Taxonomy & naming
Betta albimarginata was described by Maurice Kottelat and Peter K. L. Ng in 1994, in a paper diagnosing five new fighting-fish species from Banka Island and Borneo published in the Raffles Bulletin of Zoology. The type locality is Sungei Sanul, a tributary of Sungei Tikung within the Sungei Sebuku basin of East Kalimantan, Indonesian Borneo. Eschmeyer's Catalog of Fishes (Kottelat & Ng, 1994) is the reference for the valid name; the authority is unparenthesised because the species has always been placed in Betta.
The species belongs to the albimarginata group, a cluster of closely related small mouthbrooding bettas from the island of Borneo. Group members share the mouthbrooding reproductive strategy, modest size, and preference for shallow, soft-water forest streams that distinguish them from the larger bubble-nesting species of the splendens group. Within the albimarginata group, B. albimarginata is most readily told from its relatives by coloration rather than by meristic counts, which overlap.
The specific epithet albimarginata comes from the Latin albus (white) and marginatus (bordered or edged), a reference to the white margins on the dorsal and caudal fins that are the fish's most immediately obvious field mark. It has circulated in the hobby as the 'strawberry betta' for the red-orange body tone, though 'whiteseam fighter' maps more directly onto the scientific name.
Morphology
Betta albimarginata is a tiny fish, with standard length around 1 in — males and females alike fall in the 1 in range — making it among the smallest members of a genus that already runs small relative to the domesticated Betta splendens. The body is slender and elongate rather than deep, as befits a fish adapted to pushing through root tangles and leaf packs in very shallow water.
Coloration is the species' centrepiece. The body is a warm red-orange to brick-red on the flanks, brighter in displaying or dominant males, and the gill covers carry the same warm tones. The dorsal, caudal, and anal fins are dark — grey to nearly black at the base — and are cleanly edged with white, the white margin being widest on the dorsal and giving rise to the common name. Ventral fins are also tipped white. Females tend to be duller in body colour and have less vivid fin markings, though the white fin edges are present in both sexes. Incubating males can appear somewhat paler during the brooding period, likely as a consequence of reduced feeding and the physical demands of carrying eggs and fry.
Habitat
In the wild, Betta albimarginata occupies shallow forest streams and drainage channels within the Sebuku River system of East Kalimantan, Borneo. Seriously Fish records water depth at the type locality as extremely shallow — as little as 2–4 in — with the fish sheltering among marginal plant roots, submerged terrestrial vegetation, and deep accumulations of leaf litter on the stream bed. Overhead canopy is dense; these are shaded, tannin-stained blackwater environments far from open sunlight.
The water chemistry is soft and distinctly acidic: recorded conditions include 80 °F and pH 5.25 (Seriously Fish), consistent with the humic-acid-rich, low-conductivity water characteristic of Bornean peat-swamp and heath-forest drainage. Hardness is low (approximately 54 ppm). Flow in the micro-habitats the fish uses is moderate rather than torrential; it is not a fast-water species.
As a labyrinth fish, B. albimarginata possesses a suprabranchial air-breathing organ that allows it to extract oxygen directly from air at the surface. This adaptation is well-suited to the warm, shallow, often oxygen-depleted water of forest-floor streams, and it is a defining feature of the family Osphronemidae to which all Betta belong.
Feeding
The natural diet of Betta albimarginata consists of small invertebrates taken from the leaf litter, marginal vegetation, and the water column: small insects and their larvae, zooplankton, and other microinvertebrates that colonise the stream bottom and root tangles it inhabits. Given the extremely shallow, food-rich micro-habitat the species occupies, it is likely an opportunistic ambush predator that picks targets from close range rather than pursuing them across open water.
In the aquarium the species accepts a varied diet readily. Live and frozen foods — micro-worms, Daphnia, baby brine shrimp, tubifex, and similar small invertebrate fare — are ideal and maintain condition. Dried foods including small granules and quality flake are accepted once the fish has settled, though many experienced keepers prefer to keep at least a proportion of live or frozen food in the diet to support breeding condition and the immune needs of brooding males.
Mating
Betta albimarginata is a paternal mouthbrooder. Courtship follows the anabantoid embrace pattern: the male wraps his body around the female in a nuptial embrace that lasts seconds, during which eggs and sperm are released simultaneously. The fertilised eggs are then collected — initially caught on the male's anal fin, briefly taken into the female's mouth, and then spat into the water for the male to pick up and take into his buccal cavity, where incubation begins.
The sequence of egg-handling between the pair is brief and cooperative; once the clutch is secure in the male's mouth the female's role in that breeding bout is complete. Males display to females with fin-spreading and body colour intensification, but courtship in this small mouthbrooder is generally less elaborate and less prolonged than in the larger bubble-nesting species of the genus. Males may spawn with more than one female during a breeding season, with each clutch incubated sequentially.
Breeding
Following the nuptial embrace, the male carries the fertilised eggs in his mouth throughout incubation, eating nothing for the duration. The incubation and early brooding period lasts roughly one to two weeks; at the end of it the male releases fully formed, free-swimming fry. Because the male fast throughout the brooding period, his condition should be built up with quality live and frozen foods before a breeding attempt.
Clutch sizes are modest in keeping with the species' small size — typically in the range of a few dozen eggs. The fry are relatively large at release compared with those of egg-scattering species, and they can take newly hatched brine shrimp as a first food immediately after being released. In the aquarium the brooding male should be kept undisturbed and in a well-covered tank to prevent him from spitting the clutch prematurely under stress. A species tank or a quiet, densely planted pair-tank is strongly preferred over a busy community setup for breeding attempts. Successful breeding in captivity has been achieved by specialist keepers and the species is maintained in hobby collections, though it is not as widely bred as some other small Betta.
In the aquarium
Betta albimarginata is best kept as a species in a small, dedicated aquarium — a tank of 10–15 US gal is adequate for a pair or a small group. The tank should be heavily planted or furnished with leaf litter and driftwood to provide the cover and structure this fish relies on, and the water surface must be accessible at all times for the labyrinth organ. A tight-fitting lid is essential both to maintain the warm, humid air layer above the surface that prevents chilling of the suprabranchial organ and to prevent escape, as bettas are capable jumpers.
Water chemistry should mirror the species' blackwater origins: soft, acidic water in the pH 5.0–6.5 range, low hardness, and temperatures in the 75–82 °F band. Peat filtration, Indian almond leaves, or driftwood leaching tannins will both lower pH and add the humic acids that seem to benefit condition and colour. Filtration should be gentle — a sponge filter is ideal — as strong currents are inappropriate for a fish adapted to near-still forest drainage.
Multiple males may be kept together in a sufficiently large and structured tank, though they will display and may skirmish; a male-female pair or a single male with multiple females is generally less combative. This species is not suited to a general community aquarium: it is small, slow, and will be outcompeted or stressed by active or boisterous tankmates. The limited wild range and Endangered status make responsible captive husbandry — including maintaining healthy captive populations and breeding — genuinely conservation-relevant for dedicated keepers.
Conservation
Betta albimarginata is assessed as Endangered (EN) on the IUCN Red List, with the assessment conducted in December 2018 under criterion B2ab(v). The listing is driven by the species' extremely limited extent of occurrence — it appears to be endemic to the Sebuku River drainage of East Kalimantan, a comparatively small catchment — combined with ongoing habitat loss due to land conversion in the region. Borneo's lowland forests and peat-swamp systems have faced large-scale clearance for oil palm and other agricultural uses, and the streams B. albimarginata inhabits are directly threatened by this transformation of the surrounding landscape.
A species confined to one river drainage has no buffer: if the Sebuku system is degraded across its extent, there is no nearby source population to recolonise. While B. albimarginata is kept by specialist hobbyists in several countries and captive populations exist, the wild population's trajectory gives genuine cause for concern. Hobbyists who keep and breed the species contribute to ex-situ insurance populations; sourcing fish from captive-bred lines rather than wild-caught stock is the most direct way an aquarist can support the species.