Betta // 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 behaviour: 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. 
Siamese fighting fish (B. splendens) are frequently sold in the United States simply as "bettas". In fact, as of 2017[update], around 73 species are classified within the genus Betta. A useful distinction is that while the generic name Betta is italicized and capitalized, when used as a common name it is usually neither italicized nor capitalized. The common name of B. pugnax, for example, is thus Penang betta.
The name Betta (or betta) is pronounced //; the first part is the same as the English word bet. The name is often pronounced /ˈbeɪtə/ in American English, and may be spelled with one 't'. The name of the genus is derived from the Malay word ikan betah ("persistent fish").
Wild Betta fish are hardy and can eat almost anything in their environments, including worms, larvae of mosquitoes or other insects, and even smaller fish. Their natural environment is often resource-limited, so many Betta species have little choice of food.
While many Betta species are common and B. splendens is ubiquitous in the aquarium trade, other bettas are threatened. The IUCN Red List classifies several Betta species as Vulnerable. In addition, B. livida is Endangered, and B. miniopinna, B. persephone, and B. spilotogena are Critically Endangered.
The United Nations Environment Programme lists an unconfirmed species, Betta cf. tomi, as having become extinct in Singapore between 1970 and 1994. This likely refers to the extirpated Singaporean population of B. tomi, which continues to exist in the wild in Indonesia and Malaysia, as well as in captivity; the Red List classifies it as Vulnerable.
There are currently 73 recognized species in this genus. The currently described Betta species can be grouped into complexes for conservation purposes. (This grouping of species makes no claim at representing a phylogenetic reality.) The complexes of the associated species are:  
- B. akarensis complex:
- Betta akarensis Regan, 1910 (Akar betta)
- Betta antoni H. H. Tan & P. K. L. Ng, 2006
- Betta aurigans H. H. Tan & K. K. P. Lim, 2004
- Betta balunga Herre, 1940
- Betta chini P. K. L. Ng, 1993
- Betta ibanorum H. H. Tan & P. K. L. Ng, 2004
- Betta obscura H. H. Tan & P. K. L. Ng, 2005
- Betta pinguis H. H. Tan & Kottelat, 1998
- B. albimarginata complex:
- B. anabatoides complex:
- B. bellica complex:
- B. coccina complex:
- Betta brownorum K. E. Witte & J. Schmidt, 1992
- Betta burdigala Kottelat & P. K. L. Ng, 1994
- Betta coccina Vierke, 1979
- Betta hendra I. Schindler & Linke, 2013
- Betta livida P. K. L. Ng & Kottelat, 1992
- Betta miniopinna H. H. Tan & S. H. Tan, 1994
- Betta persephone Schaller, 1986
- Betta rutilans K. E. Witte & Kottelat, 1991
- Betta tussyae Schaller, 1985
- Betta uberis H. H. Tan & P. K. L. Ng, 2006
- B. dimidiata complex:
- B. edithae complex:
- B. foerschi complex:
- B. picta complex:
- B. pugnax complex:
- Betta apollon I. Schindler & J. Schmidt, 2006
- Betta breviobesus H. H. Tan & Kottelat, 1998
- Betta cracens H. H. Tan & P. K. L. Ng, 2005
- Betta enisae Kottelat, 1995
- Betta ferox I. Schindler & J. Schmidt, 2006
- Betta fusca Regan, 1910 (dusky betta)
- Betta kuehnei I. Schindler & J. Schmidt, 2008
- Betta lehi H. H. Tan & P. K. L. Ng, 2005
- Betta pallida I. Schindler & J. Schmidt, 2004
- Betta prima Kottelat, 1994
- Betta pugnax (Cantor, 1849) (Penang betta)
- Betta pulchra H. H. Tan & S. H. Tan, 1996
- Betta raja H. H. Tan & P. K. L. Ng, 2005
- Betta schalleri Kottelat & P. K. L. Ng, 1994
- Betta stigmosa H. H. Tan & P. K. L. Ng, 2005
- B. splendens complex:
- Betta imbellis Ladiges, 1975 (crescent betta)
- Betta mahachaiensis Kowasupat, Panijpan, Ruenwongsa & Sriwattanarothai, 2012
- Betta siamorientalis Kowasupat, Panijpan, Ruenwongsa & Jeenthong, 2012
- Betta smaragdina Ladiges, 1972 (Blue betta)
- Betta splendens Regan, 1910 (Siamese fighting fish)
- Betta stiktos H. H. Tan & P. K. L. Ng, 2005
- B. unimaculata complex:
- Betta compuncta H. H. Tan & P. K. L. Ng, 2006
- Betta gladiator H. H. Tan & P. K. L. Ng, 2005
- Betta ideii H. H. Tan & P. K. L. Ng, 2006
- Betta macrostoma Regan, 1910 (spotfin betta)
- Betta ocellata de Beaufort, 1933
- Betta pallifina H. H. Tan & P. K. L. Ng, 2005
- Betta patoti M. C. W. Weber & de Beaufort, 1922
- Betta unimaculata (Popta, 1905) (Howong betta)
- B. waseri complex:
- Betta chloropharynx Kottelat & P. K. L. Ng, 1994
- Betta hipposideros P. K. L. Ng & Kottelat, 1994
- Betta pardalotos H. H. Tan, 2009
- Betta pi H. H. Tan, 1998
- Betta renata H. H. Tan, 1998
- Betta spilotogena P. K. L. Ng & Kottelat, 1994
- Betta tomi P. K. L. Ng & Kottelat, 1994
- Betta waseri Krummenacher, 1986
- Froese, Rainer, and Daniel Pauly, eds. (2014). Species of Betta in FishBase. February 2014 version.
- Marcus Song, Caring for Betta Fish Lulu Press, 2006. ISBN 1-4116-9365-5
- Fernando, Yohan. "Betta edithae - a Pseudo Betta?". International Betta Congress Species Maintenance Program. Retrieved 2017-01-30.
- Rüber, Lukas; Britz, Ralf; Tan, Heok Hui; Ng, Peter K. L.; Zardoya, Rafael (April 2004). "Evolution of Mouthbrooding and Life-History Correlates in the Fighting Fish Genus Betta". Evolution. Society for the Study of Evolution. 53 (4): 799–813. JSTOR 3449277.
- "Betta". American Heritage Dictionary (4th ed.). Archived from the original on 2007-10-30. Retrieved 2006-06-29.
- "Common Names Summary - Betta picta". Fishbase.org. Retrieved 2012-07-31.
- "2006 IUCN Red List of Threatened Species.". Retrieved 2006-06-30.
- "Extinctions since 1970". United Nations Environment Programme World Conservation Monitoring Centre. 2006. Archived from the original on 2007-09-30. Retrieved 2006-07-01.
- Kottelat, M. (1996). "Betta tomi". 2006 IUCN Red List of Threatened Species. Retrieved 2006-07-01.
- "Betta tomi". International Betta Congress Species Maintenance Program. Archived from the original on 2013-04-14. Retrieved 2006-07-01.
- "Species Complex Management". International Betta Congress Species Maintenance Program. Retrieved 2006-07-01.
- "Betta". Integrated Taxonomic Information System. Retrieved 30 June 2006.
- Tan Heok Hui (2009). "Betta pardalotos, a new species of fighting fish (Teleostei: Osphronemidae) from Sumatra, Indonesia". The Raffles Bulletin of Zoology. 57 (2): 501–504.
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