M. ocellatus can be distinguished from congeners by the following combination of characters: caudal-fin rounded; eye stripe not connecting spot on posterior projection of opercle with eye; dorsal-fin rays filamentous posterior to a vertical through the posterior anal-fin rays in adults; 16-19 spinous dorsal-fin rays; 6-13 rakers on ceratobranchial of first gill arch; posterior tip or margin of body scales not darker than scales.

M. opercularis is a classic aquarium fish, having been introduced to the hobby by a French soldier named Gerault in 1869. Of the initial 100 specimens shipped, 22 survived and were successfully bred later that same year by another Frenchman, the Parisian Pierre Carbonnier. This gives the species the distinction of being one of the very first ornamental fish imported to Europe. It is also known as ‘Chinese fighting fish’, ‘paradise gourami’, and ‘blue paradise fish’.

M. spechti is also sometimes referred to as M. concolor, particularly in older literature. A dispute regarding conservation of that name was resolved by the ICZN in 2006, however, since M. concolor Ahl, 1937 is a junior primary homonym of M. concolor Schreitmüller, 1936 and thus permanently invalid.

This species lends its name to the Betta foerschi group of closely-related species within the genus, of which members share the following set of characters: possession of iridescent blue or green unpaired fins; rays and interradial membranes in unpaired fins not contrasting; dorsal fin with thin, bright white distal band; opercle with two, reddish-yellow, vertical bars.

Members are regarded as being closely-related to bubble nesting Betta specie…

B. pi is included in the Betta waseri group of closely-related species within the genus, an assemblage within which members share the following combination of characters: base body colouration clay yellow; opercle scales iridescent gold in mature males of all species except B. tomi; throat with black markings which join with black lower jaw in some species; no chin bar.

This species is also referred to by the vernacular name 'Malay combtail'. In terms of external characters it differs most obviously from its only congener, B. signata by its overal brownish grey (vs. reddish) body colouration and presence of a mosaic-type patterning in the unpaired fins (vs. absence).

The position of relatedness of the genus Belontia with respect to other anabantoids remains somewhat…

B. persephone can be told apart from other members of the group by the following characters: no lateral blotch; body blackish; dorsal surface of head posterior to eye sloping gently; no parallel vertical iridescent gold opercle bars; pelvic-fin falcate with white filamentous tip, remainder black to reddish; median fins covered diffusely with light green iridescence; dorsal fin-rays 9; anal-fin rays 24-27; subdorsal scales 6-7; lateral scales 27-29; predorsal scales 17-18.

This species is included in the Betta splendens complex of closely-related species within the genus, of which members share the following set of characters: head length short 22-31 % SL; often a brightly-coloured body; iris of the eye with iridescent green or blue patches; body elongate or slender; opercles parallel when head viewed dorsally; caudal-fin rays red or brown and contrasting with the iridescent interradial membranes; unpaired fins without an iridescent margin; opercle with red or blue vertical bars in males; dorsal-fin rays 0-II, 7-9; anal-fin rays II-V, 21-26.

This species’ common name is derived from the tradition of keeping the fish in order to fight against each other in organised ’bouts’ upon which participants and onlookers place wagers. It has been extensively line-bred for vigour, strength, and the ornamental trade, and hybridised with the congeners B. imbellis, B. smaragdina, and B. mahachaiensis.

This species is also traded/referred to by the vernacular name 'Ceylon fighting fish'. It differs from its only congener, B. hasselti, by its overall reddish (vs. brownish-grey) body colouration, and absence (vs. presence) of a mosaic-like patterning in the unpaired fins.

At least three putative phenotypes have been recorded. The nominative, and commonest, mostly inhabits clear streams in rainier zones…