Taxonomy & naming
Betta smaragdina was formally described by Werner Ladiges in 1972, with the type locality given as Nong Khai, on the Mekong River in north-eastern Thailand. The valid name in Eschmeyer's Catalog of Fishes is Betta smaragdina Ladiges, 1972; because it was described directly in the genus Betta, where it remains, no parenthetical authority is used.
The species belongs to the splendens group, a complex of closely related bubble-nesting bettas from mainland South-East Asia that includes the familiar Siamese fighting fish Betta splendens and several other species from Thailand and the Malay Peninsula. The boundaries within this group have long been debated, and smaragdina is known to hybridise readily with both B. splendens and B. mahachaiensis where populations come into contact — including through escape or release of fighting stock — which complicates both taxonomy and conservation genetics. Locality variants showing red-dominant or green-dominant body colour, and longer-finned mutations, are maintained in the hobby and are traded under the species name.
Morphology
Males reach around 2–2.5 in standard length, with FishBase recording a maximum total length of 3 in. The body form is typical of the splendens group: an elongated, slightly compressed fish with rounded fins and, in wild-type males, the characteristic osphronemid body plan.
Male colouration is striking. The base colour is reddish to dark brown, and each scale carries a large iridescent blue-green spot that glitters intensely in direct light, producing the emerald mosaic that gives the species its common name. The anal and caudal fins are blue with red fin rays. Females are considerably plainer — a brown fish with two darker longitudinal stripes along the flanks — though they share the hint of iridescence in good light. During spawning condition the female's flanks become paler. Meristic counts include four to five anal spines and 22–26 soft anal rays; head length is 4.0–4.5 times in standard length.
Habitat
Betta smaragdina is most strongly associated with the Khorat Plateau of north-eastern Thailand, recorded from the Mun, Chi, Songkhram, and Loei rivers — all tributaries of the Mekong — as well as from Mekong tributaries in central Laos. The IUCN documents its range across the Thai provinces of Nong Khai, Udon Thani, Khon Khaen, and Ubon Ratchathani, with probable occurrence in lowland Laos, northern Cambodia, and possibly southern Vietnam. Range boundaries remain poorly quantified.
Within that footprint the species occupies still and sluggish water bodies: rice paddies, roadside and irrigation ditches, seasonal floodplain swamps, shallow ponds, and slow-moving streams — often in shaded situations with dense emergent or floating vegetation. Dissolved-oxygen levels can be very low in these habitats, particularly during the hot season, but this poses no problem for a labyrinth fish that breathes atmospheric air directly through its suprabranchial organ. Recorded water parameters span a fairly wide range — pH 5.5–7.5 (Seriously Fish) to 6.0–8.0 (FishBase), temperature 72–82 °F (72–82 °F), and hardness from soft to moderately hard — reflecting the diversity of lowland agricultural and wetland water bodies across its range.
Feeding
In the wild, Betta smaragdina feeds primarily on aquatic and terrestrial invertebrates: insect larvae, small crustaceans, and surface-fallen insects form the bulk of the diet. As a surface-oriented predator it is well placed to exploit the invertebrate rain that falls or drifts onto still-water surfaces in tropical lowlands.
In captivity the species accepts live and frozen invertebrate foods — Daphnia, Artemia, bloodworm, and similar items — as well as good-quality prepared foods. Live and frozen foods are strongly preferred for conditioning fish for breeding and are the practical standard for the species in the hobby.
Mating
Betta smaragdina is a bubble-nester. The male constructs a raft of mucus-bound air bubbles at the water surface, typically beneath a floating leaf, an overhanging plant, or any horizontal structure that provides shelter from above. Nest size is highly variable — from a few centimetres across to covering half the surface of a small tank — and some males build little or nothing before spawning.
Courtship follows the typical osphronemid pattern. The male intensifies his iridescent display and circles the female. Once the female is receptive and the nest is ready, the pair spawn in a nuptial embrace beneath the nest: the male wraps his body around the female, she releases eggs, and the male fertilises and immediately gathers them to carry them up to the bubble raft. The female's role ends at spawning; males typically require the female to be separated or to have hiding places available once spawning is complete, though reports suggest they are often less aggressive post-spawn than B. splendens.
Breeding
Under aquarium conditions Betta smaragdina spawns at temperatures of roughly 79–81 °F (78–80 °F). Conditioning on live or frozen invertebrate foods brings both sexes into breeding condition relatively quickly. The pair need not always be introduced in the classic pre-spawn separation routine: some breeders report that the male will not tolerate the female until the nest is complete, but that extended separation is not obligatory, and the pair can often be introduced directly to a prepared breeding vessel.
After the embrace-spawn sequence the male takes sole responsibility for the nest, retrieving any eggs or newly hatched larvae that fall and replacing them in the bubble raft. Eggs hatch within 24–48 hours at breeding temperature; the larvae hang in or below the bubble nest and become free-swimming approximately three to four days after hatching (some accounts say four to five days). At that point the male should be removed, as his parental role is complete. Fry are fed newly hatched Artemia nauplii as a first food, growing at a rate similar to B. splendens. The male does not typically predate the fry while tending them.
In the aquarium
Betta smaragdina is best kept in a calm, well-planted aquarium with a tight-fitting lid — access to warm humid air above the water surface is essential for a labyrinth fish, and a cold draught across the surface can cause respiratory illness. Gentle filtration that avoids strong surface agitation suits the species' preference for still or sluggish water and protects the bubble nest during breeding attempts.
Males are territorial with one another in the manner of the splendens group and should generally be kept one per tank unless the aquarium is large enough to break lines of sight with dense planting. A single male with one or more females, or a species-only pairing, is the standard approach. The species is less aggressive overall than heavily line-bred fighting stock, and wild-caught individuals in particular can be shy and need refuge cover to settle. Water chemistry should approximate the soft to moderately hard, slightly acidic to neutral conditions of its lowland wetland habitats, though the fish tolerates a wider range in practice. Temperatures in the range 75–81 °F (75–81 °F) are appropriate for long-term maintenance.
Conservation
Betta smaragdina is listed as Data Deficient (DD) on the IUCN Red List, assessed by Vidthayanon in a 2012 publication based on a 2011 evaluation (version 3.1). The Data Deficient listing reflects genuine uncertainty about population size and trend rather than an absence of concern: the IUCN assessment notes a decreasing population trend, a generation length of approximately two years, and threats from wetland degradation driven by agricultural intensification and infrastructure development across the Khorat Plateau.
A more insidious pressure is genetic: Betta smaragdina has been extensively bred for fighting in Thailand for generations, and line-bred or feral hybrids with B. splendens and B. mahachaiensis occur. Escaped or released fighting stock can hybridise with wild populations, eroding the genetic integrity of natural smaragdina populations. The IUCN assessment recommends habitat management and awareness-raising around the risks of captive releases. Locally the species can still be found in reasonable numbers where suitable wetlands persist, but it is becoming scarce in areas affected by pollution and drainage. Aquarists keeping the species should source stock from reputable breeders who maintain verified, uncontaminated lineages.