Causative factors and conditions of the population explosion of the moon jelly Aurelia aurita in Tokyo Bay

通过野外调查和室内实验，探讨了东京湾上部月牙水母群生的影响因素和条件。从1993年4月起，连续三年每月进行一次或两次果冻实地取样和水文条件测量。测定了水母大小的季节变化以及空斑和棘的发生情况。在此基础上，利用每年几个月的时间，在不同的温度和食物条件下进行了盘藻和珊瑚虫的饲养试验。研究结果揭示了杂交和有性生殖的形成过程。其他发现包括开始结缔组织的息肉的大小，结缔组织的持续时间，结缔组织的产生速度，以及棘的数量和大小。温度和饲养条件对水螅的生长和线虫的解放时间有很大的影响。温度降低后，随着取食时间的延长，水螅体芽数增加。前一年秋天定居的珊瑚虫在1月份释放出叶黄，而秋季定居的珊瑚虫在2 - 4月份产生叶黄。在适宜的温度和食物密度下，水螅生长迅速，数量增加迅速。当温度降至15ºC或更低时，会产生振荡。如果温度降低后食物条件仍然较好，则从大量的息肉中释放出大尺寸的叶虫。在这些特殊条件下，aurita的高繁殖潜力可能是其种群爆炸的原因。

Factors and conditions to relate mass occurrence of the moon jelly Aurelia aurita in the upper part of Tokyo Bay have been examined through field survey and laboratory experiment. In the field sampling of the jelly and measurement of hydrographic conditions were carried out once or twice a month during three consecutive years from April 1993. Seasonal variation of the size of medusae and occurrence of planulae and ephyrae were determined. Based on these information, rearing experiments of planulae and polyps were carried out under different temperature and food regimes during a few months in each year. As the result, processes of formation of the strobilation and a sexual reproduction by budding were revealed. Other findings included the size of polyps to commence strobilation, duration of strobilation, production rate of strobilae, and number and size of ephyrae.Growth of polyps and timing of the ephyla's liberation are strongly influenced with temperature and feeding condition.Number of buds of polyps increase with increasing feeding duration after the temperature lowered. The polyps settled before autumn of the previous year liberate ephyrae in January, whereas those settled in autumn produce ephyrae in February to April.With favorable higher temperature and food density during the early polyp stage, the polyps growth and increase the number rapidly. The strobilation is induced when the temperature lowered to 15ﾟC or less. If food condition is still preferable after the lowering temperature, large size ephylae are liberated from large numbers of polyps. The population explosion of A.aurita could be attributed to its high reproductive potential under these special conditions.