Taxonomy & naming
The species was first described by the Austrian ichthyologist Rudolf Kner in 1855 as Doras (Oxydoras) niger, based on material from the Paraná basin. In 1862 the Dutch ichthyologist Pieter Bleeker formally raised Oxydoras to full genus rank and renamed the species Oxydoras kneri in Kner's honour, and that combination is the one recognised today by the Catalog of Fishes. It belongs to the family Doradidae, the thorny or talking catfishes, a large South American group known for their bony lateral scutes and sound-producing spines.
Oxydoras is a modest genus within Doradidae, and O. kneri is one of its larger, better-known members. Older aquarium literature and some trade labels still occasionally echo the original Doras/Oxydoras niger combination, but Oxydoras kneri is the valid name and should be used in preference to any historical synonym.
Morphology
Oxydoras kneri has the unmistakable doradid silhouette: a robust, laterally compressed body, a broad flattened head with three pairs of barbels, and a single row of large, backward-pointing bony scutes running along the midline from behind the head to the tail. These plates, together with the stiff, serrated, lockable spine at the front of the dorsal fin and each pectoral fin, are the genus's main defence — the fish has no true scales, only this bony armour, which makes it a scaleless species for medication purposes (see "In the aquarium").
FishBase records a maximum size of about 27.5 in total length and 9 kg — a genuinely large catfish, not a small or medium aquarium species. Colouration is a plain grey-brown to olive over the body, without the bold markings seen in many smaller doradids. As in other thorny catfishes, the pectoral spines can be locked and rasped against the pectoral girdle to produce an audible grunt or squeak, the source of the family's common name "talking catfishes".
Habitat
Oxydoras kneri is native to the Paraná River basin of South America, recorded from Argentina, Bolivia, Brazil, Paraguay and Uruguay, with its type locality at Cuiabá. It is a freshwater species that favours mud-bottomed stretches of rivers, streams and lakes within the basin, where it forages over the substrate and forms loose schools.
Water in this part of the Paraná drainage is warm and only mildly acidic to neutral-to-slightly-alkaline, not the soft blackwater typical of some Amazonian tributaries. Scotcat.com gives a working aquarium range of about 70–75 °F and pH 6.0–7.8, figures consistent with the moderate, temperate-leaning waters of the southern Paraná system rather than a strongly acidic blackwater habitat.
Feeding
Oxydoras kneri is an omnivore rather than a specialist predator. FishBase and hobbyist sources describe a natural diet of aquatic insects, crustaceans, molluscs and other invertebrates, supplemented with some plant material, gathered by foraging over the mud and silt bottoms it inhabits.
In the aquarium it is an easy, unfussy feeder that takes sinking pellets, wafers, and meaty frozen or fresh foods; snails, prawn and other invertebrate-based foods are readily accepted. Because it is a nocturnal bottom-forager, feeding after lights-out helps ensure this large catfish gets its share rather than being out-competed by more visible daytime fish.
Mating
Detailed observations of courtship and spawning behaviour in Oxydoras kneri are not documented in the available sources, and the species is not established as a regularly bred aquarium fish. Data is sparse on how pairs or groups behave prior to spawning in the wild or in captivity.
Breeding
No reliable account of home-aquarium breeding for Oxydoras kneri is available from the cached sources, and given its large adult size and Paraná-basin river life history, home breeding should be treated as effectively unachieved. Many large South American doradids and other big river catfish reach the aquarium trade as wild-collected or commercially propagated juveniles rather than through home spawning, and nothing in the available sources suggests O. kneri is an exception. Prospective keepers should not expect to breed this species in a home tank.
In the aquarium
Oxydoras kneri is sold as an appealing small juvenile but grows into a genuinely large fish — up to about 27.5 in and 9 kg according to FishBase — and very few home aquaria can house an adult long-term; this is a species best suited to large public displays, ponds, or exceptionally large private systems, not a standard home tank. Scotcat.com notes it does best in spacious quarters with other doradids, large cichlids such as Oscars, or similarly robust, similarly sized tankmates, and appreciates large pieces of bogwood or wide pipes to shelter in during the day.
Because it lacks true scales and instead carries bony scutes, it is a scaleless species and should be treated as medication-sensitive — dose copper-based and other treatments conservatively. It also carries stiff, serrated, lockable pectoral and dorsal spines that readily snag in a net and can jab the keeper; move this fish in a container rather than netting it. Heater guards are worth using, since a fish of this bulk can easily break an unprotected heater. Overall it is reported as a peaceful cohabitant provided tankmates are too large to be swallowed and the tank itself is sized for an adult, not a juvenile.
Conservation
Oxydoras kneri has not been evaluated by the IUCN Red List, so its status is Not Evaluated rather than a confirmed category of concern. It is a widely distributed species across the Paraná basin spanning five countries, which suggests it is not currently a species of acute conservation concern, but the absence of a formal assessment means population trends are not documented.
As with many wide-ranging Paraná basin fishes, the main pressures it likely faces are habitat degradation, damming and water-quality changes across the river system rather than aquarium collection, though specific data on this species is not available in the cached sources.