Endemic to the Korean peninsula where it occurs throughout North Korea and South Korea.

Type locality is ‘Keumsan River, near Pusan, South Korea.’

A predominantly riverine fish preferring clear, well-oxygenated, running waters with substrates of sand, gravel, rock or mud, where adults tend to form schools just above the substrate in slower-moving sections.

This species can be distinguished from other members of the genus by the following combination of characters: absence of dark spots on body in adults; body elongate with slightly convex dorsal profile; head longer than body is deep; snout much longer than postorbital head length; lips well developed, lateral lobes of lower lip broad and thick with folds, median process small; barbel thickness shorter or equal to eye diameter; dorsal spine strong, ⅔ of HL; dorsal-fin origin closer to tip of snout than caudal-fin base base; 15+ gill rakers; 6½ branched anal-fin rays; posterior simple dorsal-fin rays ossified and spinous.

Widepread in eastern Asia between the Yangtze and Amur river basins, including China (mainland and islands of Taiwan and Hainan), Korea, Mongolia, Russia, and Japan. It probably been introduced to Vietnam and Laos, or records from these countries represent another species.

Bitterlings (see ‘Notes’) exhibit an unusual spawning symbiosis in which the interlamellar spaces of the paired inner and outer gills of living unionid mussels are used as a spawning substrate.

During the reproductive period females develop a long ovipositor which is used to deposit eggs through the mussel’s exhalant siphon. Males then move in to males release milt…

A. macropterus appears in the ornamental trade on an irregular basis and should only be considered by more experienced aquarists with sufficiently large facilities.

Given its wide geographical distribution there is a possibility that it represents a group of closely-related species, with genetic studies indicating the existence of several distinct clades.

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.