358 
Fishery Bulletin 99(2) 
Table 1 
Histological characteristics of oocytes at different developmental stages in Lophius litulon. 
Developmental stage 
Oocyte 
of the oocyte 
diameter (pm) 
Histological characteristics 
chromatin nucleus 
less than 20 
Nucleus has a large nucleolus. 
perinucleus 
35-160 
Multiple nucleoli are seen toward the periphery of the nucleus; oil droplets appear 
around the nucleus and increase in number; follicle cells surrounding the oocyte 
have formed a narrow layer. 
yolk vesicle 
180-250 
Yolk vesicles appear in the peripheral region of the cytoplasm. 
primary yolk 
300-530 
Yolk globules appear between the yolk vesicles and increase in number; both granu- 
losa and thecal cell layers are clearly observed. 
secondary yolk 
480-730 
Oocytes are larger and yolk globules fill the cytoplasm. 
tertiary yolk 
750-1000 
Yolk accumulation progresses rapidly, which results in a marked increase in the size 
of oocytes. 
migratory nucleus 
950-1300 
Yolk globules begin to fuse with one another; oil droplets fuse to form larger ones. 
mature 
1450-1700 
After germinal vesicle breakdown, yolk globules form a single mass and oil droplets 
coalesce to form larger ones. 
atretic 
Early: disintegration of the nucleus and yolk globules and hypertrophy of the fol- 
licular cell layers. 
Late: presence or absence of intercellular vacuoles, flocculent material, and granular 
pigments. 
were mainly used for observations of whole gonadal struc- 
ture, whereas polymer resin sections were used for de- 
tailed observations of oocyte development, spermatogen- 
esis, and the definition of gonadal stages. In our study, 
polymer resin sections fixed in Bouin’s fluid gave the best 
results. The developmental stages of the oocytes were cat- 
egorized according to Yamamoto (1956) and Yoneda et al. 
(1998a) (Table 1). Histological classification of atretic oo- 
cytes (Table 1 ) and postovulatory follicles followed Hunt- 
er and Macewicz (1985) and Yoneda et al. (1998a). Only 
specimens larger than the minimum size at sexual matu- 
rity (see “Results” section) were used to examine seasonal 
changes in gonadal condition. 
Oocyte diameter was measured with a profile projector 
(20-100x), and the range was determined by using the 
largest and smallest oocytes in the whole projected field 
from each developmental oocyte stage. Oocyte diameter 
at each developmental stage was measured in the fol- 
lowing manner: the developmental stage of the most ad- 
vanced oocytes in each ovary was identified from histo- 
logical sections, and the diameters of 50 formalin-fixed 
oocytes forming the largest size class in ovaries at each 
developmental stage were measured (one or two random- 
ly selected specimens). 
Because the arrangement of oocytes shows a gradation 
in developmental stage within each ovigerous lamella (see 
“Results” section), the composition of oocytes within each 
ovigerous lamella was considered to be the unit within 
the ovary. To determine the size-frequency distribution of 
oocytes within each ovarian stage, 50-80 formalin-fixed 
ovigerous lamellae samples (corresponding to 300-550 oo- 
cytes) from each ovarian stage were examined. All oocytes 
>100 pm in yolk vesicle stage ovaries were measured; in 
other ovarian stages, only oocytes >200 pm were measured. 
The size-frequency distribution of oocytes at each stage rep- 
resented the ovary from one randomly selected specimen. 
Estimates of size at sexual maturity were based on the 
examination of males (193-692 mm TL [n=236]) and fe- 
males (174-1013 mm TL |n=246]) collected during the 
spawning season between February and May. Of these, 38 
females were defined as sexually immature, based on mac- 
roscopic observations (Afonso-Dias and Hislop, 1996). The 
remaining 444 specimens were classified into each gonad- 
al stage on the basis of histological observations. Sexually 
mature individuals were defined as males with testes in 
stages 3 and 4, and females with ovaries in mid-stage 2 
(oocytes at the secondary yolk stage) or at more advanced 
stages (see Results). Males with testes in stages 3 and 4 
collected in the spawning season were thought to have the 
potential to spawn because our preliminary observations 
indicated that milt ran from their genital pores on slight 
pressure. Females with ovaries with secondary yolk stage 
oocytes were considered sexually mature because an ad- 
vanced group of secondary yolk stage oocytes forms an iso- 
lated batch and increases in size in tandem with ovarian 
development. Females with ovaries in stages 2 (late), 3, 4, 
or 5 were also considered sexually mature, because these 
individuals were thought to be pre- or postspawning fish. 
Similarly, to estimate age at sexual maturity, the age of 
individual fish collected in the spawning season was deter- 
mined by counting the annual ring marks on the surface 
of the vertebral centrum (Yoneda et al., 1997). Of 482 spec- 
