Taxonomy & naming
Belontia hasselti was first described by Georges Cuvier in 1831, making it one of the earliest-described members of the suborder Anabantoidei. The parenthetical authority reflects the fact that Cuvier placed the species in a different genus at the time of description; it was later moved to Belontia, established by J.R. Norman in 1914. The valid name as recognised by Eschmeyer's Catalog of Fishes is Belontia hasselti (Cuvier, 1831).
The genus Belontia is small, containing only two accepted species: B. hasselti of the Malay Peninsula and the Sunda Islands, and B. signata from Sri Lanka. Together they form the combtail group within Osphronemidae (sometimes treated as Belontiidae in older literature), a family that encompasses the true gouramis, paradise fish, and fighting fish. The species' common names — Malay combtail, Java combtail — reflect both its geographic range and the distinctive elongated, comb-like rays of the caudal and posterior fins that characterise the genus.
Morphology
Belontia hasselti is among the larger gouramis, with males reaching 8 in standard length; females remain somewhat smaller and are more rounded in the body. The body form is robust and moderately laterally compressed, with a pointed snout and a relatively large mouth suited to a carnivorous diet. The species' signature feature is the trailing, filamentous comb-like extensions on the caudal, dorsal, and anal fins that give the genus its English name — these become most pronounced in dominant adult males.
The base coloration is bronze to olive-brown overlaid with iridescent scales that flash teal, blue, or greenish-gold depending on lighting and mood. Each scale carries a dark central spot, producing a net-like reticulated pattern across the flanks. Juveniles and subadult fish are considerably drabber and brown-toned, developing their full iridescence and fin extensions only as they mature. Males are larger and more elongate, with brighter coloration and more extravagant fin development than females — the primary basis for sexual differentiation in the species.
Habitat
Across its range — the Malay Peninsula, Sumatra, Borneo, and Java, with records also from Thailand and Vietnam — Belontia hasselti inhabits slow-moving and still waters associated with ancient lowland peat swamp forests, acidic blackwater streams, and forest-fringed rivers. It is classified by FishBase as primarily a peat habitat species, where it occupies the lower and middle water column among submerged roots, fallen timber, and dense riparian vegetation.
Water chemistry in primary peat swamp habitat is typically highly acidic (pH as low as 3–4 has been recorded), tannin-stained, soft, and low in dissolved oxygen — conditions the species tolerates thanks to its suprabranchial labyrinth organ, which allows it to breathe atmospheric air directly. In practice, Belontia hasselti shows considerable adaptability and is recorded across a wide chemistry range (pH 4.5–8.5 in captive settings). Temperature tolerance spans roughly 70–86 °F, with optimal conditions generally considered to be 70–79 °F. The species has been recorded as locally extinct in Singapore (pre-1960s habitat loss) and is possibly extinct on Java, underscoring the direct link between its survival and intact peat swamp ecosystems.
Feeding
Belontia hasselti is a confirmed carnivore. In the wild it preys on aquatic invertebrates, molluscs, and small fishes. Its relatively large, upturned mouth and robust build suit active pursuit rather than passive filtering, and wild fish take whatever animal prey is available in the peat-swamp environment — insect larvae, worms, snails, and small fish.
In the aquarium it readily accepts a wide range of carnivore-appropriate foods: frozen bloodworm, earthworm, prawn, mussel, and similar meaty items form the staple. It will also take high-quality cichlid-style pellets. Live feeder fish should be sourced with caution due to disease risk. A varied, predominantly animal-protein diet maintains condition and colour; purely plant-based or flake-only diets are unsuitable for a long-term adult.
Mating
Belontia hasselti is a bubble-nest builder. The male takes responsibility for constructing a nest of mucus-bound bubbles at the water surface, typically anchored among floating vegetation or beneath a broad leaf, and subsequently guards the site with considerable tenacity. Courtship is energetic and can be rough: the male pursues the female persistently, and in the confines of an aquarium a female that is not ready to spawn may suffer serious injury or death.
A notable feature of the species is that spawning may sometimes occur without a clearly visible bubble nest — reported by multiple sources — though the male still clusters the eggs and guards them. The combination of extreme aggression and a male's strong territorial drive means that introducing a female successfully requires careful management: a large tank with visual breaks, and monitoring the female for signs of distress. Some aquarists introduce the pair only briefly and remove the female after spawning.
Breeding
Once courtship is complete and the female has deposited eggs, the male collects and clusters the yellow eggs at or near the surface, maintaining and guarding the spawn even when a conventional bubble nest is absent. Incubation is brief: at 75–79 °F the eggs hatch in approximately 48 hours, and the fry become free-swimming after a further two days, at which point the female should be removed if she has not been already.
The male continues to guard the fry for a period, but the brood should be moved to a dedicated rearing tank once the male's tolerance of the fry begins to wane. First foods for the tiny fry include infusoria, commercial liquid fry food, and rotifers, advancing to newly hatched brine shrimp as the fry grow. Water changes must be gentle to avoid disturbing surface-hugging fry. Belontia hasselti grows relatively slowly but steadily; juveniles begin to show aggressive tendencies toward one another early, and growing fish should be thinned out to prevent casualties. The species is long-lived — captive specimens have been kept for 10–15 years.
In the aquarium
Belontia hasselti is not a community fish and is not recommended for the casual aquarist. Its size (to 8 in), extreme aggression toward conspecifics, and predatory nature toward smaller fish limit it to a single-specimen setup or a single proven pair in a large, well-furnished aquarium. Even a bonded pair may fight, and any fish small enough to fit in its mouth is a potential prey item.
A tank of at least 59 in (around 80–105 US gal) suits a single adult. Dense planting, driftwood, and other décor that breaks sight lines help reduce aggression and allow a female somewhere to retreat. Water conditions can be broadly neutral to mildly acidic (pH 5–7.5 covers the comfortable range for captive specimens), and temperature in the low-to-mid 20s Celsius suits the species well. As a labyrinth fish it must have unobstructed access to the water surface to breathe atmospheric air; a humid gap between the waterline and the tank lid prevents chilling of the suprabranchial chamber. Filtration should be efficient but not produce strong surface turbulence. For aquarists who can meet its requirements, the Malay combtail is a striking, long-lived, and genuinely impressive species.
Conservation
Belontia hasselti is assessed as Least Concern (LC) on the IUCN Red List, with the most recent assessment published in 2019 (assessment date 4 December 2018, assessor B.W. Low). The species qualifies for Least Concern primarily on the basis of its wide extent of occurrence — an estimated 1,800,0 mi² — and its presence across multiple countries and island systems.
Despite the Least Concern status, localised declines are documented and the underlying habitat trend is negative. The species has been recorded as locally extinct in Singapore, where lowland peat swamp forests were largely cleared before the 1960s, and is possibly extinct on Java, where similar land-use changes have occurred. Across its remaining range in Sumatra, Borneo, and the Malay Peninsula, peat swamp forests continue to be converted to oil palm and timber plantations at scale — the primary threat identified in the IUCN assessment. The species is also traded as a food fish locally and regionally, and enters the aquarium trade internationally, though trade volume appears to have declined. The stability of remaining populations depends directly on the persistence of intact peat swamp forest, making the fate of B. hasselti inseparable from the broader conservation of these globally threatened ecosystems.