Taxonomy & naming
Betta mahachaiensis was formally described in 2012 by Kowasupat, Panijpan, Ruenwongsa, and Sriwattanarothai in the journal Zootaxa. The holotype (THNHM-F-01630, 1.5 in SL) was collected on 30 April 2011 from the Tha Sai subdistrict of Muang District, Samut Sakhon Province, Thailand (approximately 13°34′N 100°15′E). The species is placed in the splendens group, the radiation of bubble-nesting fighting fishes that also includes B. splendens, B. imbellis, B. smaragdina, and B. siamorientalis.
The description was grounded in combined molecular and morphological analysis — COI, 16S rRNA, and ITS1 markers all support its status as a distinct lineage, with COI divergence of roughly 11% from B. splendens and 14% from B. smaragdina. Before that work, the fish had circulated in the Thai hobby under the informal tag 'Mahachai' and was at various times suspected to be a hybrid between B. splendens and other splendens-group taxa; the molecular evidence, together with the observation that a professional breeder maintained a true-breeding population for more than ten years without hybrid breakdown, put that question to rest. The valid name in Eschmeyer's Catalog of Fishes is Betta mahachaiensis Kowasupat, Panijpan, Ruenwongsa & Sriwattanarothai, 2012.
Morphology
Adults reach a maximum standard length of about 1.5 in (holotype 1.5 in SL; paratype range 1–1.5 in SL across 40 specimens); Seriously Fish records somewhat larger sizes up to around 2–2.5 in SL for aquarium-raised individuals. The body is robust and laterally compressed in the typical Betta fashion, with a rounded caudal fin and the extended pelvic-fin filaments characteristic of the splendens group.
The colour pattern sets this species apart from its closest relatives. The opercle carries two parallel iridescent bars — green to bluish-green — rather than the red or blue tones common in B. splendens. The opercular membrane itself is brown to black with no red component, a key distinction from B. splendens. Body ground colour is dark brown to black, overlaid with the same iridescent green to blue-green scale sheen. The dorsal, caudal, and anal fins are brown-black with iridescent interradial membranes; the dorsal fin shows black transverse bars. Pelvic fins are brown-black with an iridescent margin and a white tip. Females are visibly less colourful, retaining the dark ground but lacking the full iridescent intensity of breeding males.
Habitat
Betta mahachaiensis is endemic to a narrow coastal strip of Samut Sakhon Province and adjacent western Bangkok, Thailand — a total known range of 28 collection sites in Samut Sakhon (surveyed 2007–2012) and 2 sites in western Bangkok. Surveys of neighbouring coastal provinces (Samut Songkhram, Samut Prakan) found no trace of the species, underscoring the extreme endemism.
The defining habitat feature is the nipa-palm (Nypa fruticans) swamp — brackish, tidally influenced coastal channels where this feather-leafed palm forms dense stands. The water ranges from slightly brackish to moderately saline, with recorded salinity of 1.1–10.6 ppt and pH of 6.87–7.80, conditions that no other member of the splendens group tolerates as a permanent residence. Water temperature follows the Thai coastal climate, roughly 72–82 °F. Being a labyrinth fish, B. mahachaiensis breathes atmospheric air via its suprabranchial organ and thrives in the warm, oxygen-depleted, tidally slow water beneath the palm canopy — precisely the conditions most other fish avoid. Sympatric species recorded from these channels include Trichopsis vittata, Trichopodus trichopterus, Anabas testudineus, and Aplocheilus panchax.
Feeding
In the wild, B. mahachaiensis feeds on small live invertebrates and zooplankton available in the brackish swamp water column and at the surface — small crustaceans, insect larvae, and other microinvertebrates drifting through or resting on the tidal flats of the nipa channels.
In the aquarium the species accepts a range of live and frozen foods: Daphnia, Artemia (brine shrimp), bloodworm, and similar small invertebrate items are readily taken. Like most Betta it can be trained to accept high-quality pellets or frozen foods, but responds best to a diet centred on live or frozen invertebrates that reflects what it encounters in nature. A varied diet supports the iridescent colour development that makes this fish distinctive.
Mating
Betta mahachaiensis is a bubble-nester: the male constructs and maintains a floating raft of mucus-coated bubbles at the water surface, typically secured in the water-holding phytotelma — the cupped, water-retaining bracts — of nipa-palm branches. This nesting strategy is unique among Betta in using a palm structure as the primary nest site, a behavioural adaptation tightly coupled to its habitat. The bracts provide shelter and a stable microenvironment for the nest.
Courtship follows the pattern typical of the splendens group. The male displays intensely to the female, flaring the opercles and extending the fins to show the iridescent green bars to full effect. A receptive female signals with vertical banding on the body. The pair then carry out a spawning embrace beneath the bubble nest: the male wraps around the female in the characteristic osphronemid nuptial clasp, and eggs are released during each brief embrace. The female has been observed cradling some eggs briefly with her pelvic and pectoral fins before the pair place them into the nest.
Breeding
After spawning, the male takes sole responsibility for the nest, collecting any fallen eggs and returning them to the bubble raft, fanning the nest to maintain oxygen levels, and defending the immediate area from disturbance. The female plays no role in parental care and should be removed from a breeding tank once spawning is complete to prevent aggression from the male.
Fry emerge from the nest after the typical incubation period for splendens-group bettas at tropical temperatures. Very young fry require infusoria or similarly small first foods for the first few days; they graduate to microworm and freshly hatched Artemia nauplii as they grow. A key consideration for captive breeding is water chemistry: although many aquarists keep B. mahachaiensis in freshwater with success, salt content of at least a low brackish level (a small amount of marine salt, not aquarium tonic salt) better replicates the tidal-swamp conditions where the species evolved and may improve spawning success and fry survival. Captive-bred stock has been maintained as a true-breeding line by hobbyists for over a decade, confirming the species' reproductive stability.
In the aquarium
Betta mahachaiensis is kept by specialist wild-betta enthusiasts and is more demanding than the domesticated B. splendens in terms of water-chemistry requirements. A low-salinity brackish setup — achieved with a small amount of marine salt to bring conductivity up while keeping salinity well below 5 ppt — more closely mirrors the nipa-swamp habitat and appears to favour the fish's long-term health and colour. pH in the neutral to slightly alkaline range (7.0–7.8) suits the recorded field conditions. Temperature can range 72–82 °F.
As with all labyrinth fishes, unobstructed access to the water surface is essential; the fish breathes atmospheric air and will suffocate if denied it. Gentle filtration that avoids strong surface current is preferable — the nipa channels are tidally slow, not torrential. A biotope-style setup with floating or emergent vegetation, subdued lighting, and palm-leaf litter is appropriate for the species and shows its iridescent green colouration to advantage. Males are territorial toward one another in the manner of the splendens group and should generally be kept individually or in species tanks with sufficient space and visual breaks. Females are more tolerant of one another. Probable hybridisation with B. splendens, B. imbellis, and B. smaragdina has been documented in the captive-betta hobby; keeping the species isolated from close relatives maintains genetic integrity.
Conservation
Betta mahachaiensis has not been formally assessed on the IUCN Red List (status: Not Evaluated as of this writing). However, the original description identified it explicitly as a conservation priority: the species is known from roughly 30 collection sites concentrated in a single Thai province (Samut Sakhon) and the immediate western outskirts of Bangkok, with no confirmed records from neighbouring coastal provinces despite targeted surveys. That is an exceptionally restricted range for any vertebrate.
The nipa-palm swamps that constitute its entire known habitat are under severe pressure. Industrial development, housing expansion, and the landfilling of salt marshes on Bangkok's coastal fringe have destroyed and fragmented the nipa stands at a rapid pace. Pollution from industrial effluent adds a chemical stress layer to the habitat. There is no buffer population elsewhere; if the Samut Sakhon swamps are lost, so is the species. A growing community of specialist hobbyists maintains captive stock, and the species breeds reliably in captivity, which is the primary conservation safeguard currently in place. Formal IUCN assessment and designation of protected areas covering the remaining nipa-swamp patches would materially benefit the species.