Taxonomy & naming
Betta dimidiata was described by Tyson R. Roberts in 1989 as part of his comprehensive monograph on the freshwater fishes of western Borneo, published in the Memoirs of the California Academy of Sciences (volume 14, pages 1–210). The type locality is Sungai Seriang, a tributary of the Kapuas River system roughly 23 mi west of Putussibau in Kalimantan Barat (West Kalimantan), Indonesia. Eschmeyer's Catalog of Fishes records the valid name as Betta dimidiata Roberts, 1989, with authority unparenthesised because the species has not been moved from the genus in which it was originally placed.
Within the genus Betta, the species belongs to the dimidiata group — a cluster of small, mouthbrooding bettas from Borneo and Sundaland, closely related to one another and sharing the paternal mouthbrooding reproductive mode characteristic of the group. Betta dimidiata can be confused with the recently described B. krataios, from which it is distinguished by its spade-shaped rather than rounded caudal fin, gold rather than turquoise opercular scaling, a reddish-brown rather than yellowish-brown body, and reddish rather than yellowish fins.
Morphology
Betta dimidiata is a small, slender betta reaching approximately 1.5 in standard length (roughly 2 in total length), making it one of the more compact members of the genus. The body is reddish-brown overall, with iridescent gold scaling on the operculum (gill covers) that catches the light in low-angle illumination. The caudal fin is spade-shaped — broader at the tip than the base — a useful character separating it from the otherwise similar B. krataios. Unpaired fins carry a reddish tint.
Sexual dimorphism is present but subtle in juveniles and becomes clearer with maturity. Males develop a broader, more angular head, stronger iridescent opercular scaling, and more elaborately extended fins than females. As in other members of the genus, the labyrinth organ — a suprabranchial accessory breathing structure — is well developed, allowing the fish to take atmospheric air at the surface, which is essential in its low-oxygen native waters.
Habitat
Betta dimidiata is endemic to the upper Kapuas drainage and the Danau Sentarum lakes region of Kalimantan Barat, Indonesian Borneo — a mosaic of swamp forest, peat pools, and seasonally inundated floodplain habitats. The species is found in forest stream pools and swamp margins that may be only a few centimetres deep, shaded by dense closed canopy. Substrate consists of fallen leaves, submerged branches, and tree roots rather than sand or gravel; in dry periods the fish can persist in moist leaf litter as water levels recede.
The water is classic Bornean blackwater: heavily stained with humic and fulvic acids, extremely soft and mineral-poor, with pH recorded in the range of roughly 5.2–6.5, hardness as low as 18–90 ppm, and temperatures between approximately 22 and 81 °F. Dissolved oxygen levels in such habitats are low, which is why the labyrinth organ is indispensable — the fish must access surface air to survive. The combination of warm, still, acidic, oxygen-poor conditions defines the niche the species occupies, and replicating those parameters in captivity is essential to keeping it successfully.
Feeding
In nature, Betta dimidiata is an insectivore, foraging at and below the surface of its leaf-litter pools for small invertebrates — aquatic insect larvae, tiny crustaceans, and terrestrial insects that fall onto the water. Its trophic level is recorded at 3.2 on FishBase, consistent with a generalist invertebrate predator of modest size.
In the aquarium the species accepts small live and frozen invertebrate foods readily: Daphnia, Artemia nauplii, bloodworm (chironomid larvae), and similar items. As with many wild bettas whose evolution in oligotrophic, food-scarce swamp environments has produced fish with modest appetites and little metabolic reserve for digestive stress, feeding small amounts daily or every other day of varied, protein-rich foods is preferable to large, infrequent meals.
Mating
Betta dimidiata is a paternal mouthbrooder. Mating begins with the extended courtship typical of the dimidiata group: the male displays to the female with flared gill covers and spread fins, circling and approaching in a sequence of advances and retreats. When the female is receptive, the pair engages in a spawning embrace — the male wraps his body around the female in the nuptial clasp seen across the Betta genus, stimulating simultaneous release of eggs and sperm.
Unlike bubble-nesting bettas, no nest is built. Instead, once the fertilised eggs have been released, the male collects them in his mouth before they sink. The pair may embrace repeatedly in a single spawning session, the male gathering each new batch of eggs. The female typically retreats once spawning is complete, leaving the male to incubate the clutch. Males eat nothing during the mouthbrooding period and should not be disturbed by aggressive tankmates or handled unnecessarily.
Breeding
After the spawning embrace the male carries the fertilised eggs in his buccal cavity for approximately 10–14 days, depending on temperature — warmer water shortens the incubation period. Throughout this time the male is visible with a noticeably distended throat pouch. He does not eat, and a well-conditioned fish should enter incubation with sufficient body reserves to complete it comfortably.
At the end of the incubation period the male releases free-swimming fry that are already relatively well developed. The fry can accept microworm and freshly hatched Artemia nauplii from the moment they are free-swimming, which simplifies early rearing considerably compared to species that produce yolk-sac larvae. The fry and male may be separated at this point or the fry left in a species-only tank where they are unlikely to be eaten immediately. Breeders typically condition pairs in soft, warm, slightly acidic water that approximates the Kapuas blackwater, and offer a varied diet of small live or frozen invertebrates to bring fish into breeding condition.
In the aquarium
Betta dimidiata is not a community fish in any conventional sense. Its peaceful temperament toward its own species in a species-only setting, combined with its requirement for blackwater conditions that many common tankmates do not share, makes it best kept alone or with very small, similarly soft-water species that will not outcompete or harass it. Aggressive tankmates — including many other betta species — will stress or injure it.
The aquarium should replicate the Kapuas swamp environment as closely as practicable: soft, acidic, gently filtered water (pH 5.5–6.5, hardness below 90 ppm, temperature 75–81 °F), a deep layer of dried leaf litter such as Indian almond leaves, dim lighting or heavy surface planting to reduce light intensity, and minimal water movement. The labyrinth organ requires unobstructed surface access to atmospheric air; ensure the tank is covered to maintain warm, humid air above the water surface and that the fish can reach the surface easily. A small, well-aged, species-only tank of 10–15 US gal is adequate for a pair or trio.
The species is not widely available in the trade but is held by specialist wild-betta keepers and breeding groups, who have maintained it in captivity for generations. Its IUCN Vulnerable status makes captive propagation by hobbyists a meaningful contribution to its long-term security.
Conservation
Betta dimidiata is assessed as Vulnerable (VU) on the IUCN Red List, with the assessment most recently confirmed in December 2018. The listing reflects the species' restricted endemic range — the upper Kapuas basin and Danau Sentarum lakes region of West Kalimantan, Borneo — combined with ongoing threats to its peat-swamp and swamp-forest habitat from land conversion, drainage for agriculture (principally oil palm), peat fires, and logging.
The peat-swamp ecosystems of Kalimantan are among the most threatened freshwater habitats on Earth; large areas have been drained or burned in recent decades, dramatically reducing available habitat for species like B. dimidiata that are highly specialised to those conditions. Although the species' FishBase profile notes high resilience in terms of population recovery time (less than 15 months to double), that resilience is of limited value if the habitat itself disappears. Hobbyist breeding programmes that maintain the species in captivity provide a genuine buffer against further range contraction and serve an educational function in highlighting the conservation value of Borneo's swamp fisheries.