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Betta macrostoma

Betta macrostoma
Brunei Beauty Betta
Betta macrostoma has been scarce in the hobby since it was first introduced to aquarists in the early 1980s although recently is has started to become more established. It still commands a high price when available and we recommend purchasing some only if you wish to breed the species given its conservation status. This fish is included in the Betta unimaculata complex of closely-related species, an assemblage which currently includes B. patoti, B. ocellata , B. gladiator, B. ideii , Betta macrostoma, B. pallifina , B. compuncta, and of course B. unimaculata . In their 2005 paper covering the Betta species of Singapore Malaysia and Brunei Tan and Ng described the main morphological factors distinguishing Betta macrostoma from the rest of the group thus: 

"dorsal fin ocellus; body red in color; caudal fin with transverse black and red bands; opercle with large red blotch in mature male; vertebrae 31-32 (mode 32); dorsal-fin rays 9-11 (mode 10); anal-fin rays 25-28 (mode 27); pectoral-fin rays 13-14; lateral scales 32-33 (mode 33); subdorsal scales 6-8 (mode 7); predorsal scales 21-24 (mode 23); predorsal length 62.3-66.7% SL; head length 30.4-33.3% SL; anal-fin base length 43.2-49.2% SL; dorsal-fin base length 12.2-15.7% SL; lower jaw length 38.0-54.0% HL".