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Betta imbellis

Betta imbellis
Peaceful Betta
Also commonly known as the Crescent Betta, Betta imbellis is included in the Betta splendens complex of closely-related species, an assemblage which also currently includes B. smaragdina , B. stiktos, B. splendens and B. sp. "Mahachai". Betta imbellis has the widest natural range of these, suggesting that it may well be the origin species for the group. Indeed it is often confused with the other members, most notably B. splendens . This situation is not helped by the release of captive-bred fish to some of its localities as described above. In 2005 Tan and Ng published a paper entitled "The fighting fishes (Teleostei: Osphronemidae: Genus Betta) of Singapore, Malaysia and Brunei". In the paper they listed the defining morphological chracteristics of Betta imbellis as: 

"A bright red crescent on distal edge of caudal fin (vs. faint crescent or absence in B. splendens and B. smaragdina ); no caudal transverse bars (vs. presence in B. smaragdina and B. stiktos); dorsal transverse bars on distal two-thirds of dorsal fin (vs. whole dorsal fin in B. smaragdina ); iridescent blue opercle scales (vs. parallel red vertical opercle bars in B. splendens and green iridescent opercle scales in B. smaragdina ; dark brown to black body (vs. light brown body in B. smaragdina and B. stiktos); less lateral scales than B. stiktos (30-32, vs. 33-34); more slender body than B. splendens (body depth 24.2-30.1% SL, vs. 27.1-32.2); more backwards position of dorsal fin than in B. splendens (dorsal-fin origin above lateral scales 15-17, mode 16, vs. 14-16, mode 14); relatively smaller length of dorsal-fin base than B. splendens (12.9-18.9% SL, vs. 16.7-23.8); smaller height at pectoral-fin origin than B. splendens (14.8-18.9% SL, vs. 18.5-20.7)." 

Imbellis have green or blue gill plates with no wild spots on the tail rays.