硬壳蛤(Mercenaria mercenaria)繁殖生物学研究

THE REPRODUCTIVE BIOLOGY OF HARD CLAM MERCENARIA MERCENARIA (LINNAEUS,1758)

采用组织学和实验生态学方法对浙江引种养殖的硬壳蛤的性腺发育、生殖周期、胚胎发育、幼虫和稚贝培育进行了研究。结果表明,浙江养殖的硬壳蛤性腺发育过程以1年为1个周期,可分为增殖期、生长期、成熟期、排放期、休止期5个阶段;繁殖期在4月下旬至6月下旬,水温19.0—28.2℃,可大批产卵二次;肥满度最高为7.11%(6月份),最低为3.68%(1月份);水温20—22℃时,D形幼虫孵出时间约为20h,浮游幼虫经9—10天培育进入变态附着期,10—11天发育变态为稚贝。研究还发现,硬壳蛤的精子在光学显微镜下观察呈牛角形或较直的长三角形,与常见的双壳类软体动物的精子形态明显不同。文中还讨论了性腺发育与积温的关系,指出性腺发育成熟的积温和有效积温大约为1800℃和600℃。

Hard clam Mercenaria mercenaria is a commercially valued species that naturally lives along the north American Atlantic coast. In 1997, it was introduced into China as a candidate for aquaculture. Since then, scientists have conducted many researches on its habitation, ecological physiology, metabolism, artificial breeding and strain development. In this paper, the reproductive biology of hard clam M. mercenaria was studied with histological section and experimental ecology, including its reproductive cycle, fatness, gonadal development, breeding season, embryo development, larvae and post-larvae development. All our experiments were conducted in the Field Station of Zhejiang Maricuhure Research Institute （ZMRI）, located in Yueqing Bay, Zhejiang Province, southern China （28°N）. M. mercenaria seeds were originally introduced in 1998 from the Haskin Shellfish Research Laboratory, Rutgers University, New Jersey, the United States, and planted in an earth pond in the station. Sampling twice a month during March to June and once a month in the rest months from January 2002 to January 2003. Shell size, weight and fatness were measured. Gonad tissues were fixed with Carnoy＇ s solution, and tissue sections were stained with H. E solution. Spawning was induced by water flow after being shady-exposed shock for 12 hours. Hatching with aeration and larvae cultured under a stable temperature and salinity. The results showed that a complete annual reproductive cycle of M. mercenaria, cultured in the inshore of Zhejiang Province. The gonad of M. mercenaria could be classified into follicular gland consisted of follicular, gonaduct and gonadial tube, in basic trait of them. And the gonadal development is divided into five stages ： proliferation stage from early January to later February in water temperature of 7.8-13.0℃; enhancement stage under 13.0-19.0℃ from early March to mid April; mature stage under 19.0-26.2℃ from later April to mid June; breeding stage under 19.0-28.2℃ from early May to later June, and suspension stage under 29.5-7.8℃from early July to December. Its breeding season is from the end of April to June （ 19.5-25.8℃ ） and twice mass spawning in a year. The highest fatness was 7.11% in June, and the lowest fatness was 3.68% in January. Under 20--22℃ of water temperature and 25 of salinity, D-shaped larval was hatched after 20 hours to 100μm × 80μm in size. The larvae were fed with high quality of a species of Isochrypis. After 2-3 days, the D-shaped larvae developed into early-umbo larvae stage to 120μm × 90μm in size, in 5th to 6th day, developed into mid-umbo larvae stage in a size of 175μm ×140μm, in 9th to 10th days, developed into late-umbo larvae and metamorphosis stage to 220μm × 210μm. Since 11th day, the larval began to settle down and developed into post-larval, and metamorphosis was completed into spat. It was observed that the sperms of M. mercenaria were in elongated-triangle or horn-shaped under light microscope, very different from that of other common bivalve species. Study on relationship of gonadal development to maturation with accumulated temperature. It was indicated that the accumulated temperature and effective accumulated temperature were about 1800℃ and 600℃ respectively.