Annual Reproductive Cycle of Two Japanese Species of Sipunculans :

The annual reproductive cycles of the Japanese sipunculans Siphonosoma cumanense (Keferstein) and Phascolosoma scolops (Selenka, de Man & Biilow) were studied based on size, density, and frequency distribution of gametes from May 1989 to November 1990 and October 1989 to February 1991, respectively. Aliquots from the total volume of the diluted coelomic fluid from each individual were analyzed. Coelomic oocytes of S. cumanense showed a slow growth rate (i.e., 3 J-lm/month) from December to April. An accelerated rate was observed in May to July. No oocytes were encountered from September to December, suggesting that spawning occurs in July to August. Sperm clusters were encountered only from May to August, with peaks in June and July. In contrast, oocytes and sperm clusters of P. scolops were present throughout the year. From January to March small oocytes grew at an average rate of 10 J-lm/month. Oocytes showed rapid growth until July. Smaller oocytes were noted thereafter, indicating a summer spawning. Fluctuation of sperm cluster density showed a pattern similar to that of oocytes. Both species showed two major peaks in gamete density, one during oocyte proliferation and one before spawning. Stages of gametogenesis in both species are described. In studying the annual reproductive cycle of sipunculans, the usual method is to extract a sample of coleomic fluid by means of a needle and a syringe and count or measure the gametes present. Data obtained in this manner may not give an accurate picture of the reproductive cycle of the animal because the distribution of gametes may be uneven within the coelom or the bore of the needle may be limiting. Extraction of the total quantity of coelomic fluid and taking samples from it may give a better estimate and could provide absolute rather than relative counts. Although it is a tedious method, it could provide better data. This method was used in our study. In this paper we present the annual reproductive cycle of two sipunculans, Siphonosoma cumanense (Keferstein) and Phascolosoma scolops (Selenka, de Man & Biilow), collected from Nishiwaki Beach, Okayama, and Kuroshima Island, Seto Inland Sea of Japan, respectively. Parameters studied were gamete density, oocyte size frequency distribution, and oocyte growth rate.