic to neutral. It is extremely peaceful and best matched with small, calm tankmates such as tetras, small rasboras and dwarf cichlids. As with all corys, use a cup rather than a net when possible, since the fin spines snag, and transport the fish carefully — a badly stressed cory can release a skin toxin that may wipe out the group in the small volume of a bag.
Conservation
Corydoras psamathos has not yet been formally evaluated by the IUCN, so it carries no Red List category (treated here as Not Evaluated). That silence is itself a concern: the species is, on current knowledge, a narrow-range endemic of the Rio Jamanxim in the Tapajós basin, a region under intense and growing pressure from deforestation, agricultural expansion, road-building around Novo Progresso and gold mining with its attendant mercury and sedimentation. A clearwater sand specialist with so small a known range would be vulnerable to local habitat degradation, and a proper assessment — together with better data on how widely it is actually distributed — would be valuable. For now its status is genuinely unknown rather than reassuring.