601 
Abstract— Morphological development 
and growth of larval and juvenile Pacific 
bluefin tuna, Thunnus thynnus, were 
studied from laboratory-reared spec- 
imens. Average body length (BL) of 
newly hatched larvae was 2.83 mm and 
larvae grew on average 5.80 mm by 10 
days, 10.62 mm by 20 days, and 35.74 
mm by 30 days after hatching. Growth 
was especially accelerated after 20 days 
from hatching. Newly hatched larvae 
had small melanophores scattered over 
their bodies except for the finfold. On 
day 1 after hatching (3.35-3.74 mm 
BL), a characteristic melanophore pat- 
tern appeared and it was partially 
maintained until day 3 after hatching. 
At approximately 4 mm BL, larvae had 
developed melanophore patterns simi- 
lar to those of preflexion Thunnus spp. 
larvae, such as melanophores on the 
dorsum of the gut, midlateral trunk, 
and tail, at the dorsal and ventral mid- 
lines of trunk and tail, and on the lower 
jaw. Erythrophores appeared at 4.63 
mm BL in the caudal area. Jaw teeth 
appeared at 5-6 mm BL. The preoper- 
cular angle spine, anterior preoper- 
cular spine, and posttemporal spine, 
developed at approximately 7 mm BL, 
when erythrophores appeared on the 
trunk and tail. The notochord flexion 
occurred between 6 and 8 mm BL. 
At approximately 8 mm BL, erythro- 
phores disappeared and juvenile color- 
ation appeared on the trunk and tail, 
consisting of dense patches of melano- 
phores at the dorsal and anal-fin bases, 
embedded melanophores, and melano- 
phores at the periphery of the eye. The 
adult complement of fin-ray counts was 
attained at 10 mm BL, when the juve- 
nile melanophore pattern was attained, 
although the pattern was not fully 
developed. Specimens larger than 20 
mm BL did not have erythrophores. 
Squamation began at 27 mm BL and 
head spines disappeared by 38 mm 
BL. 
Manuscript accepted 10 April 2001. 
Fish. Bull. 99:601-616 (2001). 
Morphological development and growth of 
laboratory-reared larval and juvenile 
Thunnus thynnus (Pisces: Scombridae) 
Shigeru Miyashita 
Yoshifumi Sawada 
Tokihiko Okada 
Osamu Murata 
Hidemi Kumai 
Fisheries Laboratory of Kinki University, 
3153, Shirahama, 
Wakayama 649-2211, Japan 
E-mail address (for S. Miyashita): miyasita@cypress ne.|p 
The Pacific bluefin tuna is a large scom- 
brid that migrates transoceanically 
between the western and eastern Pacific 
Ocean (Orange and Fink, 1963; Fujita, 
1998). The Pacific bluefin tuna, Thun- 
nus thynnus orientalis (Tennninck and 
Schlegel), is considered a separate sub- 
species from the Atlantic bluefin tuna, 
T. thynnus thynnus Linnaeus (Gibbs 
and Collette, 1967; Collette and Nauen, 
1983). Recently, Cho and Inoue (1993) 
showed reproductive isolation between 
the two subspecies. Most recently, Col- 
lette (1999) considered these subspe- 
cies to be separate species on the basis 
of morphology and molecular data. 
Our knowledge of the early life his- 
tory of oceanic species, such as tunas, 
is incomplete. Morphological character- 
istics common to scombrid larvae are 
large head, gape, and eyes; development 
of head spination; and posterior migra- 
tion of the anus (Collette et al., 1984). 
In studies of early-stage bluefin tuna 
(e.g. Yabe et al. , 1966; Matsumoto et 
al. , 1972; Richards and Potthoff, 1974; 
Kohno et al., 1982), melanophore pat- 
terns were used to identify tuna larvae. 
Erythrophore patterns were proposed 
as another characteristic by Ueyanagi 
(1966) and Matsumoto et al. (1972). 
However, ontogenetic changes of mela- 
nophore and erythrophore patterns and 
morphological characteristics have not 
sufficiently been investigated because 
of the difficulty in obtaining a complete 
series of wild specimens in their early 
developmental stages. 
Morphological, physiological, and be- 
havioral information has recently been 
collected from laboratory-reared Pacific 
bluefin tuna specimens (Harada et al., 
1971a; Kaji et al., 1996; Takii et al., 
1997; Kumai, 1998; Miyashita et al., 
1998; Miyashita et al., 2000, in press; 
Sawada et al., 2000). The ability to pro- 
duce a complete series of early-stage 
specimens provides an opportunity to 
enhance our understanding of the ear- 
ly development of bluefin tuna. In this 
study, we describe the morphological 
development and pigment patterns of 
reared larval and juvenile bluefin tuna. 
Materials and methods 
Eggs were obtained on 10 July 1994 
from bluefin tuna that had been caught 
off Ohshima when 20-40 cm in total 
length 10-year class] and that had been 
raised for seven years naturally in a net 
cage at the Ohshima Experiment Sta- 
tion of the Kinki University Fisheries 
Laboratory. The net cage was equipped 
with a polyvinyl chloride sheet sur- 
rounding the top 2 m of the net side 
wall to prevent eggs from flowing out of 
the net. Fertilized eggs were collected 
at the surface with a 330-pm mesh net. 
Eggs were transferred to a tank in a 
land-based nursery center and kept in 
a fine mesh net at 24.5°C for approxi- 
mately 12 hours. The following morn- 
ing, eggs were transferred to a 20-m 3 
rearing tank and larvae hatched at 
