Fin-flaring, as seen here, is a behaviour typical of aggressive encounters between male fighting fish. © Chor Kiat Yeo

Scientists have shown that male Siamese fighting fish, Betta splendens, increase the frequency at which they breathe atmospheric air during aggressive interactions.

Betta species are members of the suborder Anabantoidei and possess an accessory breathing organ known as the labyrinth. So-called due to its maze-like structure this organ allows the fish to breathe atmospheric air to a certain extent.

Under normal circumstances most of the oxygen required by the fish is taken from the surrounding water via the gills.

However, during aggressive territorial encounters between rival males the combatants engage in fin and opercular flaring behaviour, the intensity of which leads to a significant increase in oxygen uptake, and the new research demonstrates that this is obtained almost entirely from the air with no significant increase in aquatic oxygen uptake.

“It seems their smaller gills, a result of living in low oxygenated water, cannot keep up with the vigour of the fight, and more air breathing is required,” Dr Steven Portugal from the Royal Veterinary College told the BBC.

Opercular flaring, as seen in this male, is thought to limit aquatic oxygen uptake in B. splendens. © Farid Laid

Opercular flaring also inhibits pumping of water over the gills due to the gill plates being held in a rigid position, and the fish thus obtain most of the additional oxygen by increasing the frequency at which they rise to the surface to gulp air.

“during fights, they simply cannot get enough oxygen from the water, they must increase their visits to the surface to breathe more oxygen from the air,” said Dr Portugal, “In a sense, they have to take a breather.”

Such events not only interrupt display behaviour and increase the probability of losing an encounter, however, but also increase the risk of predation, and male fighting fish have solved these problems by engaging in simultaneous air-breathing, with one leading the other to the surface and both taking a breath of air at the same time.

“Synchronous air-breathing in fish is typically an anti-predator defence mechanism,” said Dr Portugal.

“Coming to the surface alone makes you vulnerable to predation, and synchronous air breathing can function to reduce an individual’s susceptibility to attack.”

The study, conducted by researchers from the University of Queensland, Australia and Royal Veterinary College, London,  will be published in an upcoming volume of the journal ‘Comparative Biochemistry and Physiology, Part A’