Taxonomy & naming
Corydoras gracilis was described by the Dutch ichthyologists Han Nijssen and Isaäc Isbrücker in 1976, in the aquarium journal Tropical Fish Hobbyist, from material collected in northwestern Brazil. The type locality was originally given as a tributary of the Rio Tapajós but was corrected by the authors in 1980 to the Rio Jauna (the Rio Juma) at the Trans-Amazônica highway, about 6°09'S 59°55'W, a tributary of the Rio Aripuanã in the Rio Madeira watershed, Amazonas state. The holotype is held in Amsterdam (ZMA 114687).
For nearly fifty years it sat within Corydoras Lacépède, 1803, the vast genus of South American armoured catfishes (family Callichthyidae, subfamily Corydoradinae). Molecular and morphological work showed that genus to be unnatural, and in 2024 Dias, Tencatt and colleagues published a large revision (Zoological Journal of the Linnean Society, DOI 10.1093/zoolinnean/zlae053) that split it into several genera. Under that scheme the slender cory becomes Gastrodermus gracilis — the same lineage that received the small, slim 'elegans-type' species. The change is recent and not yet universal; the trade still says 'Corydoras gracilis,' so we keep that as the display heading while noting the current combination. The species had long been placed informally in the putative 'C. elegans group,' a cluster of small, similarly patterned corys, several of which remain scientifically undescribed and circulate under 'C' and 'CW' code numbers.
Morphology
This is a genuinely tiny catfish. FishBase gives a maximum of about 1 in standard length, and hobby sources put the total length at roughly 1–1.5 in; either way it is among the very smallest members of the group, small enough that newcomers sometimes mistake it for an Aspidoras. The body is the typical cory shape in miniature — a short, deep-bellied trunk armoured with two rows of overlapping bony plates rather than scales, a blunt helmeted head, downturned mouth and short sensory barbels.
What sets it apart is the colour pattern. Over a uniform, light base it carries a single dark, arched stripe that runs along the upper portion of each flank, from the tip of the snout, through the eye, to the base of the tail. A handful of congeners share this arched-band motif — notably C. arcuatus, C. narcissus and C. urucu — but C. gracilis is set apart by being the only one in which the stripe terminates at the base of the upper caudal-fin rays rather than continuing down onto the lower tail. It is also smaller and darker-bodied than the larger, paler arcuatus and narcissus. Like all corys it carries stiff, mildly venomous spines in the dorsal and pectoral fins; the pectoral spine can pierce skin and deliver a painful sting, so it should be handled with care.
Habitat
Corydoras gracilis is native to the central Amazon basin of northwestern Brazil. Its known range runs from the Rio Juriti in Pará state to the Rio Juma, a tributary of the Rio Aripuanã in the Rio Madeira system of Amazonas state — Eschmeyer's Catalog summarises the distribution as the middle and lower Amazon River basin. It appears to favour minor tributaries rather than main channels: small, shaded watercourses with dense riparian vegetation, submerged leaves and branches, and soft, acidic, tannin-stained 'blackwater.'
These are warm, soft, gently flowing waters. Recorded parameters span roughly 72–79 °F, with the fish tolerating a broad pH band (about 6.0–8.0 across references, though wild blackwater habitats trend distinctly acidic, to pH 5) and soft to moderately hard water. Like other corys it is a facultative air-breather, equipped with a modified, highly vascularised intestine that lets it gulp atmospheric oxygen at the surface and survive in the warm, oxygen-poor pools its habitat can become. Honest caveat: the species is thinly documented in the field, and much of what is written about its ecology is inferred from its habitat type and from close relatives rather than from dedicated study.
Feeding
Like all members of the group, Corydoras gracilis is a foraging benthic omnivore. In the wild it works soft substrate and leaf litter, sifting sand and detritus through its barbels and mouth to take small invertebrates, insect larvae, micro-crustaceans and organic matter. Its small size simply scales the menu down — it targets correspondingly tiny prey. In the aquarium it accepts small sinking dried foods alongside live and frozen fare such as micro-bloodworm, Daphnia, Tubifex and Artemia. A varied diet keeps the fish in condition; it should never be left to scavenge leftovers or expected to 'clean' the tank, and because of its size, food must be small enough and must actually reach the bottom past faster tankmates.
Mating
Corydoras are egg-laying shoalers with no pair bond and a distinctive, well-documented courtship, and the dwarf species follow the same template at miniature scale. When a female is ready, the group becomes active and a male takes up the classic 'T-position,' aligning at right angles to the female's head. In gracilis the female then clasps a very small number of eggs — typically just two to four — in a basket formed by her pelvic fins, where the male fertilises them over roughly half a minute. Only then does she swim to a cleaned site and press the intensely sticky eggs into place. The pair repeats the sequence many times until on the order of a hundred eggs have been laid in small batches across leaves, wood, fine plants or the aquarium glass.
Breeding
C. gracilis can be spawned much like other corys, though it is reckoned more demanding to raise than the hardy commercial species. Conditioning a group on live and frozen foods and skewing the ratio to two or more males per female helps; when the females are visibly full of eggs, a large (50–70%) cool water change with increased oxygenation and flow, repeated daily, is the standard trigger that mimics the onset of the rainy season. Eggs are laid on the glass, among fine-leaved plants or in spawning mops — mops being convenient because the eggs can simply be lifted out. There is no parental care, and the adults will eat the spawn, so breeders move either the eggs or the parents. Incubation runs about three to four days; once the fry absorb their yolk sacs they take microworm and Artemia nauplii. They demand excellent water quality and reportedly fare better over a thin layer of sand than in a bare tank. Most fish in the hobby are still wild-caught rather than commercially bred, a reflection of how uncommon and finicky the species is.
In the aquarium
The slender cory is an attractive but specialist's fish rather than a beginner's staple. It is a shoaling species that is visibly stressed and reclusive when kept in small numbers; keep a group of at least six (more is better) and it becomes far more confident and active. Being tiny, it does not need a large tank — a base footprint of around 60 × 12 in is ample — but it does need the right setup. A soft sand substrate is close to mandatory: corys feed by sifting, and sharp or coarse gravel abrades and infects their barbels, leaving the blunted, eroded faces that signal poor husbandry. Provide shade and cover in the form of driftwood, roots and a layer of dried leaf litter, which also suits its blackwater origins.
Water should be warm but not hot (about 72–79 °F), soft, clean and well-oxygenated, with gentle flow and regular partial changes. Because of its small size, choose tankmates with care: only small, peaceful, non-predatory species such as small tetras, pencilfish and dwarf rasboras — anything large enough to swallow or bully it is unsuitable. Two standard cory cautions apply: the fin spines snag in nets, so a cup is safer for moving them, and a badly stressed cory can release a skin toxin that, in the small volume of a bag or bucket, can kill the whole group, so transport them carefully.
Conservation
Corydoras gracilis was assessed for the IUCN Red List as Least Concern (assessment dated 7 November 2018). It is a small-ranged Amazonian endemic rather than a continent-spanning generalist, but it was judged to face no major range-wide threat at the time of assessment. That said, hobby and reference sources note real local habitat loss: since the 1990s at least some of its tributary habitats have been degraded by conversion to farmland and by dam construction, particularly around Porto Velho on the Rio Madeira in Rondônia. As with many narrowly distributed blackwater specialists, the broad pressures on Amazonian freshwaters — deforestation, agriculture, damming and pollution — bear on the small streams it depends on, and its thin field documentation means population trends are poorly known.