Infraorbital Bones
Bitterlings were found to have four to seven plate-like infraorbital bones ('io') (Figs 9-11), and could be divided into 4 groups by this total. This distribution and groupings were as follows:
Infraorbital 1 (lacrimal) was a deep pentagonal bone, the second largest in the infraorbital series, and articulated with 'io 2' ventroposteriorly, but not with any other bone dorsally and dorsoposteriorly. An infraorbital canal ran through the ventral portion of 'io 1'.
Infraorbital 2 was an elongated rectangular bone which articulated with 'io 1' anteriorly and 'io 3' posteriorly, varying in bone shape among species. In Acheilognathus longipinnis (Fig. 10H) and A. typus (Fig. 10I), the bone was small, with a clear gap between 'io 2' and 'io 3'. A small fragmentary bone with a canal tube was found between 'io 2' and 'io 3' in A. typus.
Infraorbital 3 was semicircular or crescent in shape, largest among the infraorbital series, occupying about 40% of the series total length, and articulated with 'io 2' anteriorly, and 'io 4' posteriorly. Except for Rhodeus s. sericeus and R. s. amarus (Figs. 11 G and H) where the sensory canal in 'io 3' was reduced, not extending to the posterior part of 'io 3', the infraorbital canal in 'io 3' ran from the anteriormost to the posteriormost part of 'io 3' in all other bitterlings.
Infraorbital 4 was an elongated rectangular bone, articulating with 'io 3' anteriorly and 'io 5' posteriorly. Infraorbital 4 was the last bone of the infraorbital series in R. sinensis (Fig. 11I) and a smaller specimen of R. fangi (Fig. 11F), in which it was large and with a convex posterior margin. The sensory canal in 'io 4' was reduced and did not run along the anterior portion of the bone in Rhodeus sericeus amarus (Fig. 11H).
Infraorbital 5 was the most posterior bone of the infraorbital series in most species, articulating with 'io 4' ventrally, but with no direct connection with other bones dorsally in most species. This bone could be grouped into two types by shape: (1) reduced to a bony tube as found in R. o. kurumeus (Fig. 11B), R. haradai, R. atremius suigensis (Figs. 11 D-E), and a larger specimen of R. fangi, and (2) not reduced as found in all other bitterlings (Figs. 9-10, and 11 A, C, G-H).
Infraorbital 6 was represented by a bony tube, which extended dorsoposteriorly and did not contact the lateral process of the sphenotic. When present, 'io 6' articulated with 'io 5' ventrally as was found in A. longipinnis, A. macropterus, A. melanogaster, A. t. tabira, A. tabira subsp. (a), and A. typus (Figs. 10 F-I).
On the basis of the above, bitterlings were divided into four groups by the number of infraorbital bones. Variation in number was found in A. melanogaster (5 and 6 bones) and R. fangi (4 and 5 bones).
- Fig.9.
- Infraorbital bones in Tanakia. A, Tanakia lanceolata from Japan. B, T. lanceolata from China. C, T. limbata. D, T. signifer. E, T. tanago. F, T. himantegus chii. Scale bar=1 mm.
- Fig.10.
- Infraorbital bones in Acheilognathus. A, Acheilognathus melanogaster. B, A. omeiensis. C, A. rhombeus. D, A. cyanostigma. E, A. yamatsutae. F, A. t. tabira. G, A macropterus. H, A. longipinnis. I, A. typus. Scale bar=1 mm.
Group a had 5 bones, and could be further divided into 3 subgroups by the shape of 'io 5' and the reduced sensory canal in 'io 3' and 'io 4'.
Subgroup a1 (Figs. 9, 10 A-E, and 11 A and C). A normal-sized 'io 5' was present. This was found in all Tanakia species, and 6 Acheilognathus species/subspecies as well as 4 Rhodeus species/subspecies as follows: A. cyanostigma, A. melanogaster, A. omeiensis, A. rhombeus, A. tabira subsp. (b), A. yamatsutae; R. a. atremius, R. notatus, R. o. ocellatus, and R. spinalis.
Subgroup a2 (Figs. 11 B, D-E). 'io 5' was reduced to a bony tube as found in only 4 Rhodeus species/subspecies: R. a. suigensis, R. haradai, and R. o. kurumeus, and a larger specimen of R. fangi.
Subgroup a3 (Figs. 3A and 11 G-H). 'io 3' and 'io 4' had a reduced sensory canal. This was found in only 2 Rhodeus subspecies: R. sericeus amarus and R. s. sericeus.
Group b (Figs. 10 F-H) had 6 bones as was found in 5 Acheilognathus species/subspecies: A. longipinnis, A. macropterus, A. melanogaster, A. t. tabira, and A. tabira subsp. (a).
Group c (Fig. 10I) had 7 bones as was found in A. typus.
Group d (Figs. 11 F and I) had 4 bones as found in only two Rhodeus species: R. sinensis and a smaller specimen of R. fangi.
- Fig.11.
- Infraorbital bones in Rhodeus. A, Rhodeus o. ocellatus. B, R. o. kurumeus. C, R. a. atremius. D, R. a. suigensis from Japan. E, R. a. suigensis from China. F, R. fangi. G, R. s. sericeus. H, R. sericeus amarus. I, R. sinensis. Scale bar=1 mm.