Taxonomy & naming
Corydoras knaacki was described by Luiz Tencatt and Hans-Georg Evers in 2016, in Neotropical Ichthyology, from the río Madre de Dios basin of Peru. The epithet honours Dr Joachim Knaack (1933–2012), the German physician and amateur ichthyologist who devoted more than six decades to South American catfishes and described many Corydoras himself. Before it had a scientific name the fish moved through the hobby as the 'Morse code cory' and under Ian Fuller's collection code CW032.
The authors noted that knaacki sits very close to Hoplisoma punctatum, the type species of Hoplisoma Swainson, 1838 — a placement borne out by the 2024 revision of the subfamily by Dias, Tencatt and colleagues, which resurrected Hoplisoma from the over-large genus Corydoras and assigns knaacki to it, giving the combination Hoplisoma knaacki. corydorasworld.com and FishBase both carry it as Hoplisoma. The hobby still says 'Corydoras knaacki,' so we keep that heading and note the current combination. The authors also flagged that at least two near-identical, still-undescribed species occur in the same region.
Morphology
This is a small cory, reaching about 1.5 in standard length. It has the standard Corydoras build — a deep, flat-bellied body sheathed in two rows of overlapping bony scutes, a hard bony head, three pairs of barbels around the small downturned mouth, and stiff leading spines in the dorsal, pectoral and adipose fins.
Its pattern of fine dark markings on a pale ground gave rise to the 'Morse code' nickname. It belongs to the group of corys clustered around H. punctatum, the spotted type species of the genus, and is one of several very similar Madre de Dios fish that can be hard to tell apart on colour alone. As in most corys, females grow larger and rounder than males, especially in spawning condition.
Habitat
Corydoras knaacki is known from the río Madre de Dios basin of Peru — part of the upper Madeira system on the Amazon's southwestern flank. The type locality is swampy water near the town of Santa Rita, draining toward the río Inambari. It is a fish of soft, tannin-rich, fairly acidic blackwater drains and swamps rather than open river.
These are warm, soft, tea-coloured waters over soft substrates with leaf litter. In the field, juveniles aggregate in large schools in the shallows while adults gather in smaller groups in deeper, shaded water. Detailed published chemistry is limited, but the soft, acidic, tannin-stained character of the habitat is well attested. Like all corys it is a facultative air-breather, surfacing to gulp air and absorb oxygen through a vascularised hindgut — useful in the warm, low-oxygen swamps it favours.
Feeding
Corydoras knaacki is a foraging benthic omnivore. In its swampy habitat it works the soft bottom with its barbels, sifting sediment and leaf litter for small invertebrates, larvae, worms and organic detritus. In the aquarium it is unfussy, taking sinking dried foods readily along with small live and frozen items such as bloodworm and Tubifex. As with all corys, it should be fed as a deliberately provisioned bottom-dweller rather than left to scavenge.
Mating
Corydoras knaacki spawns in the standard manner of the subfamily — egg-laying shoalers with no pair bond. A receptive female cleans a site and the group grows active; a male assumes the classic 'T-position' at right angles to her head, she appears to take his milt into her mouth, and she fertilises the small batch of eggs cradled in a basket of her pelvic fins before pressing them onto the cleaned surface. The process repeats, a few eggs at a time, until the clutch is laid.
Breeding
C. knaacki is reported as relatively easy to spawn and rear — one of the more obliging of the newer corys. Spawning follows the usual trigger of a cool, heavy water change mimicking the rains. Notably, this species is described as not strongly predatory toward its own eggs and fry: keepers have left later spawns in with the adults without losing the young, which is unusual among corys, where adults typically eat the eggs. The adhesive eggs hatch in a few days, and the fry take microworm and newly hatched brine shrimp once the yolk sac is absorbed. It is regarded as a robust, prolific aquarium fish.
In the aquarium
Corydoras knaacki is an attractive, active and easy-to-keep cory that has become reasonably established in the hobby, with tank-bred fish available. Keep it in a group of six or more over soft sand, which protects the barbels from the erosion and infection that coarse or dirty substrate causes. Provide clean, gentle, well-oxygenated water; given its blackwater swamp origins, soft, slightly acidic, warm water with some leaf litter or wood suits it best, though it is adaptable. It is wholly peaceful and pairs well with small tetras, pencilfish, dwarf cichlids and similar calm community fish. As with all corys, transport with care — a stressed fish can release skin toxins that may kill a whole group in the small volume of a bag.
Conservation
Corydoras knaacki has not been evaluated by the IUCN, so it has no formal conservation status — an honest 'not assessed' rather than a clean bill of health. It is a recently described species with a comparatively restricted known range in the Madre de Dios swamps of Peru, the kind of localised distribution that warrants caution. The broad pressures on the region — deforestation, alluvial gold mining (a serious and growing problem in Madre de Dios) and the run-off and siltation it brings — bear directly on the soft, clear blackwaters this fish depends on. A formal assessment would be needed to gauge its status properly.