Taxonomy & naming
Betta pi was described by Heok Hee Tan in 1998 in the journal Ichthyological Exploration of Freshwaters (volume 8, number 3), alongside a second new species in the same paper. The type locality is a blackwater stream in Narathiwat Province, southern Thailand — the Mae Nam Tod Deng, approximately 4 mi north of the town of Sungai Kolok near the Malaysian border. Eschmeyer's Catalog of Fishes (Catalog of Fishes, CAS) records the valid name as Betta pi Tan, 1998; because the species was described directly in Betta, the authority carries no parentheses.
The species is placed in the waseri group, a cluster of large-bodied, mouthbrooding bettas distributed across the Thai-Malay Peninsula and Borneo. The waseri group was defined on the basis of shared reproductive biology and morphological characters, and includes several other peat-swamp and blackwater specialists. GBIF recognises species key 2394008 for Betta pi, consistent with the Tan 1998 original description.
The specific epithet pi is the sixteenth letter of the Greek alphabet. Tan did not elaborate on the choice in the original description, but the convention in the waseri group of using Greek letters for species names is well-established — the group already included Betta waseri, and subsequent members have followed alphabetical or symbolic naming patterns.
Morphology
Betta pi is a large betta, with maximum standard length reported at 3–3.5 in (roughly 3–3.5 in). This places it at the upper end of the genus in terms of body size, consistent with the generally larger build of waseri-group species compared with the smaller bubble-nesting groups.
The body is elongate and laterally compressed, typical of the genus, with large fins and a pointed snout. Colouration in the waseri group tends toward iridescent greens and blues on the flanks, with reddish or brownish fins — precise colour notes for Betta pi are limited in accessible sources, but the species shares the group's characteristic pattern of metallic iridescence. Males are typically more intensely coloured than females and possess somewhat longer, more elaborate fins, which are used in the lateral displays that precede spawning. Females are plainer and usually noticeably smaller.
Habitat
Betta pi inhabits blackwater streams and peat swamps in southern Thailand and northern Peninsular Malaysia — the narrow strip of lowland forest that runs down either side of the Thai-Malaysian border. These are among the most chemically distinctive freshwater habitats in Southeast Asia: the water is stained dark brown by dissolved humic and fulvic acids leaching from deep peat layers, is extremely soft and acidic, and carries very low dissolved-oxygen concentrations relative to clearer waters. FishBase records the species as benthopelagic and tropical, with a pH around 6.0, though Seriously Fish notes the natural range extends as low as pH 4.0 in the most characteristically blackwater sites.
Temperatures in lowland peat-swamp systems in this region typically range from the low to mid 20s up to about 82 °F, with the coolest readings at greater depth or during the rainy season. As a labyrinth fish, Betta pi possesses a suprabranchial accessory breathing organ — the labyrinth — that allows it to extract oxygen directly from the air at the water surface. This adaptation is essential in the oxygen-depleted waters of peat swamps, where dissolved oxygen can be vanishingly low, and means the fish must have unobstructed access to the surface layer to breathe. The species is understood to prefer densely vegetated, slow-moving or still water with deep leaf litter and woody debris on the substrate.
Feeding
Like all wild bettas, Betta pi is a carnivore, feeding primarily on small aquatic and terrestrial invertebrates. In peat-swamp habitats this means insect larvae, small crustaceans, worms, and insects — including those that fall onto the water surface from overhanging vegetation. The fish is an opportunistic visual predator, taking prey items commensurate with its relatively large mouth.
In the aquarium, Betta pi and its waseri-group relatives accept live and frozen foods readily — bloodworm, Daphnia, brine shrimp, small earthworm pieces, and similar invertebrate fare. High-quality dried foods may be taken once the fish is settled, but live and frozen items promote best condition and colouration and are especially important for bringing a pair into breeding condition. As with most bettas, overfeeding is a risk given the species' metabolic pace in cool, acidic water; modest, varied feedings are preferable to large infrequent ones.
Mating
Betta pi is a paternal mouthbrooder. Mating begins with a protracted courtship display in which the male flares his gill covers and fans his fins toward the female, presenting his full lateral profile. Females that are receptive display a characteristic head-down posture and a darkening of the body, signalling readiness to spawn. Unreceptive females may be harassed by persistent males, and in the confines of an aquarium it is important to provide sufficient cover for a female to retreat.
Spawning takes the form of a nuptial embrace — the male wraps his body around the female in the manner of all bettas — during which eggs and sperm are released simultaneously. The male then collects the fertilised eggs from the water, taking them into his buccal cavity before the female can do so. The pair may embrace multiple times, with the male gathering eggs after each embrace. Once egg collection is complete, the male retires to a sheltered spot and the female's role in reproduction is finished.
Breeding
After collecting the eggs the male holds them in his mouth continuously for the full incubation period — typically in the range of one to two and a half weeks depending on water temperature — eating nothing throughout. He broods the eggs as they develop through embryogenesis, and continues to hold the free-swimming fry for a further period after hatching until they are competent to forage independently. At that point, or shortly before, the male releases the fry, and parental involvement ends.
In an aquarium setting, pairs are typically conditioned separately on live and frozen foods before introduction, and the breeding tank should be warm, very soft, and acidic — mimicking blackwater conditions closely, with pH in the 4.5–6.5 range, near-zero hardness, and a temperature around 79–82 °F. Tannin-stained water from Indian almond leaves or peat filtration is strongly recommended. The tank should be heavily planted or furnished with cave structures and leaf litter to give the brooding male shelter. Because the male cannot eat during brooding, he should not be disturbed. Fry, once released, are initially large enough to take live baby brine shrimp or micro worms, and grow slowly under blackwater conditions.
In the aquarium
Betta pi is an uncommon species in the hobby — not rare in the absolute sense, but infrequently encountered and held mainly by specialist keepers of wild bettas. Its care requirements reflect its origins: it demands genuinely soft, acidic blackwater, is sensitive to elevated pH or hardness, and does poorly in the neutral, moderately hard water that suits many other tropical fish. A dedicated species tank with reverse-osmosis or rain-water preparation, heavy tannin supplementation, and minimal disturbance is the right approach.
The tank should have a tight-fitting lid — bettas are accomplished jumpers, and the labyrinth organ requires access to warm, humid air at the surface; a gap between the water surface and a cold glass lid can chill and damage the organ. Filtration should be gentle, as the species is accustomed to near-still water, and lighting subdued to replicate the low-light conditions of a peat swamp. A pair or trio (one male, one or two females) can be kept together outside of breeding, though the male's persistence during courtship means the females need sufficient space and structure to avoid harassment. Given its IUCN Endangered status, sourcing captive-bred specimens from reputable breeders in the wild-betta specialist community is strongly preferred over wild-caught fish.
Conservation
Betta pi is assessed as Endangered (EN) on the IUCN Red List. The assessment reflects both the species' inherently narrow range — confined to a short stretch of the Thai-Malaysian border region — and the extensive loss and degradation of lowland peat-swamp habitat across the Malay Peninsula over recent decades. Peat-swamp forest in this region has been cleared at high rates for oil palm and timber plantations, and the hydrology of remaining fragments is often disrupted by drainage canals. The acidic blackwater habitat that Betta pi depends on is particularly sensitive to these changes: even partial drainage raises the pH and oxygen levels, altering the chemistry that defines the species' ecological niche.
The species' large body size and mouthbrooding reproductive mode mean it produces smaller broods and develops more slowly than bubble-nesting bettas, making population recovery slower following disturbance. Collection for the ornamental-fish trade adds additional pressure, though captive breeding by specialist hobbyists does maintain populations outside the wild. Habitat protection and the maintenance of intact peat-swamp hydrology in southern Thailand and northern Peninsular Malaysia are the most consequential conservation actions for this species.