海産無脊椎動物の生殖腺の成熟に関する研究-III

Abstract

This prper deals with the seasonal changes of the maturation index and histolagy of the ovary of the tanner crab, Chionoecetes opilio, (MAJIDAE, BRACHYURA), caught commercially in the Japan Sea. The materials used were obtained by a motor trawl net which was operated in Wakasa Bay on the west coast of the Japanese mainland at a depth of 175-375m from October to June of 1964-1965 and 1965-1966. During the course of our fisheries biological survey of the tanner crab, we examined 577 specimens of the 10 th molting stage, 66.2mm in carapace width, just in the last stage of the juvenescent phase, and 1, 712 specimens of the 11 th molting stage, 77.4mm, which is equivalent to the last molting adult phase. An increase in the ovarian maturation index of the 10 th molting individuals was seen from October to June (Fig. 2), and the ovarian eggs reached the ripe stage in June. At that time no sperm were detected in the seminal receptacle of th females examined. Most of these crabs were in the pre-molting condition having duplicate carapaces of outer old and inner new ones. While samples for July to September could not be obtained, the maturation index of the 10 th molting stage showed decrease again in October. Further, specimens of the 11 th molting stage with soft carapaces were caught. The ovarian histology of these old crabs showed a dissinct recovery phase of an immature picture after spawning, accompanied by many spermatophores in the seminal receptacle. From the facts mentioned above, it is natural to consider that the molt of the females in the 10 th molting stage occur in the period from July to September, and mating and further spawning take place for the first time in this species. While the ovarian maturation index of the samples in the 11 th molting stage obtained in October consisted of two components, immature and mature groups (Fig. 3), the spawning season of these two groups may occur only once a year, from January to April. Therefore, it is estimated that the fertilized eggs which are oviposited from July to September and get attached to the abdomen of adult crab itself for the first time are protected for about one a half years but eggs after the second time are protected for only one year.