Taxonomy & naming
Corydoras diphyes was described in 2003 by Thomas Axenrot and Sven Kullander, in a paper that named it together with Otocinclus mimulus as 'two new species of catfishes from Paraguay,' framed explicitly as a mimetic association — the corydoradine and the otocinclus share a similar colour pattern where they co-occur, an unusual documented case of mimicry among South American catfishes. The epithet diphyes is Greek for 'of double nature' or 'twofold,' referring to the species' own variable pattern, which ranges from a row of distinct spots to a near-continuous mid-lateral band.
For two decades it was Corydoras diphyes. In the 2024 revision of the Corydoradinae by Dias, Tencatt and colleagues (DOI 10.1093/zoolinnean/zlae053), which split the old genus Corydoras into several lineages, this round-snouted species falls within the resurrected genus Hoplisoma, and FishBase now lists it under the valid combination Hoplisoma diphyes. The change is recent and the aquarium trade still writes 'Corydoras diphyes,' so we keep that as our heading while recording the current genus.
Morphology
This is a small, short-snouted cory reaching only about 2 in in standard length. Like all members of the subfamily it is armoured with two longitudinal rows of overlapping bony plates along each flank, a hard bony head, and three pairs of barbels around the downturned mouth. It carries relatively low scute counts (roughly 23–24 dorsal-row and 20–21 ventral-row scutes, with usually three pre-adipose scutes), and its dorsal and pectoral fins are not elongated in either sex.
The striking feature is the variability that gives the fish its name: instead of two or three large dark blotches, diphyes shows a series of four to six distinct dark spots, or a continuous dark band, running along the middle of the side, over a pale ground — and it is this pattern that the co-occurring Otocinclus mimulus mimics. As in all corys the dorsal, pectoral and adipose fins each bear a stiff locking spine.
Habitat
Corydoras diphyes is restricted to a small area of eastern Paraguay: tributaries of the río Monday and río Acaray, both right-bank tributaries of the río Paraná. These are warm lowland-to-upland streams of the Atlantic-forest fringe of the upper Paraná basin, typically with sand and litter substrates and structure to shelter among.
FishBase records it as a freshwater, demersal, tropical species; precise measured water-chemistry figures for the species are limited, but its setting implies warm, soft-to-moderate, near-neutral water in the broad tropical range, and it co-occurs with the otocinclus it is associated with. As a callichthyid it is a facultative air-breather, surfacing to gulp air and absorb oxygen across a vascularised hindgut.
Feeding
Corydoras diphyes is a benthic omnivore and detritivore in the manner of the whole subfamily. It forages over the bottom, probing sand and litter with its barbels for small invertebrates, insect larvae, micro-crustaceans and organic detritus, sifting substrate through the mouth and venting it through the gills. No diet study specific to this species has been published; this is the standard corydoradine feeding mode applied to it, and there is no indication it is a specialist.
Mating
As an egg-laying corydoradine, Corydoras diphyes is expected to spawn in the group's characteristic 'T-position': shoaling fish with no pair bond, in which a male aligns at right angles to a receptive female, who clasps a few adhesive eggs in a basket of her pelvic fins and presses them onto a cleaned surface, repeating until the clutch is laid. A courtship study specific to diphyes has not been published, so this is the genus pattern applied to it rather than a documented observation of this species.
Breeding
There is no established aquarium breeding record for Corydoras diphyes; it is a localised, infrequently kept species rather than a commercial one. By inference from its relatives it would lay a modest clutch of adhesive eggs in small batches on plants, wood or glass, with no parental care — adults taking no part beyond spawning and apt to eat unguarded eggs, so a breeder would separate eggs or parents. Eggs of related corys hatch in a few days and fry take microworm and baby brine shrimp. Until firm data for diphyes are published, these figures are expectations from the genus rather than facts about the species.
In the aquarium
Corydoras diphyes is a small, attractive cory that appears in the hobby only occasionally, usually through specialist channels rather than as a standard import. It should be kept as the shoaling fish it is — a group of six or more, never singly — over soft sand, since corys sift the substrate and coarse or sharp gravel abrades and infects the barbels. Provide clean, warm (about 72–79 °F), well-filtered and well-oxygenated water with gentle flow, leaf litter and shade, and the broad pH and hardness tolerance typical of the genus. Being small and entirely peaceful, it suits a quiet community of small tetras, rasboras and dwarf cichlids and should never be housed with anything large enough to bully it. As with all corys, move it with a cup rather than a net to avoid spine-snags, and transport it carefully, since a stressed cory can release skin toxins that can prove fatal in the small volume of a bag.
Conservation
Corydoras diphyes is assessed by the IUCN Red List as Vulnerable (criterion B1ab(ii)), in an assessment dated 3 February 2021. The listing reflects its genuinely small range — a handful of río Monday and río Acaray tributaries in eastern Paraguay — combined with habitat pressures in that region: the upper Paraná drainage of eastern Paraguay has seen extensive deforestation, agricultural conversion and damming (the río Acaray is itself dammed), all of which shrink and fragment the stream habitat a narrowly distributed fish depends on. For a species this localised, such pressures matter, and the Vulnerable status is a flag that its small range leaves little margin. Collection for the aquarium trade is minor relative to those habitat-level threats.