Miyashita et al.: Morphological development and growth of Thunnus thynnus 
611 
Table 6 
for the myomere in which it occurs, n indicates the number of specimens examined at each size range. Melanophores were not observed 
Myomere 
24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 
10 
10 
14 
27 
21 
12 
10 
10 
6 
6 
11 
11 
18 
10 
10 
10 
10 
10 
10 
10 
10 
10 
10 
10 
30 
20 
30 
20 
10 
30 
20 
20 
20 
30 
10 
23 
23 
31 
23 
38 
31 
54 
38 
23 
38 
15 
38 
38 
31 
15 
15 
13 
69 
62 
69 
77 
77 
77 
77 
77 
77 
69 
62 
62 
62 
62 
46 
23 
13 
75 
75 
83 
83 
75 
83 
75 
75 
75 
75 
75 
75 
58 
58 
67 
33 
12 
90 
90 
100 
100 
90 
100 
90 
90 
90 
90 
90 
90 
70 
70 
80 
40 
10 
Table 7 
n indicates the number of specimens examined at each size range. Melanophore was not observed on the first to fourth myomeres and 
Myomere 
23 
24 
25 
26 
27 
28 
29 
30 
31 
32 
33 
34 
35 
36 
37 
38 
39 
n 
10 
10 
20 
20 
30 
20 
30 
20 
20 
30 
20 
10 
10 
10 
8 
8 
15 
15 
23 
15 
23 
15 
15 
23 
15 
8 
8 
13 
31 
46 
38 
46 
46 
38 
46 
46 
38 
38 
31 
46 
23 
8 
0 
15 
13 
83 
83 
83 
83 
83 
83 
83 
83 
83 
83 
58 
58 
58 
42 
42 
8 
12 
90 
90 
100 
100 
100 
100 
80 
80 
90 
80 
80 
70 
60 
20 
30 
10 
20 
10 
Pagrus major, and white trevally, Pseudocaranx dentex 
[Sawada unpubl. data; Fig. 5] ). Rapid growth in bluefin tu- 
na is pronounced after day 20 (if =9.34 mm BL on day 20) 
when larvae begin to metamorphose into juveniles. This 
type of growth phase seems common to scombrids, for ex- 
ample, T. albacares (Harada et al, 1971b; Kaji et al., 1999), 
Scomber japonicus (Watanabe, 1970), Auxis tapeinosoma 
(=A. rocheri , Harada et al., 1973), and Sarda orientalis 
(Harada et al., 1974) and reaches its height by the time of 
development of external physical features and the diges- 
tive system (Kohno et al., 1984; Miyashita et al., 1998), 
and by the time of development of their selective feeding 
characteristics (Sawada et al., 2000). 
On day 19 (x =10.17 mm TL), the mouth size of T. thyn- 
nus averaged 2.54 mm according to Shirota’s (1970) cal- 
culation (mouth size index=upper jaw length x 2 0 - 5 ). This 
size is comparable to that of other scombrids, such as T. 
albacares (3.3 mm) and Katsuwonus pelamis (3.0 mm) at 
10 mm TL, although much larger than that of other spe- 
cies (see Table 2 of Shirota, 1970). The digestive system of 
71 thynnus develops earlier than that of other fishes (Rich- 
ards and Dove, 1971; Miyashita et al., 1998) and attains 
an adultlike structure by the juvenile stage, thus allowing 
bluefin tuna to use food efficiently early in development. 
The development of pigment, spines, jaw teeth, and 
squamation in 71 thynnus is summarized in Figure 6. The 
jaw teeth of 7! thynnus first appeared at 5.35 mm BL (day 
10), the palatine teeth at 7 mm BL (flexion stage), and 
were fully developed at 9 mm BL (postflexion stage). At 7 
mm BL, 71 thynnus larvae fed on other fish larvae and at 8 
mm BL (postflexion stage), they were cannibalistic. Teeth 
appearance and development corresponded to the piscivo- 
rous stage during the fast-growth phase after the postflex- 
ion stage. Future improved laboratory-rearing techniques 
