For a systematic study of the subfamily Acheilognathinae, the cephalic sensory canals, trunk canal, and infraorbital bones of adult bitterlings were examined.
As this type of systematic study warranted broad ranging, we examined 27 species/subspecies of the Acheilognathinae, including representative species of all genera of the Acheilognathinae and specimens from almost of its natural distributional areas: Europe, China, Korea, and Japan.
The cephalic sensory canals were comprised of the supraorbital, infraorbital, temporal, mandibular, preopercular, and supratemporal canals. It was found that the supraorbital canal was separated from both the infraorbital and temporal canals, the preopercular canal from the temporal canal, and the right from the left supratemporal canal. Among cyprinid subfamilies, the Acheilognathinae was the only one, in which all species had the right and left supratemporal canals that did not meet. Therefore, this character was considered as an autapomorphy of the Acheilognathinae. Bitterling cephalic sensory canals were grouped into three types (I-III) by completeness or incompleteness of the infraorbital canal and the presence or absence of both temporal and supratemporal canals. Type I was defined by the complete infraorbital canal and presence of both temporal and supratemporal canals, and was found in all species/subspecies of Tanakia, Acheilognathus, and Rhodeus except R. s. sericeus, R. s. amarus, and R. sinensis. Type II was defined by the incomplete infraorbital canal, and was found in R. s. sericeus and R. s. amarus. Type III was defined by absence of the temporal and supratemporal canals, and was found in R. sinensis.
A series of four to seven infraorbital bones were found in bitterlings. The five bone series state was found in all Tanakia species, six Acheilognathus species/subspecies, and all Rhodeus species/subspecies except R. sinensis. The six or seven bone series state was found in six Acheilognathus species/subspecies, while the four bone series state was seen in two Rhodeus species, R. sinensis and a small specimen of R. fangi. Analyzing the number and shape of the infraorbital bones, bitterling infraorbital bone series could be divided into four groups and three subgroups. By a combination of degree of completeness of the trunk canal and scale size, four types of trunk canal could be distinguishable. The complete canal type was found in all Tanakia and Acheilognathus species except T. tanago and A. typus. An incomplete canal was found in T. tanago, A. typus, and all Rhodeus species: interrupted canal type in T. tanago, reduced canal within large scales type in all Rhodeus species, and reduced canal within small scales type in A. typus. Phylogenetic polarity of the cephalic sensory canal sysyem, the infraorbital bones, and the trunk canal was determined by mapping their character states on a molecular tree based on mitochondrial 12S ribosomal DNA sequences. Cephalic sensory canal system Type I was determined to be plesiomorphic, and Types II and III, apomorphic. Further an evidence that interrupted part of the canal is different in Type II and Type III (IOC was interrupted and TC present in Type II vs. IOC was complete and TC absent in Type III) suggested that Type II and Type III may have each been derived independently from Type I. Analysis of the total number of infraorbital bones revealed that a five infraorbital state was plesiomorphic, and both fewer or more than five infraorbital state, apomorphic. These latter two states may have been derived independently from the five infraorbital bone state, suggesting evolution from Tanakia to Rhodeus and from Tanakia to Acheilognathus, respectively. A complete trunk canal was determined to be the plesiomorphic state, and it could be inferred that the incomplete state may have been derived from the complete state. Evolution of Rhodes from Tanakia by progenesis was hypothesized from a bitterling phylogeny and the fact that within the Acheilognathinae, Rhodeus had the most derived 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.
Systematic significance of the cephalic sensory canals, trunk canal, barbels, branched fin-rays of the dorsal and anal fins, and the diploid chromosome number in all subfamilies of the Cyprinidae were studied to gain an overview of the family.
From this overview, the following could be discerned: 1) The cephalic sensory canal system in Cyprinidae could be divided into four types (A-D) on the basis of continuity or discontinuity between the infraorbital and supraorbital canals as well as that between the temporal and preopercular canals; 2) The Acheilognathinae was characteriged by the most reduced cephalic sensory canal system among all cyprinid subfamilies ; 3) At least two evolutionary morphogenetic pathways of cephalic sensory canals could be hypothesized (Type D → Type B → Type A, and Type D → Type C → Type A); and 4) It could also be hypothesized that the Acheilognathinae might have evolved from an ancestor of this subfamily by peadomorphosis based on the finding that the sensory canal system in all sexually mature bitterlings remained structurally similar to that in the juvenile stage of most other cyprinids.