Taxonomy & naming
Bagrichthys vaillantii was described in 1906 by the Dutch zoologist Christiaan Marinus Louis Popta, who at the time placed it as a species of the genus in use for these Southeast Asian bagrids; the Catalog of Fishes (Eschmeyer, CAS) confirms the current combination Bagrichthys vaillantii, authority (Popta, 1906), as the valid name. It belongs to the family Bagridae, the Asian bagrid catfishes, a large group spanning genera such as Mystus, Hemibagrus and Sperata across South and Southeast Asia.
The type locality is Teba (Teluk Tebaq) on the Mahakam River, eastern Kalimantan, Borneo, Indonesia, and the holotype is held under catalogue number RMNH 7839. The species appears to be endemic to the Mahakam basin. The genus Bagrichthys — literally "Bagrus-fish", combining the older catfish genus name Bagrus with the Greek ichthys, "fish" — currently holds a handful of species, the so-called lancer catfishes, distinguished by an unusually tall, blade-like dorsal spine.
Morphology
Like other Bagrichthys, B. vaillantii has an elongated, laterally compressed body, a flattened head, and long, sweeping barbels used to probe the riverbed for food in the dark or turbid water it inhabits. The dorsal fin carries a tall, stiffened spine, part of the "lancer" build shared across the genus, and the pectoral fins bear their own stout, serrated locking spine. As in all bagrids the skin is scaleless (naked), which matters directly for how the fish should be medicated.
Measured specimens range from about 3.5–7 in standard length, with most museum material falling in the 4–6 in SL range — noticeably smaller than the closely related and more commonly seen B. macracanthus (the black lancer catfish). Even at the small end of that range this is a fish that needs real swimming space, not a token "algae wafer tank"; treat the top of the recorded range as the adult size to plan around.
Habitat
Bagrichthys vaillantii is known from the Mahakam River system of eastern Kalimantan, Borneo — a large, slow-flowing, often turbid lowland river. Congeneric Bagrichthys such as B. macracanthus are documented from similar habitat: slow-moving river channels with acidic water (around pH 5–6) and soft, muddy substrate, and it is reasonable to expect B. vaillantii uses comparable water given the shared genus and region, though dedicated habitat data specific to this species is sparse.
The murky, tannin-influenced water of these rivers likely favours the nocturnal, barbel-guided foraging typical of the genus, and a dim, cover-rich tank that echoes this turbid, slow-flow environment is the safest approach for keeping it well.
Feeding
Bagrichthys species are nocturnal, benthic omnivores that patrol the riverbed after dark, taking small invertebrates, worms and organic detritus sifted from mud and silt using their long barbels rather than sight. B. vaillantii almost certainly follows the same pattern, though specific dietary studies on this species are lacking.
In the aquarium, congeners accept sinking pellets, frozen bloodworm and Tubifex, and other prepared foods once settled. Because feeding activity peaks after dark, offering food once the lights are off — rather than relying on daytime feeding — gives a nocturnal lancer catfish a fair chance against more active, light-loving tankmates.
Mating
No description of courtship or spawning behaviour for Bagrichthys vaillantii was found in the sources consulted. Bagrid catfishes as a family show a range of reproductive strategies, and without species-specific observations it would be a guess to describe pairing, territory or spawning cues here — data sparse.
Breeding
Bagrichthys vaillantii is essentially unrecorded as an aquarium or captive breeding subject; it is not established as a home-breedable species, and no hormone-induced or commercial-scale breeding program is documented for it either. This mirrors the trade status of the genus generally, where only the larger B. macracanthus is regularly seen in shops and even that species is not routinely bred by hobbyists. Anyone acquiring wild-caught B. vaillantii should assume the current supply, if any, is wild-collected rather than captive-bred.
In the aquarium
Bagrichthys vaillantii is extremely rare in the aquarium trade — only its relative B. macracanthus is commonly available — so anyone encountering this species should plan for an adult fish in the 4–6.5 in SL range, sized for a long, spacious tank rather than a small community setup. It is a naked-skinned, scaleless catfish, which means it is sensitive to copper-based medications and many standard treatments; dose conservatively (often at half strength) and monitor closely if treatment is ever needed.
It carries a stiff, serrated, locking pectoral spine typical of bagrids, which can snag in a net and jab the handler, so it is best scooped and moved in a container rather than netted. Being nocturnal and adapted to turbid water, it will do best in a dimly lit tank furnished with driftwood, pipes or other cover to retreat into by day, with soft, muddy-looking substrate and gentle-to-moderate flow echoing its slow lowland river home. Tankmates should be robust enough not to be seen as prey by a foraging lancer catfish after lights-out.
Conservation
The IUCN Red List lists Bagrichthys vaillantii as Data Deficient, reflecting how little is documented about its population size, trends or the specific pressures it faces. As a species apparently endemic to the Mahakam River basin of eastern Kalimantan, it is exposed to the broader threats affecting Bornean lowland rivers — deforestation, mining, and agricultural runoff sedimenting and degrading river habitat — but no targeted assessment of these pressures against this species has been published. Given its Data Deficient status and apparent rarity in trade, any wild collection should be treated cautiously until more is known.