Taxonomy & naming
Betta ocellata was described by the Dutch zoologist Lieven Ferdinand de Beaufort in 1933, based on material from near Bettotan, Sandakan, in what was then British North Borneo (now the Malaysian state of Sabah). The holotype is held at the Natural History Museum, London (BMNH 1959.7.7.1, ex NMS F.1126). Eschmeyer's Catalog of Fishes records the valid name as Betta ocellata de Beaufort, 1933.
The species belongs to the unimaculata group (sometimes called the unimaculata complex), a clade of large, cave- or stream-dwelling Bornean and Sumatran bettas that share mouthbrooding reproduction and generally subdued coloration relative to the fighting-fish group. Other members of the complex include Betta unimaculata and Betta macrostoma. Within the group, B. ocellata is characterised by its relatively large adult size and the presence of a distinctive ocellated spot on the caudal peduncle — the feature that gave the species its name.
Morphology
Betta ocellata reaches approximately 3–3.5 in standard length, making it one of the larger members of the genus and one of the bigger wild bettas available to hobbyists. The body is elongate and laterally compressed, with a broad, rounded head characteristic of mouthbrooding bettas in the unimaculata group — males developing a noticeably wider and heavier head than females, an adaptation that accommodates the oral incubation of eggs and fry.
Coloration is relatively understated compared with the showier Betta splendens group. The body is typically olive to brownish, often with two lateral bands in females and a less defined pattern in males. The defining diagnostic feature is an ocellus — a dark, eye-like spot ringed with a lighter border — located on the underside of the body where the caudal fin meets the peduncle; the specific epithet 'ocellata' (Latin: bearing small eyes or spots) refers directly to this mark. Fins in males are extended relative to females, and breeding males may show enhanced coloration during courtship.
Habitat
In the wild, Betta ocellata occupies a variety of freshwater habitats across northeastern Borneo, including clear, flowing headwater streams in hill and lowland forest, isolated elevated pools, ravines, ox-bow lakes, and quiet ponds — occasionally in disturbed habitats such as pools in oil palm plantation areas. It is commonly found among submerged leaf litter, marginal vegetation, and the tangle of submerged roots along stream banks, microhabitats that provide cover and foraging opportunities.
Water chemistry across the species' range is relatively variable. Seriously Fish records pH 5.5–7.5 and temperatures of 70–77 °F (70–77 °F) for wild localities, with soft water conditions (hardness 0–179 ppm). This reflects the cool, shaded streams of Bornean upland forest, which tend to be softer and slightly acidic relative to lowland habitats. Like all Betta, B. ocellata is a labyrinth fish, possessing the suprabranchial labyrinth organ that allows it to breathe atmospheric air directly — an adaptation for the warm, sometimes oxygen-depleted still-water environments the genus as a whole favours.
Feeding
Betta ocellata is a carnivore in the wild, feeding principally on terrestrial insects that fall to the water surface and on aquatic invertebrates taken from the substrate and leaf litter. FishBase notes insects and small invertebrates as the primary dietary components, a diet consistent with that of other unimaculata-group bettas that hunt by ambush among submerged debris.
In captivity the species accepts live and frozen invertebrate foods readily — Daphnia, Artemia (brine shrimp), bloodworm, and similar items suit it well. Like most wild bettas, it may initially be reluctant to accept dry foods, and a diet anchored in live or frozen material is advisable both for conditioning and to maintain the species in good health for breeding. Insects, including appropriately sized crickets, fruit flies, and similar feeder insects, are well received and reflect the natural diet.
Mating
Betta ocellata is a paternal mouthbrooder. Courtship is initiated by the female in most observed spawnings; she approaches the male and the pair engage in a series of circling displays and body-contact behaviours before spawning commences. The embrace itself is the T-position typical of anabantoids: the male wraps around the female, and fertilisation occurs as eggs and milt are released simultaneously.
After each embrace, the fertilised eggs sink or are caught before reaching the substrate, and the male gathers them into his mouth. This sequence may repeat multiple times until the full clutch has been spawned and collected. Once incubation begins, the male ceases feeding entirely for the duration of the brooding period, relying on fat reserves to sustain himself through the fast.
Breeding
Following the spawning embrace, the male incubates the fertilised eggs orally for approximately 12–17 days (typically around 14 days at typical temperatures), after which the fry are released as free-swimming juveniles capable of accepting small live foods. The incubation period varies with water temperature — cooler conditions lengthen it. The fry are relatively large and well-developed at release compared with the tiny larvae of egg-scattering species, reflecting the investment made during oral incubation.
In the aquarium, spawning occurs naturally in a well-maintained pair tank without requiring significant intervention. Breeders typically house a pair in an aquarium of around 20 US gal or larger, furnished with caves, driftwood, and dense plant cover to allow both fish retreat space. The male can be left to carry the full term; some breeders remove the female after spawning to eliminate the risk of harassment. Fry accept microworm and Artemia nauplii from first feeding and grow steadily under good conditions.
In the aquarium
Betta ocellata is considered a rewarding species for the dedicated wild-betta keeper, though it is not suited to a standard community aquarium. Its size and predatory temperament make it incompatible with small fish, and males will fight if housed together without adequate space. Pairs or trios (one male, one or two females) can be maintained in appropriately sized, well-planted tanks; groups require larger aquaria of 40 US gal or more, with visual barriers and abundant cover to reduce aggression.
Water conditions should reflect the species' Bornean headwater origins: slightly soft to moderately hard water, pH in the range 5.5–7.5, and temperatures of 70–77 °F (70–77 °F). The cooler end of the temperature range is important to note — this is not a fish for the 82 °F tanks suited to fighting fish. The labyrinth organ means it must have unobstructed access to the water surface to breathe air; tanks should not be filled to the brim, and surface agitation should be gentle. Tall plants, floating vegetation, driftwood, and leaf litter replicate the forest stream aesthetic and help the fish settle and colour up. The IBC Bettas profile notes that the species is quite tolerant of water chemistry provided the water is clean and well-filtered.
Conservation
The IUCN Red List assessed Betta ocellata as Near Threatened (NT) in 2015 (assessment ID 97218336). The listing reflects the species' restricted distribution — northeastern Borneo, encompassing parts of Sabah and East Kalimantan — combined with ongoing pressures on Bornean freshwater habitats, principally forest clearance, agricultural expansion (including oil palm development), and water-quality degradation associated with land-use change.
The species' occurrence in oil palm plantation pools (noted by Seriously Fish) suggests some degree of habitat tolerance, but the broader pattern of deforestation and stream degradation across Borneo continues to reduce the quality and connectivity of suitable habitat. No species-specific conservation measures are documented; the species benefits indirectly from protected areas within its range in Sabah. Captive breeding by wild-betta enthusiasts maintains a hobbyist population and reduces the pressure from wild collection for the ornamental trade.