Taxonomy & naming
The species was described by the British aquarist and author David Sands in 1989, originally as Corydoras elegans nijsseni — a subspecies of the elegans-type dwarf corys — and later treated at full species rank as Gastrodermus nijsseni. It belongs to the family Callichthyidae, subfamily Corydoradinae, and to the small-bodied 'elegans group' of midwater corys. The name honours Dr Han Nijssen (1935–2013), the Dutch ichthyologist who was a leading authority on the genus Corydoras.
In the 2024 phylogenomic revision of the Corydoradinae by Dias, Tencatt and colleagues, the elegans-group dwarf corys were separated from Corydoras in the strict sense and placed in the resurrected genus Gastrodermus. Under that classification this fish becomes Gastrodermus nijsseni, the combination now used by FishBase and the Catalog of Fishes. Because the change is recent and the aquarium trade still overwhelmingly says 'Gastrodermus nijsseni,' we keep that as the hobby-familiar heading while noting the current valid combination. (The name should not be confused with Apistogramma nijsseni, a dwarf cichlid that honours the same ichthyologist.)
This fish was long listed as Corydoras nijsseni; Eschmeyer's Catalog of Fishes places the species in Gastrodermus, and the current combination Gastrodermus nijsseni is followed here.
Morphology
This is one of the smallest corys, reaching only about 1.2 in standard length. It has the slimmer, more elongate build of the elegans-group dwarf species rather than the deep, stocky body of the classic short-nosed corys, and like all members of the subfamily it is armoured with two rows of overlapping bony scutes and a hard, helmeted head, with barbels fringing the small downturned mouth.
Its colour pattern is the orange-blotched type characteristic of the small corys of the upper Rio Negro drainage: a pale body broken by darker markings and warm orange tones. As in related species these black components of the pattern are notably variable, so individuals and populations differ in how strongly the dark markings are expressed. The dorsal, pectoral and adipose fins each carry the usual stiff cory spine.
Habitat
Gastrodermus nijsseni occurs in the Rio Negro basin of the Amazon — the great black-water river of northwestern Brazil and adjacent Colombia, whose forest-stained waters are warm, soft and acidic. It is grouped with the other small, orange-blotched corys of the upper Negro drainage, which share these black-water conditions: tea-coloured, mineral-poor water over sand and leaf litter in the shaded margins of streams and flooded forest.
As a callichthyid it is a facultative air-breather, periodically rising to gulp atmospheric air and absorbing oxygen through its vascularised hindgut — an adaptation well suited to the warm, oxygen-poor black-water habitats it occupies. Detailed ecological data for the species have not been published, and most information derives from collectors and aquarists working with imported fish.
Feeding
Like its relatives, G. nijsseni is a benthic micro-omnivore, foraging over the bottom and using its barbels to taste out small invertebrates, micro-crustaceans, insect larvae and organic detritus from sand and litter. Its very small size means correspondingly small prey and a need for fine foods. In the aquarium it takes small sinking foods, micro-pellets and crushed tablets, along with frozen and live foods of suitable size such as small bloodworm, daphnia and brine shrimp; care must be taken that so small a fish is not out-competed at feeding time by larger tankmates.
Mating
No formal spawning study of G. nijsseni has been published. Corydoras and their relatives are egg-laying shoalers with no pair bond and the well-known 'T-position' courtship, in which a receptive female cleans a surface, a male aligns at right angles to her head, and she clasps a few adhesive eggs in a basket formed by her pelvic fins, carrying them to the cleaned site before the sequence repeats. The dwarf elegans-group species follow this general pattern, but the specific reproductive behaviour of G. nijsseni is essentially unrecorded.
Breeding
There are no well-established captive-breeding accounts for this species, and it is an uncommon import rather than a commercially bred fish. By analogy with the genus and the elegans group, any clutch would consist of small adhesive eggs laid on firm surfaces or among plants, given no parental care — the adults may eat the eggs, so a breeder would separate eggs or parents and raise the fry on the finest foods (infusoria, microworm and freshly hatched brine shrimp) once the yolk sac is gone. The species' small adult size and scarcity in the trade mean it is seldom the subject of dedicated breeding efforts.
In the aquarium
A delicate, dwarf black-water cory, G. nijsseni is best regarded as a fish for the experienced keeper. As a shoaling species it should be kept in a group of six or more — ideally a larger shoal, given its small size — over soft sand, which lets it feed naturally and protects its barbels from the abrasion that coarse gravel causes. Soft, slightly acidic, warm water in the high-20s Celsius, kept clean and gently filtered, suits its origins in the soft black waters of the Rio Negro.
Because it is so small, tankmate choice matters: it belongs only with peaceful, similarly small fish such as small tetras, pencilfish and other dwarf corys, never with anything large enough to bully or out-eat it. Mature, stable water quality and gentle flow are important, as the small black-water species tend to be less forgiving than the hardy common corys. The usual cory cautions apply — the fin spines snag in nets, so a cup is safer, and a badly stressed fish can release a skin toxin dangerous in the confined volume of a transport bag.
Conservation
Gastrodermus nijsseni (as Gastrodermus nijsseni) is assessed by the IUCN as Data Deficient (assessed 2020): there is not yet enough information on its distribution, population and trends to evaluate its extinction risk. It is one of several small, similar corys of the upper Rio Negro whose exact ranges are imperfectly known, and it is taken only in modest numbers for the aquarium trade. The Rio Negro black-water system remains comparatively intact in parts, but deforestation and development pressures in the wider basin could affect such narrowly known fishes; until basic survey data are collected, its true status cannot be assessed.