Taxonomy & naming
Clarias kapuasensis was described in 2003 by Sudarto, Teugels & Pouyaud as part of a wider revision of Bornean walking catfishes. It belongs to the family Clariidae, the airbreathing or walking catfishes, and to the genus Clarias, a large Old World genus of more than 60 species spread across Africa and Asia. The species epithet kapuasensis refers directly to its type locality, the Kapuas River basin of West Kalimantan (Indonesian Borneo).
Clarias has a long history of taxonomic churn, with older literature sometimes placing walking catfishes under other generic names or lumping distinct Bornean populations together; C. kapuasensis was split out as morphologically distinct within this revision, diagnosed by a very short occipital process and specific dorsal and anal fin-ray counts. The genus name itself remains stable and is not disputed.
Morphology
Like other Clarias, C. kapuasensis has the unmistakable walking-catfish body plan: an elongated, cylindrical trunk, a flattened, bony head, and long, low dorsal and anal fins that run most of the length of the body without any dorsal or anal spines. Four pairs of barbels surround the mouth, used to feel out food in dark or turbid water. Maximum recorded size is 10 in standard length.
Diagnostic features separating it from other Bornean Clarias include a very short occipital process and fin-ray counts of roughly 65–72 dorsal soft rays and 51–60 anal soft rays. As in all Clarias, the skin carries no scales at all, leaving the fish more exposed to skin damage and more sensitive to certain medications than scaled fish (see "In the aquarium"). A suprabranchial accessory breathing organ lets it gulp air at the surface, the adaptation that gives the genus its "walking catfish" reputation.
Habitat
This species is endemic to the Kapuas River basin of West Borneo, Indonesia's longest river at roughly 1,89 mi, which drains an enormous expanse of peat swamp forest. C. kapuasensis lives specifically in the swampy margins bordering the Kapuas mainstem and its tributaries, rather than in open river channel.
Water in these peat-swamp systems is warm and markedly acidic to soft: temperatures run about 75–90 °F, and pH in the peatland tributaries is typically 4.0–6.3, tea-coloured blackwater stained by tannins leaching from the surrounding peat. The mainstem itself runs closer to neutral, but the swamp habitat this species favours is genuinely soft, acidic blackwater — a real exception among catfishes, most of which are not blackwater specialists.
Feeding
Specific dietary data for C. kapuasensis has not been documented, but the genus Clarias is uniformly an opportunistic carnivore, and closely related Bornean and Southeast Asian species take worms, insects, crustaceans, small fish and decaying organic matter from the swamp floor. As a large, nocturnal predator it will swallow any fish or invertebrate small enough to fit in its mouth, and tankmates must be sized accordingly — a rule that applies even more forcefully once a specimen approaches its adult length.
In captivity, related Clarias accept a wide range of meaty foods without fuss: whole or chopped fish and prawn, earthworms, and sinking pellets or pastes formulated for large predatory catfish. There is no indication this species is a fussy feeder; the challenge in keeping it is size and tankmate compatibility rather than diet.
Mating
No aquarium spawning of C. kapuasensis has been documented, and there is no published account of its courtship behaviour in the wild or in captivity. Data on this species specifically is sparse. Across the genus Clarias generally, wild spawning is associated with rising water and flooded margins during the rains, with pairs or small groups depositing adhesive eggs over submerged vegetation or detritus; some parental guarding has been reported in a few species, but this is inconsistent across the genus and unconfirmed for C. kapuasensis.
Breeding
As with the large, commercially significant Clarias species generally, C. kapuasensis and its relatives are propagated at scale by farms using hormone-induced spawning — injecting pituitary extract or synthetic hormones to trigger release of eggs and milt, which are then stripped and fertilised by hand. This is standard practice across the walking-catfish trade because natural spawning is difficult to control and the fish grow far too large, too fast, for aquarium-scale breeding programmes to be practical.
There is no record of C. kapuasensis being bred in a home aquarium, and given its adult size and the lack of any documented natural spawning behaviour for this species, it should be considered essentially not home-breedable. Anyone encountering captive-bred stock of this or a similar Clarias is almost certainly looking at farm or hormone-induced production, not a hobbyist spawn.
In the aquarium
C. kapuasensis is not a realistic long-term aquarium fish for the vast majority of keepers. It reaches a genuine adult size of around 10 in SL, and juveniles sold small will keep growing — this is a classic Clarias size trap, and a tank that looks generous for a young fish will not hold the adult. Only a very large, dedicated setup (or a pond, in warm enough climates) can responsibly house a full-grown specimen, and it should never be bought on impulse as a small juvenile without a plan for that eventual size.
Being scaleless, it is more sensitive than scaled fish to copper-based medications and some other treatments; dose conservatively, as with loaches, puffers and eels. As a facultative air-breather it needs regular access to the surface and does poorly if the tank is sealed with no airspace. Handle it with care: Clarias lack spines but are strong and slippery, and are best moved in a container rather than chased with a net once of any size. Provide dim lighting and cover, since it is most active after dark, and never house it with fish small enough to be swallowed — including tankmates that fit its mouth today but not tomorrow. Clarias are also notorious invasive species outside their native range in parts of the world; this fish must never be released into local waterways.
Conservation
The IUCN Red List assesses Clarias kapuasensis as Endangered (EN), evaluated in 2018 under criterion B1ab(i,iii), reflecting a small range confined to specific swamp habitats of the Kapuas basin and continuing decline in the extent and quality of that habitat.
The Kapuas basin's peat swamp forests, the species' specific habitat, are under sustained pressure from drainage, conversion to agriculture and plantations, and associated peat fires — threats that affect many narrow-range Bornean freshwater fishes beyond this one species. As a facultative air-breather tied specifically to swamp margins rather than open river, C. kapuasensis is more exposed to habitat loss than river-dwelling relatives with a broader range. There is no evidence this species supports an aquarium trade of any scale; conservation concern here is about habitat protection, not collection pressure.