Miyashita et al.: Morphological development and growth of Thunnus thynnus 
605 
Fin development 
The number of fin rays increased between 6 and 10 mm 
BL (Table 1). The smallest specimen possessing dorsal 
spines was 6.32 mm BL. Caudal-fin rays, second dorsal-, 
pelvic-, anal-, and pectoral-fin rays appeared at 7.10, 7.76, 
8.00, 8.47, and 8.67 mm BL, respectively. The smallest 
specimen with an adult complement of fin-ray counts (Iwai 
et al., 1965; Collette and Nauen, 1983) was 9.68 mm BL, 
whereas the largest specimen with an incomplete fin-ray 
complement was 10.02 mm BL. The number of pectoral-fin 
rays for specimens >10 mm BL varied among individuals. 
Pigmentation 
Newly hatched larvae (2.62-3.05 mm BL) had small mela- 
nophores scattered over the body, head, notochord, yolk, 
and oil globule, but not on the finfold (Fig. 2A). 
On day 1 after hatching (3.35-3.74 mm BL) (Fig. 2B), den- 
dritic melanophores were visible from snout to fore- and mid- 
brain and on the dorsum of the gut; three clusters of mela- 
nophores occurred in the dorsal midline along with punctate 
melanophores; melanophores occurred along the ventral mid- 
line (punctate melanophores formed a line at the anterior 
caudal area and a cluster of melanophores occurred in the 
posterior caudal area); and melanophores appeared along 
the lateral midline of the trunk. The eyes were pigmented. 
On day 2 after hatching (3.40-4.18 mm BL), the pattern 
of melanophores was transitional between those of 1-day 
and older preflexion larvae. Melanophores on the snout 
disappeared, whereas clusters of melanophores on the an- 
teriodorsum of the trunk (second myomere), anteriordor- 
sum of the caudal area (17th to 21st myomere) and pos- 
teriodorsum of the caudal area (30th to 39th myomere), 
as well as on the anterio- and midventral edge (14th to 
26th myomere), posteroventral edge (28th to 39th myo- 
mere), and lateral midline (posterior to the 13th myomere) 
of the caudal area began to shrink (although they were 
still large [Tables 2-5] ). 
On day 3 after hatching, forebrain melanophores disap- 
peared in all specimens >3.76 mm BL, whereas the small- 
est specimen lacking these melanophores was 3.63 mm 
BL. All specimens >3.82 mm BL had melanophores scat- 
tered on the dorsum of the gut, which formed a cap of me- 
lanophores over the gut in later stages. Clusters of mela- 
nophores on the dorsal and ventral midline, and lateral 
midline of the body now appeared punctate and additional 
punctate melanophores appeared in these areas (Tables 
2-5). Melanophores were present on the midbrain, on the 
anteriodorsal side and on the base of the hindbrain; and 
several melanophores appeared on the lower side of the 
lower jaw. The smallest specimen with melanophores on 
the lower jaw was 3.41 mm BL, whereas the largest lack- 
ing them was 9.46 mm BL (day 16). Most specimens >5.5 
mm BL had these lower jaw melanophores. On and after 
day 4 (3.92-4.43 mm BL), all specimens had the melano- 
phore pattern of preflexion larvae (Fig. 2C). 
Melanophores began to develop on several other areas 
in larvae larger than about 4.5 mm BL (Fig. 2, D-F). Em- 
bedded melanophores appeared in the lateral muscle at 
4.49 mm BL (Table 6), upper jaw tip at 5.47 mm BL, oper- 
culum and preoperculum at 6.18 mm BL, the membrane 
of the first dorsal fin at 6.32 mm BL, forebrain at 6.83 mm 
BL, and the cleithral symphysis at 10.79 mm BL. The larg- 
est specimens lacking melanophores in these areas were 
those that were 6.44 mm BL (for upper jaw tip), 6.72 mm 
BL (for lower jaw tip), 6.94 mm BL (for operculum), 9.69 
mm BL (for forebrain), 10.28 mm BL (for cleithral sym- 
physis), and 10.49 mm BL (for preoperculum). 
Melanophores forming a dorsal cap of the gut enlarged 
at about 6.5 mm BL and the rim reached the ventral sur- 
face of the gut in some specimens >7.5 mm BL. The body 
melanophore pattern changed with growth. Dorsal mid- 
line melanophores disappeared from the trunk from 4.5 
mm BL to 7 mm BL, and ventral and lateral midline me- 
lanophores disappeared from the trunk (Tables 2-5). 
Larvae showed partial juvenile pigment patterns from 
about 8 mm BL (Fig. 2F). Dorsolateral and dorsal midline 
melanophores appeared at the trunk and increased in fre- 
quency in the caudal area from 7.5 mm BL (Table 2). Em- 
bedded melanophores frequently appeared at the posterior 
trunk and caudal area (Table 6). Dense patches of mela- 
nophores appeared at the fin base of the first and second 
dorsal, and anal fin bases (Tables 7 and 8; these melano- 
phores occurred on the myosepta), as well as at the periph- 
ery of the eye, especially below and behind the eye from 
about 8.0 mm BL. Embedded melanophores appeared near 
the notochord and neural and haemal spines, and melano- 
phores at the lateral midline of body extended internally 
from about 8.2 mm BL. Generally, melanophores on the 
dorsal and ventral midline, lateral midline, and those em- 
bedded increased with BL (Tables 2-6). 
The smallest specimen having juvenile pigmentation 
(i.e. densely pigmented patches appearing on the body) 
was 13.55 mm BL (Fig. 2G). More distinct body patches 
appeared and increased in number as juveniles grew. 
In many larvae, melanophores were found in the area 
of the hypural bones and on the caudal finfold or fin rays. 
The smallest specimens having melanophores in the area 
of hypural bones and on the caudal finfold were 6.42 mm 
BL and 6.81 mm BL, respectively. 
Erythrophores first appeared at 4.63 mm BL (6 days af- 
ter hatching) on the tail: thirty-four erythrophores at the 
posteroventral edge, five at the posterolateral midline, and 
two at the posterodorsal edge (Table 9). As larvae grew, 
erythrophores appeared at the caudal finfold at 4.75 mm 
BL, on the lower jaw at 6.10 mm BL, and at the hypural 
plate at 6.96 mm BL; at the hypural plate and caudal 
finfold, erythrophores were observed only on specimens 
6.96-7.20 and 4.75-7.20 mm BL, respectively. At the dor- 
sal and ventral edge of the trunk and tail, erythrophores 
became larger and decreased in number in larvae >7 mm. 
At the ventral edge, adjacent erythrophores were united. 
At the lateral midline of the body, the number decreased 
at 8.5 mm BL. Erythrophores disappeared from the lower 
jaw at 11.26 mm BL, from the posterolateral midline and 
dorsal edge of the tail at 15.85 mm BL, and from the 
ventral edge of the tail at 19.72 mm BL. Erythrophores 
were not observed on the upper jaw. The number of eryth- 
rophores, especially the ventral edge erythrophores, de- 
