Taxonomy & naming
Clarias intermedius was described in 2001 by Guy Teugels, Sudarto and Laurent Pouyaud as part of a broader revision of Bornean walking catfishes, based on a holotype of 8.5 in and a series of eighteen paratypes ranging from 5.5–12 in. It belongs to the family Clariidae, the airbreathing or walking catfishes, and to the genus Clarias, a large Old World genus historically shuffled through combinations under names such as Silurus before Clariidae was recognised as its own family. The Catalog of Fishes treats Clarias intermedius as a currently valid species.
The genus name Clarias derives from the Greek chlaros, "lively" or "vigorous", a nod to the group's notorious ability to survive out of water and wriggle across land between pools. The species epithet intermedius is Latin for "intermediate" or "in between", referring to characters that place this species between other described Bornean Clarias in the original taxonomic revision. There is no established English common name; it is referred to in the literature simply by its scientific name.
Morphology
Clarias intermedius has the classic walking-catfish silhouette: an elongated, cylindrical, scaleless body, a long-based dorsal fin running most of the length of the back, a similarly long anal fin, and a flattened, bony head with four pairs of barbels used to feel out food in dark or turbid water. Adults reach about 7.5 in standard length, based on the type series and subsequent FishBase records, with the type specimens themselves spanning from a 5.5 in juvenile paratype up to the 8.5 in holotype and larger paratypes to 12 in total length.
As in all Clarias, the skin carries no scales, and the fish possesses an accessory suprabranchial (labyrinth-like) breathing organ that lets it gulp air at the surface and tolerate poorly oxygenated water — an adaptation to the swamp environment it calls home. The pectoral fins each carry a stiff, bony spine that can be locked outward; it is not a food fish targeted for its size, but the spine is still sharp enough to catch a net or a careless hand.
Habitat
This species is endemic to a narrow stretch of southern Borneo, known from the Seruyan River drainage eastward to the Kahayan River drainage in Central Kalimantan, Indonesia — the same peat swamp forest region that gave rise to the type series between the Sampit and Barito basins. It lives in blackwater rivers and streams draining tropical peat swamp forest, water that is intensely acidic and stained the colour of strong tea by dissolved tannins and humic acids from decaying leaf litter and peat.
FishBase records this habitat as soft, strongly acidic blackwater, typically in the pH 3 to 4 range — among the most extreme water chemistry any aquarium fish naturally experiences. Dissolved oxygen in these swamp channels is often low, which is precisely the environment its accessory air-breathing organ is built for. The species is demersal and benthic, staying near the bottom of these slow, shaded forest waterways.
Feeding
Clarias species are opportunistic, largely nocturnal predators, and Clarias intermedius fits the pattern: FishBase lists it at trophic level 3.4, consistent with a mixed diet of aquatic invertebrates, small fish and other prey it can find and swallow in the dim, tannin-stained water of its swamp habitat. Like its relatives it almost certainly forages using its barbels to locate food by touch and smell rather than sight, an advantage in the dark, cover-shaded blackwater it inhabits.
In captivity, Clarias catfishes take readily to meaty foods: chunks of fish, prawn, earthworms and sinking pellets or carnivore-formulated foods. Any tankmates offered alongside it must be too large to be swallowed — this is a predator that will eat anything that fits in its mouth, and its mouth is not small relative to its own body.
Mating
No detailed account of courtship or spawning behaviour has been published for Clarias intermedius specifically; the species was described from a taxonomic revision focused on morphology, not field observation of reproduction. Across the walking catfishes generally, spawning is associated with seasonal flooding, when adults move into flooded marginal vegetation or newly inundated swamp to scatter eggs, a pattern documented in better-studied relatives such as Clarias batrachus and Clarias gariepinus, both of which are farmed commercially using hormone-induced spawning rather than natural courtship in captivity.
Given how poorly known this species is outside its original description, and the fact that even well-studied Clarias are not bred through natural courtship in home aquaria, nothing specific can be said about pairing behaviour, courtship displays or sexual dimorphism in Clarias intermedius without overreaching the available data.
Breeding
As with other members of the genus reaching commercial size, Clarias intermedius and its relatives are propagated at scale — where they are propagated at all — through hormone-induced stripping and artificial fertilisation on fish farms, not through natural pairing in a tank. This is standard practice across the genus, most familiar from the heavily farmed Clarias gariepinus and Clarias batrachus, and there is no record of Clarias intermedius being bred in a home aquarium.
Given its restricted range, blackwater specialisation and lack of any aquarium-trade profile, this species should be treated as essentially unavailable and unbred in the hobby; anyone encountering it should assume wild or incidental collection rather than captive breeding, and should not expect to replicate spawning conditions at home.
In the aquarium
Clarias intermedius is not an established aquarium fish, and there is no meaningful hobbyist track record to draw on — but the genus-level cautions apply squarely. At an adult size around 7.5 in SL it is smaller than giants like Clarias gariepinus, yet it is still a robust, active predator that needs a long tank with a secure, weighted lid: air-breathing Clarias are proficient at pushing through gaps and wandering out of open tanks, especially at night.
Water chemistry is the real challenge: this species comes from acidic blackwater around pH 3 to 4, far more extreme than the general soft-acidic range most "blackwater" aquarium fish tolerate, and reproducing that safely (while keeping biological filtration functional) is a specialist undertaking. As a scaleless catfish it is sensitive to copper and many medications, so any treatment should be dosed conservatively. Its bony pectoral spines can lock and snag a net, so it is better scooped in a container than netted. Being a facultative air-breather, it needs unobstructed access to the surface. It is nocturnal and predatory, so tankmates must be too large to be eaten and the fish should be provided dark refuges for daytime cover. Given how narrow its known range is and how specialised its water chemistry, this is not a species to buy on impulse.
Conservation
The IUCN Red List assesses Clarias intermedius as Near Threatened, under criterion A3c, in an assessment dated 26 December 2019. The species is restricted to a single cluster of river drainages in Central Kalimantan, from the Seruyan River east to the Kahayan River, all of it dependent on tropical peat swamp forest.
The primary threat is large-scale habitat destruction and degradation of that peat swamp forest, driven by logging, canal drainage schemes that lower and alter the swamp's water table, and conversion of land to plantation agriculture — pressures that have hit Bornean peat swamp forest particularly hard in recent decades. Because the species' entire known range sits inside this single threatened habitat type, continued peat swamp loss in Central Kalimantan is a direct threat to its long-term survival.