For a study on interrelationships of bitterlings, the subfamily Acheilognathinae, the lateral line system and infraorbital bones of 27 species/subspecies in all 3 genera (Tanakia, Acheilognathus, and Rhodeus) including all Japanese species/subspecies, 12 Chinese species/subspecies, 1 Korean species, and the European bitterling, Rhodeus sericeus amarus, were examined. The bitterling cephalic sensory canal system was characterized by discontinuity between the supraorbital and infraorbital canals, discontinuity between the preopercular and temporal canals, and discontinuity between the right and left supratemporal canals. On the basis of completeness or incompleteness of these cephalic canals, three types could be defined. However, four types could be defined based on an examination of completeness in the trunk canal system. Based on the number of infraorbital bones (4 to 7), bitterlings could be divided into 4 groups. Character state analyses of the cephalic sensory canal system, infraorbital bones, and trunk canal in comparison with a molecular tree based on a mitochondrial gene revealed that a complete infraorbital canal along with the presence of both temporal and supratemporal canals, five infraorbital bones, and a complete trunk canal represent the plesiomorphic states. These suggest that Tanakia is the basal group, and both Acheilognathus and Rhodeus evolved from Tanakia. Evolution of Rhodeus from Tanakia by progenesis was hypothesized from the fact that within the Acheilognathinae, Rhodeus had the most reduced state of the lateral line system and infraorbital bones, as well as a short life span, such as with Rhodeus ocellatus which has two generations within a year. Also, as the lateral line system in all sexually mature bitterlings remained structurally similar to that in the juvenile stage of most other cyprinids, it was hypothesized that the Acheilognathinae evolved from an ancestor of this subfamily by peadomorphosis. Further, by developing an overview of the lateral line system in cyprinids, some problems on the sister group relationship between the Acheilognathinae and Gobioninae were addressed.
Key Words: Cephalic sensory canals — trunk canal — infraorbital bones — character phylogeny — heterochrony — Acheilognathinae — Cyprinidae