Taxonomy & naming
The filament cory was described by Han Nijssen and Isaäc Isbrücker in 1983, in the Revue française d'Aquariologie, from material taken in a tributary of Sisa Creek in the Río Corantijn basin of Nickerie District, Suriname. The specific epithet filamentosus refers to the produced, thread-like extension of the leading dorsal-fin ray.
It was placed, as nearly all 'cory cats' were, in Corydoras Lacépède, 1803 (family Callichthyidae). When Dias, Tencatt and colleagues revised the Corydoradinae in 2024 — splitting the long-overgrown genus into several lineages including Brochis, Hoplisoma, Osteogaster and Gastrodermus — this species was among those retained in Corydoras in the strict sense, the lineage anchored on the type species C. geoffroy. So unlike many of its relatives, its familiar name and its current valid combination are one and the same: Corydoras filamentosus.
Morphology
This is a small cory, reaching roughly 1.5 in in standard length (around 2 in total length), with females growing larger and fuller-bodied than males. It carries the standard Corydoras body plan: two overlapping rows of bony scutes down each flank, a hard helmeted head, three pairs of short barbels around the downturned mouth, and stiff, mildly venomous leading spines in the dorsal, pectoral and adipose fins.
The defining feature is the dorsal fin, whose first branched ray is drawn out into a long filament — the trait that gives the species its name and the readiest way to tell it from the several superficially similar small spotted corys of the Guianas. The body is a pale ground patterned with fine dark spotting and a darker masked region through the eye, in the general style of the spotted-cory group of the region.
Habitat
Corydoras filamentosus is endemic to the Corantijn (Corantijn/Courantyne) River basin on the Suriname–Guyana border, the type locality being a small northward-flowing tributary of Sisa Creek in the Nickerie District at a depth of under a metre. These are shallow, slow forest creeks and their margins, typically over sand with leaf litter and submerged wood.
Reported aquarium-relevant parameters for the species are a broad pH band of roughly 6.0–8.0, soft to moderately hard water (around 2–25 dH) and warm temperatures of about 72–79 °F, consistent with the warm, soft, mildly acidic to neutral lowland creeks of the region. Like other corys it is a quiet-water, bottom-dwelling fish and a facultative air-breather, able to gulp atmospheric air at the surface in oxygen-poor pools.
Feeding
The filament cory is a benthic omnivore and detritivore, foraging over the bottom and sifting sand through its barbels and gills to extract small invertebrates, insect larvae, worms, micro-crustaceans and organic detritus. It is an opportunist rather than a specialist. In captivity it takes all prepared foods that reach the bottom — sinking tablets, granules and larger flakes — and relishes live or frozen bloodworm, white worm, grindal worm, Daphnia and Tubifex, which are also the standard foods for bringing a group into spawning condition.
Mating
No spawning account specific to C. filamentosus has been published, but the species is expected to follow the well-studied Corydoras pattern: an egg-laying shoaler with no pair bond. A ready female cleans a site, the group becomes active, and a male takes up the characteristic 'T-position' at right angles to her head; she appears to take milt into her mouth, and sperm is thought to pass rapidly through her gut to fertilise the few eggs cradled in a basket of her clasped pelvic fins. She then presses those eggs onto the cleaned surface, and the sequence repeats a few eggs at a time until the clutch is complete.
Breeding
There is no published breeding report for the filament cory, which remains rare in the hobby, but it is expected to breed like its congeners. In related species spawning is typically triggered by a cool, large water change that mimics the onset of the rains, dropping the temperature a few degrees, with the group skewed toward more males than females. The adhesive eggs are given no parental care — adults will eat them — so breeders remove either the eggs or the parents. By analogy with other small corys, eggs would hatch in three to five days depending on temperature, with fry taking microworm, baby brine shrimp and powdered foods after absorbing the yolk sac.
In the aquarium
C. filamentosus is seldom traded and is essentially a collector's fish, appearing only as occasional wild imports; there are no registered keeper or breeding records of note. Its requirements are those of the genus. It is a shoaling species that should be kept in groups of six or more, and is genuinely stressed kept singly or in pairs; a group needs a tank with a generous open bottom area rather than height — around 23.5 in is a sensible minimum.
Substrate must be soft sand or very smooth, rounded fine gravel, never sharp or chipped grit, which abrades and infects the barbels these fish use to feed. Keep the bottom clean, the water warm (about 72–79 °F), well filtered and well oxygenated, with regular changes and a gentle current; it tolerates a wide pH and hardness range. It is entirely peaceful and suits a community of tetras, small rasboras, pencilfish, dwarf cichlids and the like, with nothing large enough to bully it. The usual cautions hold: net spines snag (use a cup), and a stressed cory can release a skin toxin that may prove fatal in the small volume of a transport bag.
Conservation
Corydoras filamentosus was assessed by the IUCN in 2021 as Data Deficient — there is simply not enough information on its distribution, population and threats to judge its extinction risk. It is a narrow-range endemic of a single drainage, the Corantijn basin, which makes it inherently more vulnerable than a widespread species to localised pressures such as gold-mining-driven siltation and mercury pollution, deforestation and water-quality decline that affect the rivers of the Guianas. As a Data Deficient species it is neither confirmed safe nor confirmed threatened; the honest position is that its status is unknown and that its limited range warrants attention.