efe-epaKrathum Baen, Thailand

At a rural house southwest of the capital, more than 20 men gather around two rows of square cylinder glass jars, small-value currency notes clutched in their hands ready to be placed on bets.

"I won't lose. I never do," says Wit, his gaze fixed on the aquariums inside which small Siamese fighting fish (Betta splendens) are engaged in a fierce clash, at the house in Krathum Baen, Samut Sakhon province.

"Participants bring their own fish. I have brought six today," the Thai man, who has been a regular at these fish fights for 30 years, tells EFE.

The small fish, also known as betta fish, or "pla kad" in Thai, are only up to 6.5 cm long and weigh around 270 grams, but are ferociously territorial.

Lung Chat, the referee, weighs each contestant fish to ensure they are roughly the same weight and places them inside the narrow aquariums.

The betta fish then fly at and bite each other until one concedes and retreats into a corner.

The cornered fish is then taken out and placed into another container to see if it will expand its fins once more - a sign it is ready to resume the clash.

The owner of this aquatic boxer can either accept defeat or choose to carry on - however if the fish dies, he is penalized some 200 to 400 baht ($5.80 - $11.70).

However, Lung - who says these matches can last two to three hours - claims it is unusual for the fish to die in fights.

Bets seldom exceed 500 baht, but in some cases they can go up to 3,000 baht.

Although betting on fish fights is legal in most Thai provinces - with the exception of Bangkok - the Samut Sakhon gamblers admit they sometimes have to grease the palms of local police officers.

Beer, 33, who has been breeding betta fish for 12 years, tells EFE that the fish fighting tradition involving small wagers is a very ancient one.

One website, bettafishcenter.com, says the ritual goes back to before the 19th century.

"We raise our own fish or buy them from the farm, and then train them to become stronger," Beer explains.

The Thai gave up his work as a computer engineer to raise betta fish using the techniques he had learned in southern Thailand, where these creatures are tremendously popular.

A Siamese fighting fish costs around 200 baht, although some special specimens can fetch thousands.

The fish used in fights are usually black, lightly speckled with blue, while others used in decorative aquariums are a wider range of colors, including red, orange, yellow, white and turquoise.

In November 2016, a betta fish in the colors of the Thai flag - blue, red and white - sold for a record 53,500 baht at an online auction.

Gaspar Ruiz-Canela