Shuhei Matsuura

Diurnal rhythm of oocyte development and plasma steroid hormone levels in the female red sea bream, Pagrus major, during the spawning season

Abstract Diurnal maturation rhythm in the female red sea bream, Pagrus major , which spawns every day during the spawning season and has an asynchronous-type ovary containing oocytes at various stages of development, was studied by histological investigation of the ovary. Plasma levels of estradiol-17β and testosterone corresponding to the ovarian maturation rhythm were also investigated. The fish had a diurnal ovarian maturation rhythm in which the most advanced oocytes in the ovary finished germinal vesicle breakdown at 07.00 h, arrived at a mature state at 10.00 h, and ovulation began at 13.00 h. Spawning mainly occurred between 18.00 and 19.00 h. Plasma estradiol-17β levels reached a peak of about 1200 pg/ml in fish with pre-mature stage oocytes at 07.00 h, followed by a sharp drop in ovulated fish at 13.00 h (to about 200 pg/ml); the levels showed a tendency to increase from the time of postovulation to the next germinal vesicle breakdown. Testosterone levels decreased (to about 20 pg/ml) at ovulation time (13.00 h) in parallel with the estradiol-17β levels, but were largely unchanged (70–110 pg/ml) throughout the rest of the ovarian maturation cycle. These results are discussed in relation to other studies on the role of these steroid hormones responsible for the ovarian maturation of teleosts.

Annual reproductive cycle of the captive female Japanese sardineSardinops melanostictus: Relationship to ovarian development and serum levels of gonadal steroid hormones

Gonad and blood samples were taken from the captive female Japanese sardineSardinops melanostictus between 1988 and 1989, and changes in serum levels of gonadal steroids were correlated with the annual gonadal cycle. Under captive conditions, female fish did not mature and spawn spontaneously, although oocytes developed up to the end of vitellogenic growth. Based on evidence from ovarian histology, the annual gonadal cycle of the Japanese sardine was divisible into four periods, i.e., immature (June to October), vitellogenesis (November to December), spawning (January to March), and post-spawning (April to May). The pattern of seasonal change in the gonadosomatic index (GSI) showed an inverse correlation to change in water temperature and reflected the degree of ovarian maturity. The serum estradiol-17β level increased from its lowest concentration (0.12 ng ml−1) in September to a peak (1.14 ng ml−1) in March. Serum 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-P) was detectable at low levels (<0.3 ng ml−1) between October and February, but was below the assay detection limit (0.06 ng ml−1) at all other times. Testosterone was not detectable (<0.06 ng ml−1) in the serum of any fish throughout the year. The effects of several steroids on the maturation of follicle-enclosed oocytes of sardine were examined in vitro, and 17α,20β-P was found to be the most potent inducer of maturation. This suggests that post-vitellogenic oocytes of the Japanese sardine in captivity have an ability to respond to an appropriate hormonal effector and subsequently to resume meiotic maturation.

Laboratory Studies on Molting and Growth of the Shore Crab, Hemigrapsus sanguineus de Haan, Parasitized by a Rhizocephalan Barnacle

Molting of shore crabs (Hemigrapsus sanguineus) parasitized by rhizocephalans (Sacculina senta) was observed in the laboratory, and the growth of the molted crabs was compared with that of unparasitized animals. Molting of the host was obstructed by the infestation, but was still possible. After the release of several broods of larvae, the externa (the external reproductive system of the parasite) detached from the host. Subsequent molting occurred within 40 days in about 80% of the animals, but in the remainder, it was delayed for at most 4 months. Soon after molting, a new externa protruded from the abdomen of every crab. Thus, the life-span of the externa and the molting of the host would seem to be closely connected. In the female, the molt frequency was reduced, but the molt increment of the parasitized crabs was not different from that in the unparasitized ones. In the male, however, both the molt frequency and the molt increment were reduced. Thus, the annual growth of parasitized males and females was about half that of unparasitized crabs.

Observations on ovarian follicle ultrastructure in the marine teleost, Pagrus major, during vitellogenesis and oocyte maturation

Abstract The ovarian follicle of the marine teleost, the red sea bream Pagrus major , was investigated ultrastructurally and histochemically, during vitellogenesis and oocyte maturation. Processes from the granulosa cells and from the oocyte grow and make contact with each other in the pore canals of the zona radiata during vitellogenesis, but are withdrawn as the zona radiata becomes more compact and devoid of pore canals during oocyte maturation. During vitellogenesis the special thecal cells possess organelles characteristic of steroid-producing cells, namely, mitochondria with tubular cristae and smooth endoplasmic reticulum, but the granulosa cells contain organelles typical of protein-secreting cells. Marked ultrastructural changes occur in the granulosa cells during nuclear migration. This involves a dilation of the rough endoplasmic reticulum. Histochemical reactions for Δ 5 -3 β -hydroxysteroid dehydrogenase activity are negative in both granulosa and thecal cells during vitellogenesis and oocyte maturation. Possible cellular sites for ovarian steroid hormone synthesis in this species are discussed and a comparison is made with data derived from other teleosts.

Juvenile bisexuality in the red sea bream,Pagrus major

SynopsisThe histology of the gonad of the red sea bream,Pagrus major, was examined in order to study the early gonadal development, sexual maturation and sex ratio in a natural population. A total of 1,117 fish between the ages of 4 months and 8 years were examined. Gonads of 4-month-old fish were either sexually undifferentiated with a central cavity, or ovarian in form. Gonads of 12- and 18-month-old fish were ovaries or bisexual gonads, while those of 2-year-old fish were ovaries, bisexual gonads or testes. Fish aged between 3 and 8 years had ovaries or testes, except for a few bisexual gonads found in 3- and 4-year-old fish. The chronological appearance of females, hermaphrodites and males in that order, and histological evidence, suggested that the testis originates from the ovary via a bisexual gonad in the juvenile stage. The sex ratio of females to males at the age of 2 years and over was about 1:1, suggesting that hermaphroditic red sea bream appear in about 50% of the juvenile population. The sexual pattern in this species, therefore, is concluded to be gonochorism with a bisexual juvenile stage.

Testicular development and serum levels of gonadal steroids during the annual reproductive cycle of captive Japanese sardine

Gonad and blood samples were taken throughout the year from captive males of the Japanese sardine,Sardinops melanostictus, and changes in serum levels of gonadal steroids were examined in relation to the annual gonadal cycle. On the basis of testicular histology, the annual gonadal cycle was divisible into four periods: immature (July–September), spermatogenesis (October–December), spermiation (January–April), and post-spawning (May–June). The pattern of seasonal changes in the gonadosomatic index (GSI) was inversely correlated with that of water temperature, and reflected the degree of testicular maturity. The serum testosterone level was relatively low during spermatogenesis (2.2–2.5 ng/ml), rose markedly around the time of spermiation (7.7–24.6 ng/ml), and became low after spawning and during immature periods (0.6–0.7 ng/ml). The serum 17α,20β-dihydroxy-4-pregnen-3-one level was high in males with spermatogenic or spermiating testes (0.6–1.0 ng/ml), but became low (0.2 ng/ ml during the post-spawning period and was undetectable in immature fish. Although 11-ketotestosterone was detectable in some fish, the values obtained were thought to reflect cross-reactivity of the antiserum employed with testosterone. These findings are discussed in relation to male reproduction of the Japanese sardine and steroidal regulation of spermatogenesis and spermiation in other teleosts.

Effects of steroids on germinal vesicle breakdown in vitro of intact follicles in the japanese whiting, Sillago japonica, a marine teleost

Abstract 1. 1. The effects of various steroids on germinal vesicle breakdown (GVBD) of follicle-enclosed oocytes of the Japanese whiting were examined by using an in vitro incubation technique. 2. 2. Of the stimulatory steroids, 17α,20β-dihydroxy-4-pregnen-3-one was the most effective, inducing 72% GVBD at concentrations as low as 0.1 ng/ml. The next most potent steroid was 20β-dihydroprogesterone, inducing 33% GVBD at concentrations of 0.1 ng/ml. 3. 3. The specificity for steroid-stimulated GVBD in Japanese whiting is consistent with several previous studies involving salmonids and various other teleosts, and suggests a possible physiological role for a 20β-dihydroprogestin in oocyte maturation of Japanese whiting.

Sexual maturation, annual reproductive cycle, and spawning periodicity of the shore scorpionfish, Scorpaenodes littoralis

The ovarian structure, sexual maturation, annual reproductive cycle, and spawning periodicity of the shore scorpionfish, Scorpaenodes littoralis, in Uchiura Bay, central Japan, were examined using specimens collected between May 1995 and March 1998 and fishes reared in laboratory. The ovarian stroma and blood vessels run longitudinally through the center of each ovarian lobe. The ovarian peduncles radiate from the central stroma. During the spawning season, gelatinous material is secreted from the epithelia of both the ovarian peduncle and ovarian wall, and the epithelia show morphological changes accompanying the ovarian maturation cycle. The minimum standard length at maturity was 55.2 mm for males and 40.2 mm for females. Males with mature testes were collected from March to November. Females in the mature or post spawning stages were collected between May and October, when the mean gonadosomatic indices were also high. This indicates that the spawning season of this species occurs between May and October. Four successive types of oocytes were grouped in the mature ovary, comprised of mature, late and early vitellogenic and previtellogenic oocytes respectively, suggesting that this species is a multiple spawner. Four captive females spawned repeatedly at intervals of 2–8 days over a prolonged period (4–8 months); a 2-day spawning interval was the most common for all females. This suggests that female S. littoralis have a 48-h spawning cycle in captivity.

Swimming behaviour of the pinnotherid crab, Tritodynamia horvathi observed during the low temperature season

The swimming and swarming behaviour of Tritodynamia horvathi (Brachyura: Pinnotheridae) has been considered to be a seasonal activity related to summer mating. However, little information has been available regarding other seasons of the year as the crabs have not been observed at the sea surface in the cool months from October until May. This is the first report of swimming behaviour observed during this low temperature season. The swimming individuals were observed in March 1991 in Yatsushiro Sound, Japan. Tritodynamia horvathi were observed swimming at a depth of 15 m, using their maxillipeds to feed. The crabs also swam in the aquarium and caught Artemia nauplii in the same manner. These findings suggest that their swimming behaviour is not a seasonal mating activity as previously believed, but a habitual behaviour associated with feeding.

Variation in the duration of copulation of the Japanese mitten crabEriocheir japonicus

In brachyuran crabs, the mechanisms involved in copulation have been reported for various species from different aspects (e.g. physiological, behavioral, and sociological aspect, see reviews: Hartnoll 1969; Christy 1987; Fukui 1991). Within those reports, the duration of copulation has been described for various species (e.g., Xanthidae: Wilber 1989; Cancridae: Elner et al. 1985; Majidae: Takeshita & Matsuura 1980; Ocypodidae: Kosuge et al. 1992; Grapsidae: Morita 1974) and has been used for characterizing behavioral differences (Hartnoll 1969; Kosuge et al. 1992). We observed the copulation of the Japanese mitten crab Eriocheir japonicus under laboratory conditions, and analyzed the mating system of this species (Kobayashi & Matsuura unpublished). When observing copulation, we found a large variation in the duration of copulation. Although interspecific differences in the duration of copulation have already been described for braehyurans, intraspecific variations have not yet been closely studied. Variations may be produced by the effects of environmental conditions and by the strategy or tactics used according to the social relationship between the sexes. In this study, we selected 2 probable parameters which may influence the duration of copulation, i.e. water temperature and the carapace width of each sex, and investigated how these parameters influenced the variations.