Taxonomy & naming
Betta macrostoma was described by Charles Tate Regan in 1910, and the valid name as recognised by Eschmeyer's Catalog of Fishes (Catalog of Fishes, California Academy of Sciences) is Betta macrostoma Regan, 1910. It belongs to the unimaculata group, a clade of large paternal mouthbrooders distributed across Borneo and adjacent islands, whose members share a generalised, robust body plan and lack the elaborate finnage of the bubble-nesting splendens group.
The specific epithet macrostoma is from the Greek makros (large) and stoma (mouth), referencing the broad gape that accommodates the male's role as an oral incubator. The species has historically been placed in the subgenus Betta alongside other large Bornean mouthbrooders, though the precise internal phylogeny of the unimaculata group continues to be refined as molecular and morphological datasets expand. The common name 'Brunei Beauty' reflects both the species' stronghold in Brunei Darussalam and the esteem in which it is held by collectors.
Morphology
Betta macrostoma is among the largest members of the genus, with standard lengths recorded up to 2.5 in by FishBase and up to 3.5–4 in by Seriously Fish for exceptional wild specimens, making it considerably more robust than most wild bettas encountered in the hobby. The body is elongate and moderately compressed, typical of the unimaculata group, with a broad, blunt head and a noticeably large mouth — the feature that inspired the specific epithet.
Males in breeding condition are spectacular. The body ground colour is deep chestnut to burnt orange-red, overlaid with rows of iridescent blue-green scales that give the flanks a metallic sheen. A bold, dark horizontal band runs from the snout through the eye and along the midline of the body. The dorsal fin carries a prominent dark spot edged with iridescence — the 'spotfin' of one common name — and the caudal fin is rounded with orange and blue-green patterning. Females are more subdued but still attractive, with a similar banding pattern on a less intensely coloured body. Both sexes show the dark eye-stripe clearly. Colour varies between populations, and hobbyists maintain locality-specific lines that should be kept separate to preserve that variation.
Habitat
Betta macrostoma is endemic to northern Borneo, with the core range centred on Brunei Darussalam and extending into the Baram district of North Sarawak (Malaysia) near Marudi. The IUCN assessment places the extent of occurrence at approximately 429 mi² — an extremely small area of occupancy for a vertebrate with a declining population. It is sympatric in some localities with Rasbora tubbi.
The species inhabits shallow, still to slow-moving water in lowland rainforest: pools that form above and alongside waterfalls, shaded forest streams, and near-stagnant pools with clay or gravel substrates and accumulations of leaf litter. Water is acidic — pH 4.3 to 5.7 — and temperatures range from 68–79 °F, with some Seriously Fish records suggesting cooler conditions than many tropical bettas prefer. The water is slightly tea-coloured from dissolved humic acids but otherwise clear, with hardness near zero. Like all members of the genus, B. macrostoma is a labyrinth fish, breathing atmospheric air via its suprabranchial organ, which allows it to exploit shallow, oxygen-poor pools that exclude most other fish.
Feeding
In nature, Betta macrostoma feeds on insects, zooplankton, small aquatic invertebrates, and shrimp that fall into or inhabit its shallow forest-pool habitat. The diet reflects the species' position as an opportunistic predator in a species-poor, nutrient-limited rainforest ecosystem — it takes whatever small prey is available.
In the aquarium the species is straightforward to feed once settled. It accepts live and frozen foods readily — Daphnia, brine shrimp, mosquito larvae, small earthworms, and shrimp — and most individuals will eventually take high-quality pellets or freeze-dried foods. A diet with substantial live or frozen invertebrate content is best for breeding condition and colour. The species is not a vigorous surface feeder and may be outcompeted by faster tankmates at feeding time; target feeding is advisable in mixed setups.
Mating
Betta macrostoma is a paternal mouthbrooder, and its courtship behaviour reflects that reproductive strategy in ways that set it apart from the bubble-nesting bettas most hobbyists know. Males do not build nests; instead they invest in post-spawning oral incubation. Courtship involves prolonged circling and mutual display between the sexes, with the male flaring fins and intensifying colour to advertise fitness. Pairs may engage in extended periods of display before spawning commences.
Spawning occurs in a characteristic embrace: the male wraps around the female in the typical anabantoid manner, and the pair release eggs and sperm simultaneously. After each embrace, the male collects the fertilised eggs in his large mouth. This sequence is repeated over a spawning session that may last several hours, until the male is carrying a full clutch. Colour variation between locality populations is considered significant by serious hobbyists, and mixed-locality breeding is discouraged.
Breeding
Once the male has gathered the full clutch into his mouth, he ceases feeding entirely and incubates the eggs and early fry for 14 to 35 days — a notably long incubation period that produces large, well-developed fry capable of taking Artemia nauplii immediately. During this time the male should be disturbed as little as possible; stress can cause him to swallow or expel the brood prematurely.
The female should ideally be removed to a separate tank after spawning to reduce pressure on the holding male. When the male finally releases fry, they are relatively large and robust compared to the fry of egg-scattering species. First foods are newly hatched brine shrimp nauplii and micro-worms; the fry grow steadily but colour development in males takes many months. Captive breeding of B. macrostoma is achieved by dedicated hobbyists worldwide, and captive-bred stock is the only ethical source for the aquarium trade given its protected status in Brunei. Water for breeding should replicate the species' soft, acidic, warm-to-cool conditions (pH 5–6, temperature 72–77 °F).
In the aquarium
Betta macrostoma is a specialist species best suited to experienced keepers who can replicate its exacting soft, acidic water requirements and provide a calm, uncrowded environment. Water parameters should target pH 4.4–5.7, very soft (near-zero hardness), and temperatures of 68–77 °F — cooler than many tropical fish rooms, which can be a logistical challenge. Stability matters greatly; fluctuating pH or temperature is a common cause of failure with this species.
A dedicated species tank or a pairing with a compatible, equally undemanding companion is far preferable to a community setup. The tank should be well-shaded, heavily planted with floating cover, and filtered with a gentle sponge filter that provides water movement without current — as a labyrinth fish it must reach the surface to breathe, and it requires access to warm, humid air above the waterline. Leaf litter (Indian almond leaves) and driftwood help maintain acidity and provide shelter. Pairs can be kept together outside of breeding, but the male should be watched for aggression. Given the species' rarity, its protected status in Brunei, and the availability of captive-bred stock from specialist breeders, sourcing wild-caught specimens is both unnecessary and discouraged.
Conservation
Betta macrostoma is assessed as Vulnerable (VU) on the IUCN Red List under criterion B1ab(iii,v), assessed 28 December 2018. The IUCN evaluation identified an extent of occurrence of only approximately 429 mi², with fewer than six known locations, a decreasing population trend, and ongoing habitat loss. The primary threats are overharvesting for the international aquarium trade — wild pairs have fetched USD 150–180 on the collector market — and accelerating conversion of Bornean rainforest to oil palm plantation, which degrades and eliminates the shallow forest pool habitat on which the species depends.
The species is legally protected in Brunei Darussalam, and export from Brunei is banned, though enforcement is described as loose. Populations in Sarawak, Malaysia are not legally protected under equivalent measures. The IUCN assessment notes that captive-bred stock is potentially available despite the export ban, and the conservation community broadly supports captive-breeding programmes as a buffer against further wild population decline. Aquarists who keep this species should source captive-bred individuals from reputable specialist breeders, maintain locality-pure lines, and consider participating in organised studbook or conservation-breeding efforts through betta and anabantoid societies.