Fish
Siamese Fighting Fish
Betta splendens · also called betta, betta fish
The Siamese fighting fish is one of the most familiar aquarium animals in the world — the jewel-coloured fish with trailing fins sold in a cup. Behind that image is a genuinely unusual animal: an air-breather from Southeast Asian floodplains, a devoted nest-building father, and, in its wild form, a threatened species.
Overview
Wild Betta splendens comes from the central floodplains of Thailand — rice paddies, marshes, and sluggish canals in the Chao Phraya and Mae Klong basins. This is warm, shallow, often stagnant water that dwindles in the dry season, and the betta is built for exactly those conditions. The wild fish is a modest brown-green with short fins and only flashes of colour; everything vivid and flowing about the pet betta is the product of human breeding.
The labyrinth organ
Bettas are labyrinth fishes. Above the gills they carry a folded, blood-rich labyrinth organ that works like a simple lung, letting the fish gulp air from the surface. This is what allows it to live where dissolved oxygen is almost gone — and it means a betta must be able to reach the surface to survive. The common image of a betta thriving in a tiny sealed vase is a misconception; the fish needs space, warmth, clean water, and air.
Bubble nests and fatherhood
Breeding male bettas build a floating nest of bubbles coated in saliva. After an embrace-and-release spawning beneath the nest, the male catches the sinking eggs in his mouth and spits them up into the bubbles, then guards the nest alone, retrieving any egg or hatchling that drops until the fry can swim freely.
The fighting
Male bettas are intensely territorial. Confronted with a rival — or their own reflection — they flare: gill covers spread wide, fins fully extended, colour at maximum. In open water this display usually decides the contest without injury. Confined together, they fight, and the domestic strains fight hard, because bettas were first selectively bred over a thousand years ago in Thailand precisely to supply staged fighting contests, which are still a legal, regulated form of gambling there. Only later did breeders turn to producing the enormous fins and colours of the ornamental fish.
Status
As a pet, the betta is superabundant. As a wild species, Betta splendens is Vulnerable and declining — its floodplain habitat is being drained and polluted, and escaped ornamental fish interbreed with wild stock and erode its genetic distinctness. Several wild relatives in the same group are already Endangered or Critically Endangered.
Siamese Fighting Fish: common questions
Why can't two male bettas be kept together?
Male Betta splendens are territorial to the point that two in one tank will fight, often until one is badly injured or dies. In the wild the flaring displays — spread fins, extended gill membranes, intense colour — usually settle a dispute without much contact, because the loser can swim away. In a tank there is nowhere to retreat, so the display escalates to biting and fin-shredding. Centuries of breeding for combativeness, originally to supply the Thai betting scene, made the domestic strains especially aggressive. Females are less territorial but can also fight.
Do bettas really breathe air?
Yes. Bettas belong to the labyrinth fishes, which have a maze-like folded organ above the gills lined with blood vessels, allowing them to absorb oxygen directly from air gulped at the surface. This evolved for life in warm, still, oxygen-poor water — rice paddies, ditches, swamps — where gills alone are not enough. A betta actually needs access to the surface and can drown if denied it, and it can survive brief periods in very little water, though the popular idea that it lives happily in a puddle or a tiny vase is a myth that leads to sick fish.
How does a betta breed?
The male builds a floating raft of saliva-coated bubbles, often anchored to a leaf. He then courts a female with flaring displays; if she is receptive he wraps his body around hers beneath the nest in a series of embraces, and she releases a few eggs each time, which he fertilises and catches in his mouth. He spits each egg into the bubble nest, guards it fiercely, and retrieves any eggs or newly hatched fry that sink. After the fry are free-swimming, in a few days, the male's job is done and he will no longer tolerate them.
Is the Siamese fighting fish endangered?
The pet betta is one of the most abundant aquarium fish on Earth, but the wild species it comes from is a different story. Wild Betta splendens is assessed as Vulnerable and declining — its floodplain habitat in central Thailand is being drained, built over, and polluted, and released or escaped ornamental bettas interbreed with wild populations and dilute their distinct genetics. Several of its close wild relatives in the betta group are Endangered or Critically Endangered for the same reasons.