” Betta /ˈbɛtə/ is a large genus of small, often colorful, freshwater ray-finned fishes in the gourami family (Osphronemidae). By far the best known Betta species, however, is B. splendens, commonly known as the Siamese fighting fish. All the Betta species are small fishes, but they vary considerably in size, ranging from under 2.5 cm (1 in) total length in B. chanoides to 12.5 cm (5 in) in the Akar betta (B. akarensis).
Bettas are anabantoids, which means they can breathe atmospheric air using a unique organ called the labyrinth. This accounts for their ability to thrive in low-oxygen water conditions that would kill most other fish, such as rice paddies, slow-moving streams, drainage ditches, and large puddles.
The bettas exhibit two kinds spawning behavior: some build bubble nests, such as B. splendens, while others are mouthbrooders, such as B. picta. The mouthbrooding species are sometimes called “pseudo bettas”, and are sometimes speculated to have evolved from the nest-builders in an adaptation to their fast-moving stream habitats.
A phylogenetic study published in 2004 concluded tentatively that bubble-nesting was the ancestral condition in Betta, and that mouthbrooding has evolved on more than one occasion in the history of the genus. However it was unable to establish a correlation with any of three habitat variables studied: whether a species was found in lowland or highland streams, whether it was found in peat swamp forests, and whether it was found in water with fast or slow currents. Mouthbrooding species tend to exhibit less sexual dimorphism, perhaps because they do not need to defend a territory as the bubble-nesters do. ” https://en.wikipedia.org/wiki/Betta