サメ類の研究―2 : ホシザメの生殖生態について

The sexual cycle of Japanese dogfish, Mustelus manazo, landed at Nagasaki fish market was studied in the present study. Study on Mustelus manazo distributed in the waters off eastern coast of Kyushu has been done to some extent, lacking, however, very important portions in the sexual cycle, i.e., observation upon collection during the period from May to August. Investigation including the above-mentioned period was carried out on the materials caught in the waters off western coast of Kyushu. This species belongs to viviparous non-placental type, and dose not develop the uterine compartments during gestation. The left ovary of this species is rudimentary throughout the life, and only the anterior portion of the right ovary is functional. During the ovulating period (June to August), of the developing ova, 8-10 mature eggs are ovulated in the peritoneal cavity, and are, one by one, received at a common ostium where they are distributed to the both oviducts and moved to the nidamental glands. The eggs fertilized in the nidamental gland are enclosed in the embryonic membrane and descend into the uterus to develop as embryos. The uterus of this species is a semi-transparent thin membrane, and the placenta is not developed at all stages. The posterior end of the uterus is not closed even when large-sized embryos being contained. The embryo, being small, is nourished by the yolk stored in the yolk sac through the yolk stalk. However, when yolk is completely utilized, the yolk stalk disappears. In these waters, the parturition season is April through May and the embryo at birth is approximately 30 cm in total length. This species appears to have two months' resting period after parturition, and after this period, starts another mating, ovulation, fertilization and pregnancy. This species reaches maturity when it becomes about 60 cm in total length. The number of embryos contained in the both uteri is 2 to 8 , increasing proportionally with the body length of the female parent. Gestation period is about 10 months.Study on Japanese dogfish, Mustelus manazo, caught by the bottom long liners in the waters off Goto Islands is summarized as follows: 1. This species belongs to the viviparous non-placental type, and neither the uterine compartments nor the placenta are developed in the uterus throughout gestation. 2. The left ovary of this species is rudimentary throughout the life, and only the anterior portion of the right ovary is functional. 3. Parturition of this species occurs during the period from April to May and the embryo at birth is approximately 30cm in total length. 4. This species has two months' resting period after parturition, and then undergoes ovulation, mating and fertilization in June to July. This means that gestation period is about 10 months. 5. Females of this species reach sexual maturity at the total length between 62 and 64cm, and males at the length of about 60cm. 6. During ovulating period, 8-10 mature eggs of about 18mm in diameter are ovulated in the peritoneal cavity from the right ovary. The ovulated eggs are received at a common ostium where they are distributed to the both oviducts and moved to the nidamental gland. The fertilization appears to occur in the nidamental gland. 7. Each fertilized egg is enclosed in an embryonic membrane in the nidamental gland, and descends into the uterus to develop as an embryo. The embryo is enclosed by the embryonic membrane until birth, and no connections exist between this membrane and the uterine wall. 8. This species does not possess any embryonic membrane-store chamber. 9. Although the embryo is nourished with yolk through the yolk stalk during early gestation, the yolk stalk disappears upon consumption of yolk when the embryo grows up to some developmental stage 10. The uterine wall of this species is thin and semi-transparent. The posterior end of the uterus is open towards the vagina throughout gestation. 11. The number of embryos in a gestation is relative to the adult body length. The adult being 60-90cm in total length, the embryos contained in both uteri numbered within the range of 2-8, accordingly